Factor causing Methicilin-resistance in MRSA?

Factor causing Methicilin-resistance in MRSA?

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I think the main reason is the natural selection that is causing methicillin-resistance. However, I am not completely sure what this means practically.

Here, the original question:

MRSA was isolated in the throat secretion of patient hospitalized with bronchitis. Which of the following statements best characterizes this microorganism?

  • a. its MIC is increased to methicillin, but not to penicillin
  • b. methicillin-resistance is associated with production of β-lactamases
  • c. Methicillin-resistance is caused by changes in configuration (mutation) of PBP

What is the right answer?

I think you can normally think about similar pressures leading to extended-spectrum beta-lactamase (ESBLs) and methicillin/oxacillin-resistant Staphylococcus aureus (MRSA).

Going to the core of your question:

"Currently, the reported mechanism of methicillin resistance in S. aureus is the production of a distinctive penicillin binding protein 2a (PBP2a), which exhibits low affinity toward β-lactams"

But to really read on it you need to back to Liu et al.

Again Todar's is a great source for a good broad reading on staph including MRSA.

Response to Edit [inclusion of full question]:

TSDR: The answer is C.

Let's break down the three options given in the question and learn about MRSA along the way.

a. its MIC is increased to methicillin, but not to penicillin

First, for people who may not know, MIC is short for minimum inhibitory concentration. Simply, the MIC is a basic measurement of how much of any given agent is need to stop the growth of a bacterial colony. Often we think of current medical antibiotics when thinking of MIC, but even simpler things like table salt and sugar have a MIC for a given species. Thus this sentence is making the assertion that the bacteria found in the patent have an increased resistance to methicillin but not to penicillin. This leads us to ask the obvious question:

Can Staphylococcus aureus (staph going forward) be methicillin resistant but not penicillin resistant?

This is a bit of a trick question, but let's break it down to the needed components: What is Methicillin, what is/are penicillin(s), and how are staph resistant to them.

First that it should be noted that penicillins can actually refer to a whole class of antibiotics which all use β-Lactam and the specific set of antibiotics:

benzylpenicillin (penicillin G), procaine benzylpenicillin (procaine penicillin), benzathine benzylpenicillin (benzathine penicillin), and phenoxymethylpenicillin (penicillin V). (from wiki)

It should be noted that more correct way to address the whole class of antibiotics would be β-Lactam antibiotics, not penicillins, and that if you wanted to talk about more specifically penicillin derived compounds you would be discussing penams. For even further clarification, penicillin as drug most likely refers to benzylpenicillin, and for the rest of this answer I will use penicillin to refer to benzylpenicillin.

Thus we should think of penicillin as an early antibodic that work by preventing the dividing/genesis of cell walls and certain organelles via binding to penicillin binding proteins (PBPs).

Methicillin is also a β-Lactam antibiotic, and it's MOA is similar to penicillin. It was developed/discovered after penicillin and was seen a answer to Gram-positive bacteria that were breaking down penicillin via β-lactamase. Methicillian still works by binding to PBPs, but it escapes the bacteria's counter to penicillin.

This leads us to address how penicillin resistance and methicillin resistance commonly occur in staph. First, many staph strains and other bacteria use β-lactamases to breakdown antibiotics so they no longer can bind to PBP's (1). But methicillin is particularly suited by its side chains to not be degraded by β-lactamases. In their ground breaking work on the subject, Hartman and Tomasz identified that methicillin resistance was not in the acquisition of a β-lactamase, but in a mutation in PBP's that prevented methicillin binding (2). There they tested 4 strains of staph, two were methicillin resistant (MR), and two were methicillian susceptible (MS). You will note that all but 1 didn't have β-lactamase activity, and where more susceptible to penicillin than methicillin (ibid).

BUT this does not mean that MR strains are not also penicillin resistant, instead it shows that the resistance can be independent of each other. Therefore "A" is wrong because it tries to draw a correlation that is not there. In reality many MR strains are also β-lactamase positive.

This also address the problem with "B." While it is possible that a β-lactamase could bind to methicilin and lead to degradation of the antibiotic, the main mechanism of methicillin resistance is the mutation of PBPs (ibid, 3), in particular BPB2a (4, 5).

We are then left with the task of figuring out why "C" is the correct choice. Indeed "C" is the given reason for the failure of "A" and "B," but we can still go deeper into how and perhaps why BPB2a mutated.

For that I think it is best that we turn back to Chambers' review of the subject. Forgive my over quoting, but it's done so well there and the text is now open.

Methicillin resistance is associated with production of a novel PBP that is not present in susceptible staphylococci. Resistant strains of S. aureus produce an additional 78- kilodalton PBP (Fig. 1), termed PBP2a or PBP2' (assumed to be identical for the purposes of this review), that has a low binding affinity for beta-lactam antibiotics.

PBP2a is highly conserved. Limited proteolysis of PBP2a from unrelated strains of S. aureus (123) and coagulasenegative staphylococci (31), whether homogeneous or heterogeneous, generates remarkably similar peptide fragments.

Presumably PBP2a can substitute for essential PBPs when these have been saturated by drug and can perform the functions necessary for cell wall assembly (22, 122).

In some strains, PBP2a is inducible by beta-lactam antibiotics and its production differs according to growth conditions (34, 122, 125, 159).

Unfortunately, they didn't quite have the staphylococcal cassette chromosome mec (SCCmec) figured out at that point. The genetics is quite complex.

How does MRSA genetically accomplish resistance?

As we already established, the resistance comes from the production of an alternate PBP, PBP2a. SCCmec is interesting for several reasons. First of all, it's much larger than a plasmid, and contains information for several genes. That's why it's called cassette chromosome. Further it normally incorporates into the same part of the genome in staph, in an area know as OrfX (6). This means that even during horizontal transfer, that the cassette has to direct it's integration into the genome, which is exceedingly uncommon, or at least there are not many other know examples (7, 8). This cassette can be spread horizontally between staph, and even with other species (ibid, 9). Even if the integration site (integration site sequence, ISS) is slightly different, this specificity is carried out by cassette chromosome recombinases (ccr), wich are also on SCCmec (8). This is carried out by ccr-medated recombination of the target chromosome, and further details on the process are considered outside the scope of this question.

The actual gene that encodes PBP2a is called mecA. But as we mentioned above, PBO2a production can be induced and regulated. It is likely less favorable to produce it in the absence of antibodies, and after serial passage of bacteria in antibiotic free broth, you find that PBP2a expression can drop drastically. Therefore regulatory and other useful proteins encoded by SCCmec. When placed in a β-Lactamase environment, MecR1 causes a single transduction cascade to start transcription of mecA (10). Conversely, MecI provides a negative feedback loop to MecR1, and in the absence of β-Lactamase, will lead to the down regulation of mecA (ibid, 11). The actual action of MecR1 is to cleave MecI, thereby remove the suppression of mecA by MecI. I actually learned something new when reading the wiki on MRSA, but didn't do further research on the subject:

mecA is further controlled by two co-repressors, BlaI and BlaR1. blaI and blaR1 are homologous to mecI and mecR1, respectively, and normally function as regulators of blaZ, which is responsible for penicillin resistance. The DNA sequences bound by MecI and BlaI are identical; therefore, BlaI can also bind the mecA operator to repress transcription of mecA.

This represents the general pattern of resistance of MRSA, but there is a rich diversity in the particulars of how each strain manages expression. Two of the main identifiers are how the mecA gene complex and ccr gene complex are configured (carriage). The other two identifiers are the ISS and whether or not the ISS is repeated in the target chromosome (and how many times it's repeated) (8). If we just consider the mecA and ccr carriage, then we get a great summary from IWG-SCC (ref 8):

The mec gene complex is composed of mecA, its regulatory genes, and associated insertion sequences. The class A mec gene complex (class A mec) is the prototype complex, which contains mecA, the complete mecR1 and mecI regulatory genes upstream of mecA, and the hypervariable region (HVR) and insertion sequence IS431 downstream of mecA. The class B mec gene complex is composed of mecA, a truncated mecR1 resulting from the insertion of IS1272 upstream of mecA, and HVR and IS431 downstream of mecA. The class C mec gene complex contains mecA and truncated mecR1 by the insertion of IS431 upstream of mecA and HVR and IS431 downstream of mecA. There are two distinct class C mec gene complexes; in the class C1 mec gene complex, the IS431 upstream of mecA has the same orientation as the IS431 downstream of mecA (next to HVR), while in the class C2 mec gene complex, the orientation of IS431 upstream of mecA is reversed. C1 and C2 are regarded as different mec gene complexes since they have likely evolved independently. The class D mec gene complex is composed of mecA and ΔmecR1 but does not carry an insertion sequence downstream of ΔmecR1 (as determined by PCR analysis).


ccr gene complex.The ccr gene complex is composed of the ccr gene(s) and surrounding open reading frames (ORFs), several of which have unknown functions. Currently, three phylogenetically distinct ccr genes, ccrA, ccrB, and ccrC, have been identified in S. aureus with DNA sequence similarities below 50% (Fig. 2 and 3). The ccrA and ccrB genes that have been identified to date have been classified into four allotypes. In general, ccr genes with nucleotide identities more than 85% are assigned to the same allotype, whereas ccr genes that belong to different allotypes show nucleotide identities between 60% and 82%. All ccrC variants identified to date have shown ≥87% similarity; thus, there is only one ccrC allotype. We suggest describing their differences as alleles by using previously used numbers, e.g., ccrC1 allele 2 or ccrC1 allele 8.

All of this is nicely summarized in their table:

egin{array} {|r|r|r|} hline SCCmec ~type &ccr ~gene ~complex &mec ~gene ~complex hline I &1 ~(A1B1) &B hline II &2 ~(A2B2)&A hline III &2 ~(A3B3)&A hline IV &2 ~(A2B2)&B hline V &5 ~(C)&C2 hline VI &4 ~(A4B4)&B hline VII &5 ~(C)&C1 hline VIII &4 ~(A4B4)&A hline end{array}

To sum up, methicillin-resistance is caused by the encoding of a novel PBP, PBP2a, along with other factors, which is both genetically stable and transferable via the use of SCCmec.


(1) Pathak A et al. High prevalence of extended-spectrum β-lactamase-producing pathogens: results of a surveillance study in two hospitals in Ujjain, India. Infect Drug Resist. 2012;5:65-73. doi: 10.2147/IDR.S30043. Epub 2012 Apr 5. [Note that many of the great sentinel studies on drug resistant bacteria occur in the massive hospitals in India].

(2) B Hartman and A Tomasz. Altered penicillin-binding proteins in methicillin-resistant strains of Staphylococcus aureus. Antimicrob Agents Chemother. 1981 May; 19(5): 726-735.

(3) Liu, H. et al. Detection of borderline oxacillin-resistant Staphylococcus aureus and differentiation from methicillin-resistant strains. Eur J Clin Microbiol Infect Dis. 1990 Oct;9(10):717-24.

(4) Tawil N. et al. The differential detection of methicillin-resistant, methicillin-susceptible and borderline oxacillin-resistant Staphylococcus aureus by surface plasmon resonance. Biosens Bioelectron. 2013 Nov 15;49:334-40. doi: 10.1016/j.bios.2013.05.031. Epub 2013 Jun 4.

