An MRSA strain is confirmed by the cefoxitin disc diffusion method (inhibition zone ≤21 mm). The molecular basis of methicillin resistance in MRSA is:
- A Acquisition of mecA gene encoding altered penicillin-binding protein PBP2a (PBP2') ✓
- B Overproduction of penicillinase (beta-lactamase) encoded by blaZ
- C Efflux pump overexpression reducing intracellular oxacillin concentration
- D Porin loss preventing beta-lactam entry into the cell
Correct answer: A. Acquisition of mecA gene encoding altered penicillin-binding protein PBP2a (PBP2')
Explanation
MRSA carries the mecA gene (or the variant mecC) on the Staphylococcal Chromosomal Cassette mec (SCCmec), encoding an altered penicillin-binding protein PBP2a (PBP2') with very low affinity for beta-lactam antibiotics, conferring resistance to all penicillins, cephalosporins, and most beta-lactams. Cefoxitin disc diffusion is the best phenotypic surrogate because it reliably induces mecA expression. blaZ encodes penicillinase (only breaks penicillins). Porin loss and efflux are Gram-negative resistance mechanisms.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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