In disk diffusion susceptibility testing (Kirby-Bauer), an MRSA isolate gives a cefoxitin disk zone of 18 mm (CLSI breakpoint: susceptible ≥22 mm). To confirm the mecA gene-mediated resistance mechanism, which molecular test is most specific?
- A Latex agglutination for PBP2a (PBP2')
- B Oxacillin MIC by broth microdilution
- C PCR for mecA/mecC gene ✓
- D MRSA chromogenic agar culture
Explanation
Cefoxitin disk diffusion is the screening test of choice for MRSA detection because it reliably induces mecA expression. Confirmation of the resistance mechanism requires PCR for the mecA gene (or mecC in livestock-associated MRSA), which directly detects the gene encoding the altered penicillin-binding protein PBP2a with nearly 100% sensitivity and specificity. PBP2a latex agglutination is a validated phenotypic surrogate for mecA with high accuracy. Oxacillin MIC is used in clinical decision-making but is less sensitive than cefoxitin for MRSA screening. Chromogenic agar is used for screening clinical samples, not confirmation.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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