(5) Chambers HF. Methicillin-resistant staphylococci. Clin Microbiol Rev. 1988 Apr;1(2):173-86.

(6) Huletsky A et al. New real-time PCR assay for rapid detection of methicillin-resistant Staphylococcus aureus directly from specimens containing a mixture of staphylococci. J Clin Microbiol. 2004;42(5):1875-84. DOI: 10.1128/JCM.42.5.1875-1884.2004

(7) Schoenfelder SM et al. Success through diversity - how Staphylococcus epidermidis establishes as a nosocomial pathogen. Int J Med Microbiol. 2010 Aug;300(6):380-6. doi: 10.1016/j.ijmm.2010.04.011. Epub 2010 May 6.

(8) IWG-SCC. Classification of staphylococcal cassette chromosome mec (SCCmec): guidelines for reporting novel SCCmec elements. Antimicrob Agents Chemother. 2009 Dec;53(12):4961-7. doi: 10.1128/AAC.00579-09. Epub 2009 Aug 31.

(9) Stürenburg E. Rapid detection of methicillin-resistant Staphylococcus aureus directly from clinical samples: methods, effectiveness and cost considerations. Ger Med Sci. 2009 Jul 6;7:Doc06. doi: 10.3205/000065.

(10) Jensen SO and Lyon BR. "Genetics of antimicrobial resistance in Staphylococcus aureus". Future Microbiol 4 (5): 565-82. doi:10.2217/fmb.09.30. PMID 19492967

(11) Oliveira DC, de Lencastre H. Methicillin-resistance in Staphylococcus aureus is not affected by the overexpression in trans of the mecA gene repressor: a surprising observation. PLoS One. 2011;6(8):e23287. doi: 10.1371/journal.pone.0023287. Epub 2011 Aug 2.

Acquisition of a new PBP (eg methicillin resistance in staphylococcus aureus), source Murray.

So I think the answer is C: change in configuration (mutation) of PBP, which can interpreted as acquisition of a new PBP.

Therapeutic Areas II: Cancer, Infectious Diseases, Inflammation & Immunology and Dermatology Community-Acquired Methicillin-Resistant Staphylococcus aureus

CA-MRSA are characterized as strains of SCCmec types IV and V, 26 which typically do not have the genetic elements that encode resistance to other antibiotic classes like the HA-MRSA, and are thus primarily resistant only to β-lactams. 54 The overall prevalence of CA-MRSA strains in the community is uncertain, but is apparently on the rise. 34 A recent report found the incidences in the two US cities of Baltimore, MD, and Atlanta, GA, to be 18 and 26 cases per 100 000 residents, respectively. 36 Recommended effective treatment regimens for skin infections with CA-MRSA following incision and drainage are linezolid, vancomycin, or trimethoprim/sulfamethoxazole plus rifampin. 55

Methicillin Resistant Staph: Superbug Infections

A lot of media attention has been given to the topic of “superbugs” and the threat that they pose. Accounts often border on sensationalism, seemingly with the intent to provoke fear, rather than to educate the public. One of the most talked about “superbugs” is MRSA – methicillin-resistant Staphylococcus aureus. MRSA used to be associated with hospital-acquired (nosocomial) infection, but is now being encountered more and more commonly in community-acquired infection. Is MRSA a superbug? For that matter, what is a superbug?

When I hear the term superbug, my imagination creates an image of a germ in a super-hero cape. And, of course, this super-hero represents evil rather than good. If I wanted to carry this fantasy a little further, and if I relied on drugs as a cure all, as it seems our pharmaceutical companies would have us believe, I would envision MRSA as the arch-rival of Rx-Marvel or some other caped drug who fights to eradicate the evil MRSA.

Presumably, superbugs are those with special virulence factors that result in the ability to cause infection more readily, cause more severe infection, and/or be extremely difficult to treat.

Had we coined the term superbug in the 1960s, penicillin-resistant Staphylococcus aureus, commonly referred to as “staph”, would have been considered a superbug. The emergence of methicillin resistance is similar to the emergence of penicillin resistance in staphylococcus. I’m not sure how many of you can remember back that far, but if you can you may also remember that staph infections occurred just as commonly in those days, and were usually treated with penicillin.

Eventually, Staphylococcus aureus developed resistance to penicillin, and today most strains of the organisms are untreatable using penicillin. Today, we are experiencing the same trend with MRSA, and although infections involving MRSA are still more commonly encountered in a hospital or healthcare environment – hospital acquired MRSA (HAMRSA) – the organism is increasingly being isolated from infections acquired in the community – community-acquired MRSA (CAMRSA). To date, CAMRSA infections have been associated with infection in marginalized populations including prison inmates, homeless people and injection drug users however, infections are increasingly being reported in sports team participants, and are also being reported in the general population in a number of countries. This trend will no doubt occur in Canada as well.

Spread of Infection and Pathology

S. aureus lives harmlessly in the mucous membranes of the nose on about 33% of the general population (14). When the organism is able to penetrate the skin or mucous barriers it can lead to infection. The most common methods of spread outside of a hospital setting include: contact with pus from an infection site, skin-to-skin contact or sharing towels, clothing or athletic equipment with an infected individual. The compound hyaluronidase, which is produced by S. aureus, destroys soft tissue and allows for easier spread once inside the body.

Once an infection has occurred, the first, most common symptom is a significant abscess on the epidermis (Figure 4). From this abscess, proteolytic enzymes produced by S. aureus enable it to disperse throughout the body and cause secondary infections. The organism is now able to cause pneumonia and also infection of the joints, bones and heart valves. CA-MRSA strains also produce enterotoxins, which can cause serious illnesses. These toxins can be the cause of certain forms of food poisoning, Staphylococcal Scalded Skin Syndrome (SSSS), and Toxic Shock Syndrome (TSS). SSSS is a rare disease that usually affects infants by the production of an exfoliative enterotoxin. TSS is the most widely known byproduct of CA-MRSA infections. Improper use of tampons has been the cause of the vast majority of TSS cases, as the tampon can create a nutrient-rich environment for S. aureus to live. Symptoms include hypotension, fever, rash and can lead to hepatic and renal dysfunction, membrane hyperemia and thrombocytopenia (11).

Outbreaks of CA-MRSA are most common in athletic teams, prisons and military training centers and often occur in large clusters. As previously discussed, athletic teams will typically spread the infection rapidly based on skin-to-skin contact during participation in competition and/or sharing of towels or equipment in the locker room. Thus, one infected individual can rapidly pass the infection on to multiple teammates by means of this mechanism. In 2000, ten members of a Pennsylvania college football team presented with CA-MRSA infections, with seven of the members requiring hospitalization. Aforementioned risk factors explained the relatively rapid and significant spread of the infection. A fencing team reported that five of its members had been infected with MRSA. Equipment and towel sharing was reported as common. Electronic sensing wires were worn underneath clothing during competition and were not routinely cleaned. These pathways are probable culprits for the infection spread (4).

In recent years, professional teams have experienced MRSA outbreaks. Since 2003, the Cleveland Browns, Washington Redskins and St. Louis Rams have all had multiple members infected by S. aureus. There have been several notable cases of MRSA in Ohio, including several professional and high school-level incidents around the Cleveland area. In 2005, two members of the Toronto Blue Jays presented with MRSA colonies. In all cases, each sports organization sterilized locker rooms, replaced outdated whirlpools, provided surgical scrub soap to combat turf burn-induced infections and increased education regarding MRSA. A 2003 study of a MRSA outbreak in a Georgia prison revealed that the likelihood of MRSA infection was inversely associated with hand-washing and showering frequency. Individuals who were colonized by S. aureus were found to be at a higher risk for infection. Furthermore, the study found that younger ages as well as obesity are risk factors for MRSA infection. Younger age was hypothesized to result in a more active lifestyle, which in turn caused more abrasions, etc. in this setting. Obesity may lead to differing patterns of MRSA skin colonization, which can increase the potential for infection (15).

MRSA outbreaks are also problematic at military training facilities where close contact and sharing of equipment exist as significant risk factors. During a MRSA outbreak at a military training facility from October 2000 to July 2002 monthly incidence rates fluctuated from approximately 4.9 to 11 cases per 1,000 recruits. The recruits, between 17-25 years of age, most frequently presented with MRSA colonies on the extremities (73.7%). It was noted in this study that there were more cases of infection during warmer months as well as during high infection-risk training weeks (Figure 5). S. aureus infections are known to be spread more readily in warmer months (2), but other reasons for this seasonal spread were theorized. Some recruits complained of bug bites that lead to infection, which are seasonal. More frequently exposed skin is also a likely source of transmission in this case, particularly in instances where training put recruits in close physical contact. In 2002, administrators placed anti-bacterial hand soaps at every recruit sink, extended time allotted for showering and cleaning, prohibited towel and razor sharing, and recommended that hand-washing be performed as frequently as possible. The outbreak subsided shortly after these measures were implemented (12).


Risk of MRSA infection is elevated among children, elderly individuals, athletes, military personnel, individuals who inject drugs, persons with an indigenous background or in urban, underserved areas, individuals with HIV or cystic fibrosis, those with frequent health-care contact and those in institutionalized populations, including prisoners 37,38,39,40 . Rates of MRSA infection increased rapidly between the 1990s and early 2000s. Since 2005, parallel decreases in MRSA infections have been confirmed in multiple US and European populations, especially among bloodstream and soft tissue infections 10,41,42,43,44 . Paediatric trends mirror those seen in adults in the United States 45 . Although specific factors responsible for the changing rates of MRSA infection remain uncertain, advances in molecular epidemiology are informing an increasingly complex understanding of MRSA population dynamics.

Between the first reports of MRSA in 1961 and in the 1990s, infection was generally associated with health-care contact. By the 1990s, cases of MRSA infection emerged in individuals who had no prior hospitalization, leading to separate definitions for HA-MRSA and CA-MRSA. CA-MRSA isolates were initially distinguished by lower rates of clindamycin resistance (particularly in the United States), increased likelihood of PVL expression, a predominance of SCCmec type IV or type V and strain types ST5 or ST8 (refs 5,46 ). However, since the 1990s, genotypic differences by site of acquisition have begun to homogenize, demonstrating that CA-MRSA and HA-MRSA can each invade the other’s niche 47 .

The molecular epidemiology of S. aureus is largely characterized by the successive emergence of regionally predominant strain types. Penicillin-resistant phage type 80 or type 81 of S. aureus surged between 1953 and 1963. After originating in hospitals, it spread to communities in North America, the United Kingdom and Australia before inexplicably receding again 48 . With the emergence of MRSA in the 1960s, HA-MRSA began to affect hospitals in North America, the United Kingdom, Australia and Japan while spreading in the Scandinavian countries to a lesser extent. Sporadic reports of MRSA occurring without prior health-care contact began to appear in the 1980s and 1990s, just before the widespread emergence of regionally dominant CA-MRSA strains.

