MecA is carried on a mobile genetic element, the staphylococcal cassette chromosome mec (SCCmec) of which four forms have been described that differ in size and genetic composition. Many MRSA strains are multiply resistant and are susceptible only to glycopeptide antibiotics, such as, vancomycin. There are also additional repressor and inducer genes, mecR and mecI that can be associated with mecA. These and several other genes known as factor essential for methicillin resistance or fem or auxiliary or aux genes that are found in both susceptible resistant strains affect the expression of methicillin resistance in S. aureus.
This multiplicity of genetic expression that is translated into resistance pattern of the bacteria is perhaps causing an increase in the absolute number of the MRSA infections in the hospital setting, and genetic alterations of the bacteria is causing adaptability of these strains in the community and leading to an increased number of community-associated MRSA infections. As discussed earlier, MRSA are resistant to methicillin and other ? -lactamase resistant penicillins and cephalosporins because they produce this unique low-affinity penicillin-binding protein called PBP2a that is encoded by mecA contained within the chromosomal region mec.
These strains are fully resistant to methicillin, and this type of resistance is called intrinsic resistance. The phenotypic expression of methicillin resistance among MRSA isolates varies considerably. Resistance levels are dependent on efficient PBP2a production and modulated by chromosomal factors (Enright, M. C. et al. , 2002). Resistance Pattern: Most MRSA isolates demonstrate, therefore, heterogeneous grades of resistance. This happens due to segregation of a more highly resistant subpopulation on challenge with methicillin.
Not surprisingly, only a small proportion of the bacterial population in a given culture would manifest high-level resistance under standard test conditions. Maximal expression of resistance by synthesis of PBP2a requires the efficient and appropriate synthesis of the precursor peptidoglycan. Bacterial genes that are involved in cell wall precursor formation and turn over, regulation, transport, and signal transduction may determine the level of resistance that is expressed in a particular strain in that isolate.
Manipulation of physical condition can increase resistance expression, but isolation and subculture of a highly resistant minority population on increased concentration of methicillin produces bacterial colonies that are again heterogeneous. CA-MRSA: Community associated MRSA has unique characteristics that separates it form methicillin-susceptible Staphylococcus aureus and MRSA from the hospital setting. CA-MRSA has different epidemiological risk factors, clinical manifestations, and microbiological characteristics than healthcare-associated (HA) MRSA.
CA MRSA is a problem with long-term care facilities and intravenous drug abusers. Risk factors for newly identified community cases of MRSA include recent hospitalization, admission from another hospital, prior antimicrobial use, a child in a day-care center, and underlying illnesses such as cardiovascular or pulmonary disease, diabetes, malignancy, and chronic skin diseases (Millar, B. C. , Prendergast, B. D. , and Moore, J. E. , 2008). Microbiology: MRSA is readily isolated in routine culture media.
However, they can be difficult to detect when mixed with other microbial flora in a clinical specimen, and many different media have been recommended for the isolation of MRSA from screening specimens. An ideal screening method will allow the growth of all MRSA, inhibit or differentiate other organisms, and allow direct identification tests to be performed on colonies. British Society of Antimicrobial Chemotherapy, The Hospital Infection Society, and The Infection Control Nurses Working Party recommended enrichment broth such as nutrient broth or cooked-meat medium containing 7% added sodium chloride.
When isolating a staphylococcus from a clinical or screening specimen, it is of utmost importance to ensure that it is in fact S. aureus, not coagulase-negative staphylococcus. Apart from the conventional microbiological techniques, the ideal and the most precise methods for the identification of methicillin resistance in MRSA are the direct detection of mec, mecA, or the product PBP2a. Relatively simple screening tests for MRSA are now available; these test PBP2a and mecA. These yield results that are as or more accurate than testing for drug resistance.
The standard identification methods still remain as standards of identification of intrinsic methicillin-resistant phenotype (Brown, D. B. J. et al. , 2005). Susceptibility Testing: The MIC determined by a dilution method has been the gold standard for susceptibility testing. More recently, MIC methods have been replaced by molecular methods, which detect the mecA gene as the test for determination of methicillin resistance in S. aureus. Although this method bypasses heterogeneity of the different strains of MRSA, resistance not associated with mecA would not be detected (Brown, D. B. J. et al. , 2005).