A clinical isolate of Staphylococcus aureus is identified with a MIC of 4 µg/mL for oxacillin. The resistance mechanism in this organism involves a gene encoding a modified penicillin-binding protein. Which PBP and gene are responsible?

• A PBP2a encoded by mecA gene on Staphylococcal Cassette Chromosome mec (SCCmec) ✓
• B PBP1 encoded by pbp1 gene with hypermethylation of the active site
• C PBP3 encoded by a plasmid-borne blaZ gene producing extended-spectrum beta-lactamase
• D PBP4 encoded by mecC gene on the chromosome conferring oxacillin susceptibility

Explanation

MRSA (methicillin-resistant S. aureus) resistance is mediated by mecA, which encodes PBP2a (also called PBP2'), a low-affinity penicillin-binding protein that can catalyse cross-linking of peptidoglycan even when all normal PBPs are inhibited by beta-lactams. The mecA gene is carried on the mobile genetic element SCC mec (Staphylococcal Cassette Chromosome mec). The blaZ gene encodes staphylococcal beta-lactamase (hydrolytic, not a modified PBP), which causes narrow-spectrum penicillin resistance. mecC is a homologue conferring resistance, but PBP2a from mecA is the classic and most common mechanism.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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