Perhaps the most infamous of these strains, USA300 (an ST8 or CC8 derivative), rapidly overtook other circulating strain types as a dominant cause of CA-MRSA skin and soft tissue infections across the United States in 2000 (ref. 49 ). Some of the earliest cases of USA300 arose with an outbreak of CA-MRSA skin infections among a group of football players in Pennsylvania, followed shortly thereafter by a similar cluster in a Mississippi prison, establishing the first epidemiologic associations between MRSA, athletes and those in prisons 50,51 . As USA300 spread, it also proved capable of causing invasive infection at a wide range of body sites, perhaps most notably necrotizing pneumonia following influenza virus infection 52 . In contrast to its rapid spread across North America and despite multiple introductions into other continents, USA300 has not achieved the same dominance globally 53 . Although it has become regionally established across the globe in several countries outside of North America, emerging evidence suggests that the total burden of infection from USA300 is finally beginning to slow or even decline in parallel with an overall decrease in MRSA incidence rates 54,55 . It is worth noting that a parallel CA-MRSA epidemic occurred in South America with the related strain USA300-LV (Latin American variant), though this strain appears to have arisen from a common ancestor rather than by direct spread of USA300 (ref. 56 ).

Other well-described MRSA strain types show similar patterns of regional epidemic spread. Unlike USA300 in North America, however, MRSA isolates exhibit greater genetic diversity globally (Fig. 2). Epidemic methicillin-resistant S. aureus 15 (EMRSA-15) ST22 (CC22) and EMRSA-16 ST36 (CC30) emerged as predominant HA-MRSA strain types in the United Kingdom in the late 1990s 57,58,59 . The same strain types, ST22 and ST30, predominate among HA-MRSA isolates in continental Europe 60 . ST30 (CC30) has also successfully spread through the Asia–Pacific regions and parts of the Americas 48,61,62 . Beyond the wide spread of CC30 strains, this particular clonal complex has been associated with relatively higher invasive infection rates and mortality 63 . The ST22 strain appears to be gradually overtaking ST239, another widely distributed HA-MRSA strain (from CC8) that has been found in Europe, the Middle East, Asia and the Pacific 64,65 . European CA-MRSA exhibits a fair amount of diversity, though ST80 has been well described in parts of western Europe with some spread to northern Africa and the Middle East 59,64,66 . Just as USA300 has achieved only limited spread beyond the United States, even relatively successful European strains, such as ST30, ST22 and ST80, remain rare in the United States 8 .

a | Map of major strain type distributions. Regional strain prevalence is summarized from select studies performed in Africa 167 , Asia 67,68,69,71,168,169,170 , Australia 70 , Europe 57,58,59,60,66,73,171 , the Middle East 172 , North America 5,74,173,174,175 and South America 56,62 . The map provides an overview of strain diversity and cannot comprehensively display all relevant strain types within each region. As strain prevalence may vary by region and setting, the prevalence displayed from selected studies may not reflect strain prevalence throughout the entire region. b | Maximum likelihood SNP dendrogram for 60 Staphylococcus aureus isolates representing relationships between major clonal complexes. SNPs for each genome were concatenated to form SNP pseudosequences and used to generate a phylogenetic tree using the HKY93 algorithm with 500 bootstrap replicates. Notably, isolate grouping by multilocus sequence type is largely congruent with strain clustering by the SNP dendrogram. Part b is reproduced from ref. 176 , CC-BY-ND (

Strain maps of isolates from Asia and the Pacific are especially diverse, with ST72 (CC8) well described in Korea, ST8 or ST30 in Japan, ST59 in Taiwan, and greater diversity still in China 61,67,68,69 . ST93 is well described as a major cause of CA-MRSA skin and soft tissue infections specifically among indigenous populations in central Australia, whereas ST772 (the Bengal Bay clone) has spread from its namesake Bay of Bengal in the Indian Ocean to parts of Pakistan and Nepal, confirming the ongoing emergence of distinct, regionally predominant clones 70,71 .

The One Health approach has also drastically informed MRSA epidemiology with the recognition of CA-MRSA transmission between livestock and humans 9,72 . ST398 (CC398) has been well reported as a cause of livestock-associated CA-MRSA in Europe since 2005 (ref. 73 ). ST398 has since been confirmed as a cause of livestock-associated CA-MRSA also in Asia, Australia and the Americas, though it is not the only strain to occur in livestock 69,74 . Interspecies transmission may impose additional evolutionary constraints on MRSA, as particular genetic markers associated with immune evasion, such as scn, chp and sak, appear to exhibit divergent selection, being positively correlated with human infection but negatively associated with livestock colonization 75 .

Insights from genomics

One interesting feature of MRSA epidemiology is that despite substantial overall diversity, relatively few strains dominate. Whole genome sequencing has allowed the reconstruction of the spread of MRSA within both health-care systems and communities. Phylogenetic reconstructions from whole genome sequencing data of CA-MRSA isolates confirm that USA300 emerged through rapid clonal expansion rather than convergent evolution 6 . Whole genome sequencing has even provided sufficient resolution to determine that household clustering (for example, co-occurrence of multiple cases of MRSA infection within one familial dwelling) likely had a substantial role in the transmission of USA300 within the community 32,76 . Similarly, detailed phylogenetic reconstructions suggest that individuals colonized by circulating community strains subsequently introduced USA300 into hospitals, resulting in the eventual intermixing of CA-MRSA and HA-MRSA strains 47 .

Modern genomic approaches have provided new insights into the factors driving the emergence and spread of successful CA-MRSA strains. Subsequent attempts to identify the factors responsible for the success of USA300 identified multiple candidates, broadly categorized into MGEs and core genome components. Early attention focused on MGEs. Whole genome sequencing of USA300 isolates quickly identified multiple MGEs carrying a range of virulence factors (including PVL) and drug resistance determinants. Among these, the arginine-catabolic mobile element (ACME) appeared unique to USA300. One of the key enzymes in the arginine catabolism pathway, arginine deiminase, inhibits innate and adaptive immune responses and improves pathogen survival. By increasing the expression of multiple genes within this pathway, ACME was hypothesized to increase the fitness of USA300 relative to other S. aureus strains 77 . However, further genome-level comparative studies demonstrated that MGEs do not explain the entirety of the success of USA300. Instead, it seems that upregulation of the virulence regulatory gene agr mediates increased expression of PSMs and α-toxin, correlating with increased virulence in USA300 independent of any MGEs 34,78 .

Genomic methods have also helped to map the spread of HA-MRSA across the globe, exemplified by the story of HA-MRSA in the United Kingdom. EMRSA-16 (CC30) was the predominant HA-MRSA strain in the United Kingdom beginning in the 1990s, before being overtaken by EMRSA-15 (CC22) in the early 2000s 59 . As with CA-MRSA, whole genome sequencing permitted the detailed tracking of the spread of each strain. EMRSA-16, for example, appears to have spread from major urban centres, such as London and Glasgow, outwards to regional and local hospitals — likely carried by patients transferring from one facility to another 7 . Acquisition of antimicrobial resistance (particularly to fluoroquinolones and antiseptics, such as quaternary ammonia compounds) correlates with the spread of EMRSA-16, linking strain success to an ability to survive strong selective pressures within the health-care environment where antibiotics and antiseptics are commonplace. A remarkably similar series of events was seen with the successor of EMRSA-16, EMRSA-15 (ST22), which has since spread beyond the United Kingdom to other parts of Europe, Asia, Australia and Africa. EMRSA-15 has also acquired resistance to additional antibiotics as it has spread — again likely contributing to its success in the highly selective hospital environment. Although many antibiotic resistance markers appear to have been acquired on multiple occasions, fluoroquinolone resistance is the most stable and consistent trait among successful isolates 79 .

Drastic shifts in epidemiology rarely arise as a result of a single genetic trait, however, and more recent analysis suggests that at least some of the clonal expansion of HA-MRSA preceded the widespread acquisition of fluoroquinolone resistance 32,54,79 .

Additionally, there is emerging evidence that declining HA-MRSA infection rates are strain-specific and preceded the deployment of enhanced infection control and antibiotic stewardship measures in the United Kingdom 11 . This pattern suggests that selection pressures imposed by human efforts have been less influential than originally thought. Although genomics has substantially expanded our understanding of MRSA epidemiology, the factors contributing to the success of particular strains are not fully understood. It is likely that strain dominance results from the complex interplay of both genetic adaptations and host genetic variability and from the broader context shaped by the environment, health-care practices and social and geographic factors.

The Problem

The incidence of disease caused by MRSA bacteria is increasing worldwide. Thirty years ago, MRSA accounted for 2 percent of staph infections. By 2003, 64 percent of staph infections were caused by MRSA. According to a report by the Centers for Disease Control and Prevention (CDC) in the United States in 2005, more 94,000 people developed life-threatening infections caused by MRSA nearly 19,000 people died during hospital stays related to these MRSA infections. The majority of MRSA cases, 85 percent, were associated with healthcare facilities, while approximately 14 percent occurred in individuals with no known exposure to healthcare.

The staph bacterium continues to evolve and is beginning to show resistance to additional antibiotics. In 2002 the first staph strains were found that are resistant to vancomycin, an antibiotic that is one of the few available treatments used as a last resort against MRSA. Although vancomycin-resistant staph strains are currently still quite rare, it is feared that these strains will become more widespread over time and further reduce the limited number of antibiotics that are effective against MRSA.

The rising problem of resistance of staph bacteria to methicillin and other antibiotics is part of a larger issue that greatly concerns healthcare professionals. The emergence of antimicrobial-resistant organisms is making it more difficult to treat a variety of infectious diseases. Besides MRSA, the treatment of other diseases complicated by drug resistance include HIV, tuberculosis, influenza, and malaria.

Drug resistance occurs because microbes, such as staph bacteria, need to reproduce to ensure their survival. When this ability is threatened, as when they are exposed to antibiotics, microbes adapt and evolve to overcome the block to their reproduction. This can occur naturally, and microbes become genetically altered in ways which allow them to survive in the presence of antimicrobial drugs. However, drug resistance adaptations can be accelerated by human actions, particularly by the overuse and inappropriate use of antibiotics. The escalating use of antimicrobials in humans, animals, and agriculture is increasing the problem of drug resistance.

The consequences of antimicrobial resistance pose a significant concern to scientists and medical professionals. Infection with drug-resistant organisms can lead to increased and longer hospital stays, more complicated treatment, more deaths, and higher healthcare costs.


