MRSA: Introduction,Resistance Mechanism and Its Detection

MRSA detection method

Introduction of Methicillin Resistant Staphylococcal aureus 

MRSA stands for  Methicillin-Resistant Staphylococcal aureus (MRSA). Although antibiotics like oxacillin, nafcillin, and Cloxacillin have replaced methicillin in clinical practice, the term methicillin-resistant S. aureus is still being used for resistance to all these antibiotics. MRSA is defined as an isolate having a MIC of greater than 4 µg of oxacillin/ml. Methicillin sensitive  S. aureus (MSSA) is defined as an isolate with a MIC of less than 2ug of oxacillin/ ml. Isolates having MIC of 2-4 μg/ml are defined as borderline resistant S. aureus (BoRSA).  methicillin-resistant S. aureus refers to isolates that are resistant to all currently available β- lactam antibiotics, including penicillins and cephalosporins.

Predisposing factors for MRSA

Prolonged hospital stay, frequent contact with health care environment, surgical wounds, proximity to an infected or colonized patient, receipt of antibiotics in the previous 60 days, and surgical procedures are known risk factors for nosocomial acquired MRSA. Community-acquired MRSA has been seen even in patients without any risk factors.

Mechanisms of Resistance

Resistance of S. aureus to methicillin is mediated by the production of a penicillin-binding protein (PBP), which has a low affinity for binding with β-lactam antimicrobials. The gene that encodes PBP present on the chromosome of all isolates of MRSA is mecA. This type of resistance is called intrinsic resistance. MRSA is usually also resistant to other antibiotics, e.g. aminoglycosides, erythromycin, clindamycin, trimethoprim-sulfamethoxazole, etc. this has been attributed to the DNA, which flanks the mec A gene and has many potential sites for the integration of transposons (genetic elements mediating resistance to antibiotics).

Detection of MRSA

Various methods of methicillin-resistant S. aureus detection are as follows-

  1. Oxacillin 1μg disk diffusion method
  2. Oxacillin 5 μg disk diffusion method
  3. Oxacillin agar screen test
  4. Cefoxitin disk diffusion test
  5. MIC oxacillin by agar dilution method
  6. E-test
  7. PCR (for mec A gene)
  8. Immunochromatographic test
  9. Latex agglutination test
  10. Quenching fluorescence method

Further Readings



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