A 55-year-old diabetic man has a rapidly spreading necrotizing skin infection. Gram stain of wound exudate shows Gram-positive cocci in clusters. The isolate is coagulase-positive and is resistant to oxacillin by disc diffusion (zone of inhibition < 10 mm for cefoxitin disc). The MIC for vancomycin is 1 µg/mL. Which term best categorizes this isolate?
- A Vancomycin-intermediate S. aureus (VISA)
- B Methicillin-resistant S. aureus (MRSA) ✓
- C Vancomycin-resistant S. aureus (VRSA)
- D Heterogeneous VISA (hVISA)
Correct answer: B. Methicillin-resistant S. aureus (MRSA)
Explanation
Oxacillin or cefoxitin disc diffusion resistance confirms MRSA (mecA gene encoding PBP2a). Vancomycin MIC of 1 µg/mL is within the susceptible range (MIC ≤ 2 µg/mL per CLSI), so this is not VISA (MIC 4–8 µg/mL) or VRSA (MIC ≥ 16 µg/mL). hVISA is a phenotypic precursor with subtle vancomycin tolerance not detected by routine MIC. Vancomycin remains the treatment of choice for MRSA skin and soft tissue infections; daptomycin or linezolid are alternatives.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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