In humans, Staphylococcus aureus is part of the normal microbiota present in the upper respiratory tract, [2] and on skin and in the gut mucosa. [3] However, along with similar bacterial species that can colonize and act symbiotically, they can cause disease if they begin to take over the tissues they have colonized or invade other tissues the resultant infection has been called a "pathobiont". [2]

After 72 hours, MRSA can take hold in human tissues and eventually become resistant to treatment. The initial presentation of MRSA is small red bumps that resemble pimples, spider bites, or boils they may be accompanied by fever and, occasionally, rashes. Within a few days, the bumps become larger and more painful they eventually open into deep, pus-filled boils. About 75 percent of CA-MRSA infections are localized to skin and soft tissue and usually can be treated effectively. [4]

A select few of the populations at risk include:

  • People with indwelling implants, prostheses, drains, and catheters [1][5]
  • People who are frequently in crowded places, especially with shared equipment and skin-to-skin contact [6]
  • People with weak immune systems (HIV/AIDS, lupus, or cancer sufferers transplant recipients severe asthmatics etc.) [1][7] users [8][9]
  • Regular contact with someone who has injected drugs in the past year [10]
  • Users of quinolone antibiotics[5][11]
  • Elderly people [5][12]
  • School children sharing sports and other equipment
  • College students living in dormitories [6]
  • People staying or working in a health-care facility for an extended period of time [5][6]
  • People who spend time in coastal waters where MRSA is present, such as some beaches in Florida and the West Coast of the United States[13][14]
  • People who spend time in confined spaces with other people, including occupants of homeless shelters, prison inmates, and military recruits in basic training[15][16]
  • Veterinarians, livestock handlers, and pet owners [17]
  • People who ingest unpasteurized milk [18]
  • People who are immunocompromised and also colonized [19] : 249
  • People with chronic obstructive pulmonary disease[5]
  • People who have had thoracic surgery [5]

As many as 22% of people infected with MRSA do not have any discernable risk factors. [20] : 637

Hospitalized people Edit

People who are hospitalized, including the elderly, are often immunocompromised and susceptible to infection of all kinds, including MRSA an infection by MRSA is called healthcare-associated or hospital-acquired methicillin-resistant S. aureus (HA-MRSA). [1] [5] [21] [22] Generally, those infected by MRSA stay infected for just under 10 days, if treated by a doctor, although effects may vary from person to person. [23]

Both surgical and nonsurgical wounds can be infected with HA-MRSA. [1] [5] [21] Surgical site infections occur on the skin surface, but can spread to internal organs and blood to cause sepsis. [1] Transmission can occur between healthcare providers and patients because some providers may neglect to perform preventative hand-washing between examinations. [11] [24]

People in nursing homes are at risk for all the reasons above, further complicated by their generally weaker immune systems. [12] [25]

Prison inmates and military personnel Edit

Prisons and military barracks [18] can be crowded and confined, and poor hygiene practices may proliferate, thus putting inhabitants at increased risk of contracting MRSA. [17] Cases of MRSA in such populations were first reported in the United States and later in Canada. The earliest reports were made by the Centers for Disease Control and Prevention in US state prisons. In the news media, hundreds of reports of MRSA outbreaks in prisons appeared between 2000 and 2008. For example, in February 2008, the Tulsa County jail in Oklahoma started treating an average of 12 S. aureus cases per month. [26]

Animals Edit

Antibiotic use in livestock increases the risk that MRSA will develop among the livestock strains MRSA ST 398 and CC398 are transmissible to humans. [18] [27] Generally, animals are asymptomatic. [1]

Domestic pets are susceptible to MRSA infection by transmission from their owners conversely, MRSA-infected pets can also transmit MRSA to humans. [28]

Athletes Edit

Locker rooms, gyms, and related athletic facilities offer potential sites for MRSA contamination and infection. [29] Athletes have been identified as a high-risk group. [18] A study linked MRSA to the abrasions caused by artificial turf. [30] Three studies by the Texas State Department of Health found the infection rate among football players was 16 times the national average. In October 2006, a high-school football player was temporarily paralyzed from MRSA-infected turf burns. His infection returned in January 2007 and required three surgeries to remove infected tissue, and three weeks of hospital stay. [31]

In 2013, Lawrence Tynes, Carl Nicks, and Johnthan Banks of the Tampa Bay Buccaneers were diagnosed with MRSA. Tynes and Nicks apparently did not contract the infection from each other, but whether Banks contracted it from either individual is unknown. [32] In 2015, Los Angeles Dodgers infielder Justin Turner was infected while the team visited the New York Mets. [33] In October 2015, New York Giants tight end Daniel Fells was hospitalized with a serious MRSA infection. [34]

Children Edit

MRSA is becoming a critical problem in children [35] studies found 4.6% of patients in U.S. health-care facilities, (presumably) including hospital nurseries, [36] were infected or colonized with MRSA. [37] Children and adults who come in contact with day-care centers, [18] playgrounds, locker rooms, camps, dormitories, classrooms and other school settings, and gyms and workout facilities are at higher risk of contracting MRSA. Parents should be especially cautious of children who participate in activities where sports equipment is shared, such as football helmets and uniforms. [38]

Injection drug abusers Edit

Needle-required drugs have caused an increase of MRSA, [39] with injection drug use (IDU) making up 24.1% (1,839 individuals) of Tennessee Hospital's Discharge System. The unsanitary methods of injection causes an access point for the MRSA to enter the blood stream and begin infecting the host. Furthermore, with MRSA's high contagion rate, [10] a common risk factor is individuals who are in constant contact with someone who has injected drugs in the past year. This does still depend how strong the non-infected individual's immune system is and how long both individuals remain in contact.

Antimicrobial resistance is genetically based resistance is mediated by the acquisition of extrachromosomal genetic elements containing genes that confer resistance to certain antibiotics. Examples of such elements include plasmids, transposable genetic elements, and genomic islands, which can be transferred between bacteria through horizontal gene transfer. [40] A defining characteristic of MRSA is its ability to thrive in the presence of penicillin-like antibiotics, which normally prevent bacterial growth by inhibiting synthesis of cell wall material. This is due to a resistance gene, mecA, which stops β-lactam antibiotics from inactivating the enzymes (transpeptidases) critical for cell wall synthesis. [41]

SCCmec Edit

Staphylococcal cassette chromosome mec (SCCmec) is a genomic island of unknown origin containing the antibiotic resistance gene mecA. [42] [43] SCCmec contains additional genes beyond mecA, including the cytolysin gene psm-mec, which may suppress virulence in HA-acquired MRSA strains. [44] In addition, this locus encodes strain-dependent gene regulatory RNAs known as psm-mecRNA. [45] SCCmec also contains ccrA and ccrB both genes encode recombinases that mediate the site-specific integration and excision of the SCCmec element from the S. aureus chromosome. [42] [43] Currently, six unique SCCmec types ranging in size from 21–67 kb have been identified [42] they are designated types I–VI and are distinguished by variation in mec and ccr gene complexes. [40] Owing to the size of the SCCmec element and the constraints of horizontal gene transfer, a minimum of five clones are thought to be responsible for the spread of MRSA infections, with clonal complex (CC) 8 most prevalent. [42] [46] SCCmec is thought to have originated in the closely related Staphylococcus sciuri species and transferred horizontally to S. aureus. [47]

Different SCCmec genotypes confer different microbiological characteristics, such as different antimicrobial resistance rates. [48] Different genotypes are also associated with different types of infections. Types I–III SCCmec are large elements that typically contain additional resistance genes and are characteristically isolated from HA-MRSA strains. [43] [48] Conversely, CA-MRSA is associated with types IV and V, which are smaller and lack resistance genes other than mecA. [43] [48]

These distinctions were thoroughly investigated by Collins et al. in 2001, and can be explained by the fitness differences associated with carriage of a large or small SCCmec plasmid. Carriage of large plasmids, such as SCCmecI–III, is costly to the bacteria, resulting in a compensatory decrease in virulence expression. [49] MRSA is able to thrive in hospital settings with increased antibiotic resistance but decreased virulence – HA-MRSA targets immunocompromised, hospitalized hosts, thus a decrease in virulence is not maladaptive. [49] In contrast, CA-MRSA tends to carry lower-fitness cost SCCmec elements to offset the increased virulence and toxicity expression required to infect healthy hosts. [49]

MecA Edit

mecA is a biomarker gene responsible for resistance to methicillin and other β-lactam antibiotics. After acquisition of mecA, the gene must be integrated and localized in the S. aureus chromosome. [42] mecA encodes penicillin-binding protein 2a (PBP2a), which differs from other penicillin-binding proteins as its active site does not bind methicillin or other β-lactam antibiotics. [42] As such, PBP2a can continue to catalyze the transpeptidation reaction required for peptidoglycan cross-linking, enabling cell wall synthesis even in the presence of antibiotics. As a consequence of the inability of PBP2a to interact with β-lactam moieties, acquisition of mecA confers resistance to all β-lactam antibiotics in addition to methicillin. [42] [50]

mecA is under the control of two regulatory genes, mecI and mecR1. MecI is usually bound to the mecA promoter and functions as a repressor. [40] [43] In the presence of a β-lactam antibiotic, MecR1 initiates a signal transduction cascade that leads to transcriptional activation of mecA. [40] [43] This is achieved by MecR1-mediated cleavage of MecI, which alleviates MecI repression. [40] mecA is further controlled by two co-repressors, blaI and blaR1. blaI and blaR1 are homologous to mecI and mecR1, respectively, and normally function as regulators of blaZ, which is responsible for penicillin resistance. [42] [51] The DNA sequences bound by mecI and blaI are identical [42] therefore, blaI can also bind the mecA operator to repress transcription of mecA. [51]

Arginine catabolic mobile element Edit

The arginine catabolic mobile element (ACME) is a virulence factor present in many MRSA strains but not prevalent in MSSA. [52] SpeG-positive ACME compensates for the polyamine hypersensitivity of S. aureus and facilitates stable skin colonization, wound infection, and person-to-person transmission. [53]

Strains Edit

Acquisition of SCCmec in methicillin-sensitive S. aureus (MSSA) gives rise to a number of genetically different MRSA lineages. These genetic variations within different MRSA strains possibly explain the variability in virulence and associated MRSA infections. [54] The first MRSA strain, ST250 MRSA-1, originated from SCCmec and ST250-MSSA integration. [54] Historically, major MRSA clones ST2470-MRSA-I, ST239-MRSA-III, ST5-MRSA-II, and ST5-MRSA-IV were responsible for causing hospital-acquired MRSA (HA-MRSA) infections. [54] ST239-MRSA-III, known as the Brazilian clone, was highly transmissible compared to others and distributed in Argentina, Czech Republic, and Portugal. [54]

In the UK, the most common strains of MRSA are EMRSA15 and EMRSA16. [55] EMRSA16 has been found to be identical to the ST36:USA200 strain, which circulates in the United States, and to carry the SCCmec type II, enterotoxin A and toxic shock syndrome toxin 1 genes. [56] Under the new international typing system, this strain is now called MRSA252. EMRSA 15 is also found to be one of the common MRSA strains in Asia. Other common strains include ST5:USA100 and EMRSA 1. [57] These strains are genetic characteristics of HA-MRSA. [58]

Community-acquired MRSA (CA-MRSA) strains emerged in late 1990 to 2000, infecting healthy people who had not been in contact with healthcare facilities. [58] Researchers suggest that CA-MRSA did not evolve from HA-MRSA. [58] This is further proven by molecular typing of CA-MRSA strains [59] and genome comparison between CA-MRSA and HA-MRSA, which indicate that novel MRSA strains integrated SCCmec into MSSA separately on its own. [58] By mid-2000, CA-MRSA was introduced into healthcare systems and distinguishing CA-MRSA from HA-MRSA became a difficult process. [58] Community-acquired MRSA is more easily treated and more virulent than hospital-acquired MRSA (HA-MRSA). [58] The genetic mechanism for the enhanced virulence in CA-MRSA remains an active area of research. The Panton–Valentine leukocidin (PVL) genes are of particular interest because they are a unique feature of CA-MRSA. [54]

In the United States, most cases of CA-MRSA are caused by a CC8 strain designated ST8:USA300, which carries SCCmec type IV, Panton–Valentine leukocidin, PSM-alpha and enterotoxins Q and K, [56] and ST1:USA400. [60] The ST8:USA300 strain results in skin infections, necrotizing fasciitis, and toxic shock syndrome, whereas the ST1:USA400 strain results in necrotizing pneumonia and pulmonary sepsis. [54] Other community-acquired strains of MRSA are ST8:USA500 and ST59:USA1000. In many nations of the world, MRSA strains with different genetic background types have come to predominate among CA-MRSA strains USA300 easily tops the list in the U.S. and is becoming more common in Canada after its first appearance there in 2004. For example, in Australia, ST93 strains are common, while in continental Europe ST80 strains, which carry SCCmec type IV, predominate. [61] [62] In Taiwan, ST59 strains, some of which are resistant to many non-beta-lactam antibiotics, have arisen as common causes of skin and soft tissue infections in the community. In a remote region of Alaska, unlike most of the continental U.S., USA300 was found only rarely in a study of MRSA strains from outbreaks in 1996 and 2000 as well as in surveillance from 2004–06. [63]

A MRSA strain, CC398, is found in intensively reared production animals (primarily pigs, but also cattle and poultry), where it can be transmitted to humans as LA-MRSA (livestock-associated MRSA). [57] [64] [65]

Diagnostic microbiology laboratories and reference laboratories are key for identifying outbreaks of MRSA. Normally, a bacterium must be cultured from blood, urine, sputum, or other body-fluid samples, and in sufficient quantities to perform confirmatory tests early-on. Still, because no quick and easy method exists to diagnose MRSA, initial treatment of the infection is often based upon "strong suspicion" and techniques by the treating physician these include quantitative PCR procedures, which are employed in clinical laboratories for quickly detecting and identifying MRSA strains. [66] [67]

Another common laboratory test is a rapid latex agglutination test that detects the PBP2a protein. PBP2a is a variant penicillin-binding protein that imparts the ability of S. aureus to be resistant to oxacillin. [68]

Microbiology Edit

Like all S. aureus (also abbreviated SA at times), methicillin-resistant S. aureus is a Gram-positive, spherical (coccus) bacterium about 1 micron in diameter. It does not form spores and it is not motile. It is frequently found in grape-like clusters or chains. [69] : 390 Unlike methicillin-susceptible S. aureus (MSSA), MRSA is slow-growing on a variety of media and has been found to exist in mixed colonies of MSSA. The mecA gene, which confers resistance to a number of antibiotics, is always present in MRSA and usually absent in MSSA however, in some instances, the mecA gene is present in MSSA but is not expressed. Polymerase chain reaction (PCR) testing is the most precise method for identifying MRSA strains. Specialized culture media have been developed to better differentiate between MSSA and MRSA and, in some cases, such media can be used to identify specific strains that are resistant to different antibiotics. [69] : 402

Other strains of S. aureus have emerged that are resistant to oxacillin, clindamycin, teicoplanin, and erythromycin. These resistant strains may or may not possess the mecA gene. S. aureus has also developed resistance to vancomycin (VRSA). One strain is only partially susceptible to vancomycin and is called vancomycin-intermediate S. aureus (VISA). GISA, a strain of resistant S. aureus, is glycopeptide-intermediate S. aureus and is less suspectible to vancomycin and teicoplanin. Resistance to antibiotics in S. aureus can be quantified by determining the amount of the antibiotic that must be used to inhibit growth. If S. aureus is inhibited at a concentration of vancomycin less than or equal to 4 μg/ml, it is said to be susceptible. If a concentration greater than 32 μg/ml is necessary to inhibit growth, it is said to be resistant. [20] : 637

Screening Edit

In health-care settings, isolating those with MRSA from those without the infection is one method to prevent transmission. Rapid culture and sensitivity testing and molecular testing identifies carriers and reduces infection rates. [70] It is especially important to test patients in these settings since 2% of people are carriers of MRSA, even though in many of these cases the bacteria reside in the nostril and the patient won’t present any symptoms. [71]

MRSA can be identified by swabbing the nostrils and isolating the bacteria found there. Combined with extra sanitary measures for those in contact with infected people, swab screening people admitted to hospitals has been found to be effective in minimizing the spread of MRSA in hospitals in the United States, Denmark, Finland, and the Netherlands. [72]

Handwashing Edit

The Centers for Disease Control and Prevention offers suggestions for preventing the contraction and spread of MRSA infection which are applicable to those in community settings, including incarcerated populations, childcare center employees, and athletes. To prevent the spread of MRSA, the recommendations are to wash hands thoroughly and regularly using soap and water or an alcohol-based sanitizer. Additional recommendations are to keep wounds clean and covered, avoid contact with other people's wounds, avoid sharing personal items such as razors or towels, shower after exercising at athletic facilities, and shower before using swimming pools or whirlpools. [73]

Isolation Edit

Excluding medical facilities, current US guidance does not require workers with MRSA infections to be routinely excluded from the general workplace. [74] The National Institutes of Health recommend that those with wound drainage that cannot be covered and contained with a clean, dry bandage and those who cannot maintain good hygiene practices be reassigned, [74] and patients with wound drainage should also automatically be put on “Contact Precaution,” regardless of whether or not they have a known infection. [75] Workers with active infections are excluded from activities where skin-to-skin contact is likely to occur. [76] To prevent the spread of staphylococci or MRSA in the workplace, employers are encouraged to make available adequate facilities that support good hygiene. In addition, surface and equipment sanitizing should conform to Environmental Protection Agency-registered disinfectants. [74] In hospital settings, contact isolation can be stopped after one to three cultures come back negative. [77] Before the patient is cleared from isolation, it is advised that there is dedicated patient-care or single-use equipment for that particular patient. If this is not possible, the equipment must be properly disinfected before it is used on another patient. [78]

To prevent the spread of MRSA in the home, health departments recommend laundering materials that have come into contact with infected persons separately and with a dilute bleach solution to reduce the bacterial load in one's nose and skin and to clean and disinfect those things in the house that people regularly touch, such as sinks, tubs, kitchen counters, cell phones, light switches, doorknobs, phones, toilets, and computer keyboards. [79]

Restricting antibiotic use Edit

Glycopeptides, cephalosporins, and in particular, quinolones are associated with an increased risk of colonisation of MRSA. Reducing use of antibiotic classes that promote MRSA colonisation, especially fluoroquinolones, is recommended in current guidelines. [11] [24]

Public health considerations Edit

Mathematical models describe one way in which a loss of infection control can occur after measures for screening and isolation seem to be effective for years, as happened in the UK. In the "search and destroy" strategy that was employed by all UK hospitals until the mid-1990s, all hospitalized people with MRSA were immediately isolated, and all staff were screened for MRSA and were prevented from working until they had completed a course of eradication therapy that was proven to work. Loss of control occurs because colonised people are discharged back into the community and then readmitted when the number of colonised people in the community reaches a certain threshold, the "search and destroy" strategy is overwhelmed. [80] One of the few countries not to have been overwhelmed by MRSA is the Netherlands: an important part of the success of the Dutch strategy may have been to attempt eradication of carriage upon discharge from hospital. [81]

Decolonization Edit

As of 2013, no randomized clinical trials had been conducted to understand how to treat nonsurgical wounds that had been colonized, but not infected, with MRSA, [21] and insufficient studies had been conducted to understand how to treat surgical wounds that had been colonized with MRSA. [1] As of 2013, whether strategies to eradicate MRSA colonization of people in nursing homes reduced infection rates was not known. [25]

Care should be taken when trying to drain boils, as disruption of surrounding tissue can lead to larger infections, including infection of the blood stream. [82] Mupirocin 2% ointment can be effective at reducing the size of lesions. A secondary covering of clothing is preferred. [79] As shown in an animal study with diabetic mice, the topical application of a mixture of sugar (70%) and 3% povidone-iodine paste is an effective agent for the treatment of diabetic ulcers with MRSA infection. [83]

Community settings Edit

Maintaining the necessary cleanliness may be difficult for people if they do not have access to facilities such as public toilets with handwashing facilities. In the United Kingdom, the Workplace (Health, Safety and Welfare) Regulations 1992 [84] require businesses to provide toilets for their employees, along with washing facilities including soap or other suitable means of cleaning. Guidance on how many toilets to provide and what sort of washing facilities should be provided alongside them is given in the Workplace (Health, Safety and Welfare) Approved Code of Practice and Guidance L24, available from Health and Safety Executive Books, but no legal obligations exist on local authorities in the United Kingdom to provide public toilets, and although in 2008, the House of Commons Communities and Local Government Committee called for a duty on local authorities to develop a public toilet strategy, [85] this was rejected by the Government. [86]

Agriculture Edit

The World Health Organization advocates regulations on the use of antibiotics in animal feed to prevent the emergence of drug-resistant strains of MRSA. [27] MRSA is established in animals and birds. [18]

Antibiotics Edit

Treatment of MRSA infection is urgent and delays can be fatal. [19] : 328 The location and history related to the infection determines the treatment. The route of administration of an antibiotic varies. Antibiotics effective against MRSA can be given by IV, oral, or a combination of both, and depend on the specific circumstances and patient characteristics. [4] The use of concurrent treatment with vancomycin or other beta-lactam agents may have a synergistic effect. [20] : 637

Both CA-MRSA and HA-MRSA are resistant to traditional anti-staphylococcal beta-lactam antibiotics, such as cephalexin. CA-MRSA has a greater spectrum of antimicrobial susceptibility to sulfa drugs (like co-trimoxazole (trimethoprim/sulfamethoxazole), tetracyclines (like doxycycline and minocycline) and clindamycin (for osteomyelitis). [4] MRSA can be eradicated with a regimen of linezolid, [87] though treatment protocols vary and serum levels of antibiotics vary widely from person to person and may affect outcomes. [88] The effective treatment of MRSA with linezolid has been successful [87] in 87% of people. Linezolid is more effective in soft tissue infections than vancomycin. [89] [1] This is compared to eradication of infection in those with MRSA treated with vancomycin. Treatment with vancomycin is successful in approximately 49% of people. [1] Linezolid belongs to the newer oxazolidinone class of antibiotics which has been shown to be effective against both CA-MRSA and HA-MRSA. The Infectious Disease Society of America recommends vancomycin, linezolid, or clindamycin (if susceptible) for treating those with MRSA pneumonia. [4] Ceftaroline, a fifth-generation cephalosporin, is the first beta-lactam antibiotic approved in the US to treat MRSA infections in skin and soft tissue or community-acquired pneumonia. [90]

Vancomycin and teicoplanin are glycopeptide antibiotics used to treat MRSA infections. [91] Teicoplanin is a structural congener of vancomycin that has a similar activity spectrum but a longer half-life. [92] Because the oral absorption of vancomycin and teicoplanin is very low, these agents can be administered intravenously to control systemic infections. [93] Treatment of MRSA infection with vancomycin can be complicated, due to its inconvenient route of administration. Moreover, the efficacy of vancomycin against MRSA is inferior to that of anti-staphylococcal beta-lactam antibiotics against methicillin-susceptible S. aureus (MSSA). [94] [95]

Several newly discovered strains of MRSA show antibiotic resistance even to vancomycin and teicoplanin. These new strains of the MRSA bacterium have been dubbed vancomycin intermediate-resistant S. aureus (VISA). [96] [97] Linezolid, quinupristin/dalfopristin, daptomycin, ceftaroline, and tigecycline are used to treat more severe infections that do not respond to glycopeptides such as vancomycin. [98] Current guidelines recommend daptomycin for VISA bloodstream infections and endocarditis. [4]

This left vancomycin as the only effective agent available at the time. However, strains with intermediate (4–8 μg/ml) levels of resistance, termed glycopeptide-intermediate S. aureus (GISA) or vancomycin-intermediate S. aureus (VISA), began appearing in the late 1990s. The first identified case was in Japan in 1996, and strains have since been found in hospitals in England, France, and the US. The first documented strain with complete (>16 μg/ml) resistance to vancomycin, termed vancomycin-resistant S. aureus (VRSA) appeared in the United States in 2002. [99] However, in 2011, a variant of vancomycin was tested that binds to the lactate variation and also binds well to the original target, thus reinstating potent antimicrobial activity. [100]

Oxazolidinones such as linezolid became available in the 1990s and are comparable to vancomycin in effectiveness against MRSA. Linezolid resistance in S. aureus was reported in 2001, [101] but infection rates have been at consistently low levels. In the United Kingdom and Ireland, no resistance was found in staphylococci collected from bacteremia cases between 2001 and 2006. [102]

Skin and soft-tissue infections Edit

In skin abscesses, the primary treatment recommended is removal of dead tissue, incision, and drainage. More information is needed to determine the effectiveness of specific antibiotics therapy in surgical site infections (SSIs). [4] Examples of soft-tissue infections from MRSA include ulcers, impetigo, abscesses, and SSIs. [89] In surgical wounds, evidence is weak (high risk of bias) that linezolid may be better than vancomycin to eradicate MRSA SSIs. [1]

MRSA colonization is also found in nonsurgical wounds such as traumatic wounds, burns, and chronic ulcers (i.e.: diabetic ulcer, pressure ulcer, arterial insufficiency ulcer, venous ulcer). No conclusive evidence has been found about the best antibiotic regimen to treat MRSA colonization. [21]

Children Edit

In skin infections and secondary infection sites, topical mupirocin is used successfully. For bacteremia and endocarditis, vancomycin or daptomycin is considered. For children with MRSA-infected bone or joints, treatment is individualized and long-term. Neonates can develop neonatal pustulosis as a result of topical infection with MRSA. [4] Clindamycin is not approved for the treatment of MRSA infection, but it is still used in children for soft-tissue infections. [4]

Endocarditis and bacteremia Edit

Evaluation for the replacement of a prosthetic valve is considered. Appropriate antibiotic therapy may be administered for up to six weeks. Four to six weeks of antibiotic treatment is often recommended, and is dependent upon the extent of MRSA infection. [4]

Respiratory infections Edit

CA-MRSA in hospitalized patients pneumonia treatment begins before culture results. After the susceptibility to antibiotics is performed, the infection may be treated with vancomycin or linezolid for up to 21 days. If the pneumonia is complicated by the accumulation of pus in the pleural cavity surrounding the lungs, drainage may be done along with antibiotic therapy. [4] People with cystic fibrosis may develop respiratory complications related to MRSA infection. The incidence of MRSA in those with cystic fibrosis increased during 2000 to 2015 by five times. Most of these infections were HA-MRSA. MRSA accounts for 26% of lung infections in those with cystic fibrosis. [103]

There is insufficient evidence to support the use of topical or systematic antibiotics for nasal or extra-nasal MRSA infection. [104]

Bone and joint infections Edit

Cleaning the wound of dead tissue and draining abscesses is the first action to treat the MRSA infection. Administration of antibiotics is not standardized and is adapted by a case-by-case basis. Antibiotic therapy can last up to 3 months and sometimes even longer. [4]

Infected implants Edit

MRSA infection can occur associated with implants and joint replacements. Recommendations on treatment are based upon the length of time the implant has been in place. In cases of a recent placement of a surgical implant or artificial joint, the device may be retained while antibiotic therapy continues. If the placement of the device has occurred over 3 weeks ago, the device may be removed. Antibiotic therapy is used in each instance sometimes long-term. [4]

Central nervous system Edit

MRSA can infect the central nervous system and form brain abscess, subdural empyema, and spinal epidural abscess. Excision and drainage can be done along with antibiotic treatment. Septic thrombosis of cavernous or dural venous sinus can sometimes be a complication. [4]

Other infections Edit

Treatment is not standardized for other instances of MRSA infection in a wide range of tissues. Treatment varies for MRSA infections related to: subperiosteal abscesses, necrotizing pneumonia, cellulitis, pyomyositis, necrotizing fasciitis, mediastinitis, myocardial, perinephric, hepatic, and splenic abscesses, septic thrombophlebitis, and severe ocular infections, including endophthalmitis. [4] Pets can be reservoirs and pass on MRSA to people. In some cases, the infection can be symptomatic and the pet can suffer a MRSA infection. Health departments recommend that the pet be taken to the veterinarian if MRSA infections keep occurring in the people who have contact with the pet. [79]

Worldwide, an estimated 2 billion people carry some form of S. aureus of these, up to 53 million (2.7% of carriers) are thought to carry MRSA. [105]


In a US cohort study of 1,300 healthy children, 2.4% carried MRSA in their nose. [106] Bacterial sepsis occurs with most (75%) of cases of invasive MRSA infection. [4] In 2009, there were an estimated 463,017 hospitalizations due to MRSA, or a rate of 11.74 per 1,000 hospitalizations. [107] Many of these infections are less serious, but the Centers for Disease Control and Prevention (CDC) estimate that there are 80,461 invasive MRSA infections and 11,285 deaths due to MRSA annually. [108] In 2003, the cost for a hospitalization due to MRSA infection was US$92,363 a hospital stay for MSSA was $52,791. [89]

Infection after surgery is relatively uncommon, but occurs as much as 33% in specific types of surgeries. Infections of surgical sites range from 1% to 33%. MRSA sepsis that occurs within 30 days following a surgical infection has a 15–38% mortality rate MRSA sepsis that occurs within one year has a mortality rate of around 55%. There may be increased mortality associated with cardiac surgery. There is a rate of 12.9% in those infected with MRSA while only 3% infected with other organisms. SSIs infected with MRSA had longer hospital stays than those who did not. [1]

Globally, MRSA infection rates are dynamic and vary year to year. [109] According to the 2006 SENTRY Antimicrobial Surveillance Program report, the incidence of MRSA bloodstream infections was 35.9 per cent in North America. MRSA blood infections in Latin America was 29%. European incidence was 22.8%. The rate of all MRSA infections in Europe ranged from 50% per cent in Portugal down to 0.8 per cent in Sweden. Overall MRSA infection rates varied in Latin America: Colombia and Venezuela combined had 3%, Mexico had 50%, Chile 38%, Brazil 29%, and Argentina 28%. [89]

The Centers for Disease Control and Prevention (CDC) estimated that about 1.7 million nosocomial infections occurred in the United States in 2002, with 99,000 associated deaths. [110] The estimated incidence is 4.5 nosocomial infections per 100 admissions, with direct costs (at 2004 prices) ranging from $10,500 (£5300, €8000 at 2006 rates) per case (for bloodstream, urinary tract, or respiratory infections in immunocompetent people) to $111,000 (£57,000, €85,000) per case for antibiotic-resistant infections in the bloodstream in people with transplants. With these numbers, conservative estimates of the total direct costs of nosocomial infections are above $17 billion. The reduction of such infections forms an important component of efforts to improve healthcare safety. (BMJ 2007) [ citation needed ] MRSA alone was associated with 8% of nosocomial infections reported to the CDC National Healthcare Safety Network from January 2006 to October 2007. [111]

The British National Audit Office estimated that the incidence of nosocomial infections in Europe ranges from 4% to 10% of all hospital admissions. As of early 2005, the number of deaths in the United Kingdom attributed to MRSA has been estimated by various sources to lie in the area of 3,000 per year. [112]

In the United States, an estimated 95 million people carry S. aureus in their noses of these, 2.5 million (2.6% of carriers) carry MRSA. [113] A population review conducted in three U.S. communities showed the annual incidence of CA-MRSA during 2001–2002 to be 18–25.7/100,000 most CA-MRSA isolates were associated with clinically relevant infections, and 23% of people required hospitalization. [114]


In a US cohort study of 1,300 healthy children, 2.4% carried MRSA in their noses. [106] There are concerns that the presence of MRSA in the environment may allow resistance to be transferred to other bacteria through phages (viruses that infect bacteria). The source of MRSA could come from hospital waste, farm sewage, or other waste water. [4]


Livestock associated MRSA (LA-MRSA) has been observed in Korea, Brazil, Switzerland, Malaysia, India, Great Britain, Denmark, and China. [18]

In 1961, the first known MRSA isolates were reported in a British study, and from 1961 to 1967, infrequent hospital outbreaks occurred in Western Europe and Australia, [16] with methicillin then being licensed in England to treat resistant infections. Other reports of MRSA began to be described in the 1970s. [1] Resistance to other antibiotics was documented in some strains of S. aureus. In 1996, vancomycin resistance was reported in Japan. [20] : 637 In many countries, outbreaks of MRSA infection were reported to be transmitted between hospitals. [69] : 402 The rate had increased to 22% by 1995, and by 1997 the level of hospital S. aureus infections attributable to MRSA had reached 50%.

The first report of community-associated MRSA (CA-MRSA) occurred in 1981, and in 1982, a large outbreak of CA-MRSA occurred among intravenous drug users in Detroit, Michigan. [16] Additional outbreaks of CA-MRSA were reported through the 1980s and 1990s, including outbreaks among Australian Aboriginal populations that had never been exposed to hospitals. In the mid-1990s, scattered reports of CA-MRSA outbreaks among US children were made. While HA-MRSA rates stabilized between 1998 and 2008, CA-MRSA rates continued to rise. A report released by the University of Chicago Children's Hospital comparing two periods (1993–1995 and 1995–1997) found a 25-fold increase in the rate of hospitalizations due to MRSA among children in the United States. [115] In 1999, the University of Chicago reported the first deaths from invasive MRSA among otherwise healthy children in the United States. [16] By 2004, the genome for various strains of MRSA were described. [116]

The observed increased mortality among MRSA-infected people arguably may be the result of the increased underlying morbidity of these people. Several studies, however, including one by Blot and colleagues, that have adjusted for underlying disease still found MRSA bacteremia to have a higher attributable mortality than methicillin-susceptible S. aureus (MSSA) bacteremia. [117]

A population-based study of the incidence of MRSA infections in San Francisco during 2004–05 demonstrated that nearly one in 300 residents suffered from such an infection in the course of a year and that greater than 85% of these infections occurred outside of the healthcare setting. [118] A 2004 study showed that people in the United States with S. aureus infection had, on average, three times the length of hospital stay (14.3 vs. 4.5 days), incurred three times the total cost ($48,824 vs. $14,141), and experienced five times the risk of in-hospital death (11.2% vs 2.3%) than people without this infection. [119] In a meta-analysis of 31 studies, Cosgrove et al., [120] concluded that MRSA bacteremia is associated with increased mortality as compared with MSSA bacteremia (odds ratio= 1.93 95% CI = 1.93 ± 0.39 ). [121] In addition, Wyllie et al. report a death rate of 34% within 30 days among people infected with MRSA, a rate similar to the death rate of 27% seen among MSSA-infected people. [122]

In the US, the CDC issued guidelines on October 19, 2006, citing the need for additional research, but declined to recommend such screening. [123] According to the CDC, the most recent estimates of the incidence of healthcare-associated infections that are attributable to MRSA in the United States indicate a decline in such infection rates. Incidence of MRSA central line-associated blood-stream infections as reported by hundreds of intensive care units decreased 50–70% from 2001–2007. [124] A separate system tracking all hospital MRSA bloodstream infections found an overall 34% decrease between 2005 and 2008. [124] In 2010, vancomycin was the drug of choice. [4]

Across Europe, based mostly on data from 2013, seven countries (Iceland, Norway, Sweden, the Netherlands, Denmark, Finland, and Estonia, from lowest to highest) had low levels of hospital-acquired MRSA infections compared to the others, [125] : 92–93 and among countries with higher levels, significant improvements had been made only in Bulgaria, Poland, and the British Isles. [125] : 40

A 1,000-year-old eye salve recipe found in the medieval Bald's Leechbook at the British Library, one of the earliest known medical textbooks, was found to have activity against MRSA in vitro and in skin wounds in mice. [126]

MRSA is frequently a media topic, especially if well-known personalities have announced that they have or have had the infection. [127] [128] [129] Word of outbreaks of infection appears regularly in newspapers and television news programs. A report on skin and soft-tissue infections in the Cook County jail in Chicago in 2004–05 demonstrated MRSA was the most common cause of these infections among those incarcerated there. [130] Lawsuits filed against those who are accused of infecting others with MRSA are also popular stories in the media. [131] [132]

MRSA is the topic of radio programs, [133] television shows, [134] [135] [136] books, [137] and movies. [138]

Various antibacterial chemical extracts from various species of the sweetgum tree (genus Liquidambar) have been investigated for their activity in inhibiting MRSA. Specifically, these are: cinnamic acid, cinnamyl cinnamate, ethyl cinnamate, benzyl cinnamate, styrene, vanillin, cinnamyl alcohol, 2-phenylpropyl alcohol, and 3-phenylpropyl cinnamate. [139]

The delivery of inhaled antibiotics along with systematic administration to treat MRSA are being developed. This may improve the outcomes of those with cystic fibrosis and other respiratory infections. [103] Phage therapy has been used for years in MRSA in eastern countries, and studies are ongoing in western countries. [140] [141]

MRSA will be included in experiments and cultured on the International Space Station to observe the effects of zero gravity on its evolution. [142] [143]

A 2015 Cochrane systematic review aimed to assess the effectiveness of wearing gloves, gowns and masks to help stop the spread of MRSA in hospitals, however no eligible studies were identified for inclusion. The review authors concluded that there is a need for randomized controlled trials to be conducted to help determine if the use of gloves, gowns, and masks reduces the transmission of MRSA in hospitals. [144]

Prevalence of Methicillin-Resistant Staphylococcus aureus and Associated Risk Factors among Patients with Wound Infection at Referral Hospital, Northeast Ethiopia

Background. The spectrums of infections due to methicillin-resistant Staphylococcus aureus are manifold and are associated with worse outcomes. A study on the prevalence of these pathogens and their sensitivity patterns will give updated information which is very helpful for health personnel responsible in the management of patients and timely monitoring of the emergence of resistant bacteria. Hence, the study aimed at assessing the prevalence of methicillin-resistant Staphylococcus aureus and associated factors among patients with wound infection at Dessie Referral Hospital. Method. A cross-sectional study was conducted among 266 patients at Dessie Referral Hospital from February to May 2016. Wound swab samples were collected aseptically using Levine’s technique and transported to Dessie Regional Laboratory by using brain-heart infusion transport media. Isolation of Staphylococcus aureus was done based on cultural and biochemical profiles. Drug susceptibility test was performed using the disc diffusion technique as per the standard and interpreted based on the Clinical and Laboratory Standards Institute guidelines. The data were entered and analyzed by using SPSS version 20. Result. Staphylococcus isolates from 266 processed wound swabs were 92 (34.58%). Of these, 26 (28.3%) were identified as methicillin-resistant S. aureus and 66 (71.7%) were methicillin-sensitive S. aureus. The overall prevalence of methicillin-resistant S. aureus among the study population was 9.8%. The isolated methicillin-resistant S. aureus showed full resistance to penicillin (100%) followed by erythromycin and ciprofloxacin (16, 61.5%) and cotrimoxazole and gentamicin (14, 53.8%). From the total S. aureus isolates, 20 (21.7%) of them showed multidrug resistance. Of these methicillin-resistant S. aureus, 18 (69.8%) showed high multidrug resistance. Patients who are farmers in occupation (AOR = 6.1, 95% CI (1.086–33.724)), admitted in the hospital (AOR = 3.56, 95% CI (1.429–8.857)), and have low BMI (<18.5) (AOR = 13.89, 95% CI (4.919–39.192)) were among the risk factors significantly associated with wound infection due to methicillin-resistant S. aureus. Conclusion. All methicillin-resistant S. aureus isolates were 100% resistant to penicillin and showed high multidrug resistance. Therefore, antibiotic susceptibility test should be performed prior to treatment.

1. Background

Wound is a break in the skin and exposes the underlying tissue to the outside environment. Loss of skin integrity by wounding provides a moist, warm, and nutritious environment for microbial colonization, proliferation, and infection [1, 2]. Common bacterial skin infections include Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Streptococcus pyogenes, Proteus species, Streptococcus species, and Enterococcus species [3, 4]. Among the most frequent members of wound infection, Staphylococcus aureus [5] is a leading cause of nosocomial infections (NI) and surgical wound infections [6]. It develops resistance to many antibiotics in recent years.

Methicillin-resistant S. aureus acquires its resistance via the methicillin resistance gene mecA, which encodes a low-affinity penicillin-binding protein (PBP2a) that is absent in susceptible S. aureus strains [7, 8]. This resistant penicillin-binding protein receptor does not bind well to most β-lactams and therefore allows MRSA to grow in their presence [8]. Methicillin-resistant S. aureus strains were recently classified as two groups by epidemiologic as well as molecular characteristics, namely, community-associated (CA) MRSA and healthcare-associated (HA) MRSA. Community-associated MRSA isolates are usually less resistant than HA-MRSA isolates [9].

Methicillin-resistant S. aureus is a major problem worldwide causing hospital-acquired infections [10]. It is estimated that MRSA infections within the healthcare setting alone affected more than 150,000 patients annually in the European Union, with an additional cost of 380 million euros [11]. The widespread and prolonged use of antibiotics leads to the emergence of resistant bacterial pathogens during wound infections contributing to high morbidity and mortality [12]. The spectrums of infections due to MRSA are manifold [13, 14] and are associated with worse outcome in addition to prolonged hospital stay, higher cost of treatment, and increased mortality [15, 16].

In some parts of Africa, 80% of S. aureus infection was resistant to methicillin, rendering treatment with standard antibiotics ineffective [17]. Even though different studies across the region in Ethiopia showed that the burden of MRSA constitutes a major public health problem [18–20], prevention and control strategies are not well established to minimize MRSA. In addition, antibiotics are widely and inappropriately used results in the increased prevalence of drug resistance strain bacteria such as MRSA, so that a study on the prevalence of these pathogens and their sensitivity patterns in healthcare facility will give updated information which is very helpful for health personnel responsible in the management of patients and timely monitoring of the emergence of resistant bacteria. In general, the current study results can also be used as input data to establish a guideline to minimize the burden of MRSA.

2. Methods

A cross-sectional study was conducted from February 2016 to April 2016 to assess the antibiotic resistance pattern of methicillin-resistant Staphylococcus aureus isolated from wound infection and associated risk factors in Dessie Referral Hospital (DRH). Dessie Referral Hospital is found in Dessie town with a distance of 400 km from the capital city of the county Addis Ababa and 471 km far from Bahir Dar which is the capital city of Amhara regional state. In Dessie town, there are one referral hospital, three private general hospitals, three health centers, five higher private clinics, and one regional health research laboratory where culture and susceptibility tests are performed. All patients suspected of wound infections and who have not taken antibiotics for the last two weeks prior to the study period were included in this study. The sample size was determined using a single population proportion formula at 19.6% [18] and 95% confidence interval (CI), and the total sample size was 266.

Sociodemographic related data and associated risk factors were collected by using a structured questionnaire.

2.1. Sample Collection and Processing

Wound samples were collected using Levine’s technique [21]. The wound surface was cleaned with sterile gauze moistened with 70% alcohol. Dressed wounds were cleansed with sterile normal saline after removing the dressing. Aseptically, the end of a sterile cotton-tipped applicator was rotated over 1 cm 2 area for 5 seconds with sufficient pressure to express fluid and bacteria to surface from within the wound tissue as technique stated by Levine and Gardner [22]. Samples from closed wound were collected after the skin was cleansed with 70% alcohol. Double wound swabs were taken from each wound at a point in time to increase the chance of recovering bacterial pathogens. All collected specimens were labeled and transported by using brain-heart infusion transport media to Dessie Regional Health Research Laboratory for culturing and antimicrobial susceptibility testing within 1 hour. Each wound specimen was inoculated on blood (Oxoid, Ltd., Basingstoke, Hampshire, England) and subcultured on mannitol salt agar. All plates were incubated in aerobic atmosphere at 35–37°C for 24 h.

Staphylococcus aureus was identified based on Gram-positive cocci in clusters, β-hemolytic colonies on blood agar, catalase and coagulase production, and yellow colony surrounded by yellow zone on mannitol salt agar [21].

2.2. Antimicrobial Susceptibility Test

Antimicrobial susceptibility test was carried out on each bacterial isolate using the disc diffusion method on Muller Hinton agar (MHA). Three to five pure colonies of each bacterium were picked and transferred to a tube containing 5 ml sterile nutrient broth. The preparation was mixed thoroughly to make the suspension homogeneous. The suspension was incubated at 37°C until the turbidity of the suspension adjusted to a 0.5 McFarland turbidity standard (bacterial concentration of 1.5 × 10 8 colony-forming unit/ml) [23]. A sterile swab was dipped in the suspension, and the entire surface of the MHA plates was uniformly flooded with the suspensions and allowed to dry for about 15–30 minutes.

The antimicrobial impregnated disks were placed on the media using sterile forceps in such a way that each disk was placed at least 24 mm away from each other to avoid the overlapping zone of inhibition. After the disk was placed on the inoculated media, the plates were allowed to stand for 30 minutes so that the antibiotic will diffuse into the media. The plates were inverted and incubated at 35 ± 2°C for 24 h and observed for the zone of inhibition.

The selected antibiotic disks used were (Oxoid UK) penicillin (10IU), ciprofloxacillin (5 μg), cotrimoxazole (1.25/23.75 μg), doxycycline (30g), erythromycin (15 μg), clindamycin (2 μg), chloramphenicol (30 μg), and gentamicin (10 μg). Susceptibility pattern was interpreted by comparison of the zone of inhibition according to the Clinical and Laboratory Standards Institute (CLSI, 2014) guideline and reported as sensitive, intermediate, and resistant [24]. Standard strains of S. aureus (ATCC25923) were used as controls on the biochemical tests and agar plates including MHA with antimicrobial discs to assure the testing performance of antimicrobial discs.

Data were entered and analyzed using SPSS version 20 for Windows. Stepwise logistic regression model was considered to determine factors associated with wound infection. Adjusted odds ratio and 95% CI were calculated to measure the strength of the association.

values <0.05 were considered as statistically significant.

3. Results

3.1. Sociodemographic Characteristics of Study Participants

In this study, a total of 266 study participants were included. Of these, 180 (67.7%) were male and 86 (32.3%) were female. The mean ages of the study participants were 33.2 ± 17.8 years (range from 5 to 81 years). One-fourth of the study participants had no formal education, while the majorities were lived in urban (205, 77.1%) (Table 1).

3.2. Prevalence of MRSA

Out of 266 patients suspected of developing wound infection, 92 (34.58%) have culture-confirmed S. aureus wound infections. Of these, 26 (28.3%) were MRSA. The overall prevalence of MRSA among the study population was 9.8% (26/266). Among 82 inpatients and 184 outpatients suspected of wound infection, 41.5% (34/82) and 31.5% (58/184) were culture-positive for S. aureus, respectively. The overall prevalence of MRSA in inpatients and outpatients was 19.5% (16/82) and 5.4% (10/184), respectively (Figure 1).

3.3. Antibiotic Resistance Pattern

Out of the 92 S. aureus isolated from wound swab including MRSA, 78 (84.8%) showed a high level of resistance to penicillin and 4 (4.3%) showed a low level of resistance to clindamycin while MRSA showed full (100%) resistance rate to penicillin followed by erythromycin and ciprofloxacin (16, 61.5%) and cotrimoxazole and gentamicin (14, 53.8%) (Table 2).

3.4. Multidrug Resistance Pattern of MRSA

In this study, a high prevalence of multidrug resistance (MDR) to MRSA was observed as compared to methicillin-sensitive Staphylococcus aureus (MSSA) which accounted for 18 (69.2%) and 2 (3%), respectively none of the strains were resistant to all antibiotics tested. However, 10 (15.2%) MSSA were sensitive to all antibiotics tested (Table 3).

3.5. Bivariate and Multivariate Analysis of Factors Associated with MRSA among Wound Infection Individuals

In a bivariate logistic regression analysis, wound infection due to MRSA showed significant association with occupation, history of recent admission, history of recent surgery, being diagnosed in the inpatient department, and low body mass index (BMI) (<18.5). However, other factors such as age, sex, education, residence, history of previous antibiotic use, and chronic illness did not show statistically significant association.

In a multivariate logistic regression analysis, the abovementioned associated factors remained associated with wound infection due to MRSA except recent history of admission. Farmers were 6 times more likely to develop MRSA wound infection (AOR = 6.1 95% CI (1.086–33.724)) than housewives. Patients who have low BMI were 13.9 times more likely to develop MRSA wound infection (AOR = 13.89 95% CI (4.919–39.192)) than their counterparts. In addition, those inpatients were 3.6 times more likely to be infected with MRSA (AOR = 3.56 95% CI (1.429–8.857)) as compared to those diagnosed in OPD (Table 4).

4. Discussion

Wound infection due to MRSA was a major concern in resource-limited countries, in particular, Ethiopia, where there are poor infection prevention and control measures [25]. In this study, the prevalence of S. aureus wound infection was 34.5%. This finding is in line with the study conducted in Debre Markos (39.7%) [18] and Cameroon (28.9%) [26]. On the other hand, this finding is higher than studies conducted in Jimma (23.6%) [19], Nigeria (26.6%) [27], Tanzania (26.7%) [28], and Brazil (20%) [29]. However, the prevalence reported in the current study is lower than a study reported in Addis Ababa (57.8%) [30] and Uganda (41%) [31]. The variation in prevalence might be due to variation in the study subjects, study conducted time, and the method employed for the detection of S. aureus.

In our study, the overall prevalence of MRSA was 9.8%. This finding is similar to the results reported from studies in Addis Ababa (13.2%) [20], Eretria (9%) [32], and Cameroon (13.16%) [26] and lower than the results reported from previous studies in Ethiopia such as Debre Markos (19.6%) [18] and Jimma (17.4%) [19] and other African countries: Uganda (41%) [31] and Libya (31%) [33]. On the other hand, this study finding is higher than study reports from Nigeria (5.8%) [27], Brazil (5.6%) [29], and Tanzania (4.3%) [28]. The observed high prevalence of MRSA in our study may be due to the high rate of certain antibiotics use either due to availability or cost-effectiveness issues.

Regarding the possible associated risk factors, MRSA wound infections were significantly associated with occupation (farmers), patients with low BMI, and those patients who are currently admitted (inpatient) as compared to their counterparts. This might be because farmers may not have knowledge of utilizing healthcare services in addition, their occupation may expose them to wound infection and make them use antibiotics without prescription. High prevalence of MRSA in admitted patients may be attributed by resistant strain bacterial cross-contamination in health institutions. Patients who have low BMI had higher odds of developing wound infection due to MRSA. Healthy people may carry MRSA asymptomatically for long periods of time, but patients with compromised immune system are at a significantly greater risk of symptomatic infections [13, 14].

Concerning the antimicrobial resistance profile of the isolates, in the present study, S. aureus isolates showed resistance to penicillin (84.8%), gentamicin (15.2%) ciprofloxacillin (18.4%), clindamycin (4.3%), erythromycin (26.1%), cotrimoxazole (16.3%), doxycycline (9.7%), and chloramphenicol (8.7%). The resistance profile of S. aureus to penicillin in our study is similar to the results obtained within DRH (82.2%) [18]. In other studies, resistance to penicillin done in Tanzania [28] and Jimma, Ethiopia [34], is slightly higher and reported as 97% and 100%, respectively. The resistance to clindamycin in the current study is similar to other studies in Ethiopia in which the resistance is less than 50% [20, 30]. We noticed that the resistance rate of other antibiotics listed above is varied in different studies conducted in Ethiopia [18, 19].

The current study showed MRSA isolates were resistant to penicillin (100%), gentamicin (53.8%), ciprofloxacillin (61.5%), clindamycin (7.7%), erythromycin (61.5%), cotrimoxazole (53.8%), doxycycline (30.8%), and chloramphenicol (26.9%). Similarly, studies conducted in different areas showed MRSA isolates were 100% resistant to penicillin [18, 35]. In this study, the resistance of MRSA isolates to gentamycin is higher compared to the study done in Yekatit 12 Hospital in Addis Ababa (38.2%) [20]. Similarly, the resistance to ciprofloxacin is slightly higher compared to the results reported in Tanzania (54%) [28]. In contrast, the resistance to clindamycin, erythromycin, cotrimoxazole, and chloramphenicol is lower compared to the studies done in Ethiopia [18, 20].

The main variation in drug resistance patterns among different studies might be due to the indiscriminate use and availability of these antibiotics in a certain area. The variation of resistance rate among different areas indicates the resistance pattern of antibiotics varies according to regional and geographical location and also changes through time.

Furthermore, high prevalence of multidrug-resistant MRSA (69.2%) was reported in the study area. This finding is concordant with the study conducted in northern India whereby 73% of MRSA strains were multidrug-resistant [36]. Likewise, a study conducted in Debre Markos showed all MRSA strains isolated were resistant to ≥3 antibiotics [18]. High prevalence of multidrug resistance may predispose patients to infection with intractable isolates, emphasizing the need for improved infection control practices and guidelines for the use of antibiotics in this setting.

The current study results could not show MRSA is community-associated or healthcare-associated.

5. Conclusion

Out of 266 patients suspected with wound infection, 92 (34.58%) have culture-confirmed S. aureus. Of these, 26 (28.3%) were MRSA. Wound infection due to MRSA showed significant association with occupation, being diagnosed in the inpatient department, and body mass index. Greater than 50% of MRSA isolates were resistant to gentamicin, ciprofloxacillin, cotrimoxazole, and erythromycin.


BMI:Body mass index
CA-MRSA:Community-associated methicillin-resistant Staphylococcus aureus
CoNS:Coagulase-negative Staphylococcus aureus
HA-MRSA:Healthcare-associated methicillin-resistant Staphylococcus aureus
MDR:Multidrug resistance
MRSA:Methicillin-resistant Staphylococcus aureus
MSSA:Methicillin-sensitive Staphylococcus aureus.

Data Availability

The data used to support the results of this research are available from the corresponding author upon request.

Ethical Approval

The study was ethically approved by the ethical review committee of the School of Biomedical and Laboratory Sciences, University of Gondar.


Permission was obtained from DRH. Informed written consent was obtained from participants before data collection. All the information obtained from the study subjects was coded to maintain confidentiality. When the participants were found to be positive for S. aureus, they were informed by the hospital clinician and received proper treatment.

Conflicts of Interest

The authors have no conflicts of interest.

Authors’ Contributions

YT and TG conceived this research topic and objectives. YT, TG, AA, MA, and BG participated in the designing of the study and data analysis and performed statistical analysis. MM and ST prepared and critically revised the manuscript for its scientific content. All authors read and approved the final manuscript.


The authors would like to acknowledge staff members of Dessie Regional Health Research Laboratory who helped a lot during this research work. The authors would also like to acknowledge all the data collectors and study participants who have been involved in this study. This study was financially supported by the Amhara Regional Health Bureau and the necessary materials to accomplish this work were provided by Dessie Regional Health Research Laboratory.


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Copyright © 2020 Yeterefwork Tsige et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


This study highlights the unintended consequences of widespread antibiotic use, and how when new drugs are introduced to bypass known resistance mechanisms, they may already be rendered ineffective due to unrecognised adaptations accrued in response to prior selective pressures exerted by other antibiotics. This remains one of the many challenges in tackling the growing problem of AMR and serves to emphasise the importance of continual surveillance of pathogen populations for evidence of emerging adaptations and resistance patterns in the context of prescribing practice.

Immune Response

The innate immune response is the leading defense against MRSA infections. Neutrophils are recruited to the site of infection by Pathogen Recognition Receptors binding to the bacteria, causing them to release interleukin-8 (IL-8, CXCL8), GROα (CXCL1), granulocyte chemotactic protein 2 (GCP2, CXCL6), and complement component C5a. These chemokines help to recruit neutrophils to the site of infection in order to assist macrophages in phagocytosis of the bacteria. The triggering of the innate immune response and neutrophil recruitment also serve to trigger complement activation and inflammation, which contribute to the destruction of the pathogens [11] [12]