S. aureus methicillin resistance is mediated by the mecA gene encoding PBP2a. A clinical isolate of MRSA is being tested for sensitivity. The minimum inhibitory concentration (MIC) of vancomycin is 2 µg/mL. According to CLSI breakpoints, this isolate would be classified as:
- A Vancomycin-susceptible (VSSA)
- B Vancomycin-intermediate (VISA) ✓
- C Vancomycin-resistant (VRSA)
- D Heterogeneous VISA (hVISA)
Correct answer: B. Vancomycin-intermediate (VISA)
Explanation
CLSI breakpoints for S. aureus vancomycin susceptibility: MIC ≤2 µg/mL = susceptible (VSSA), 4–8 µg/mL = intermediate (VISA), ≥16 µg/mL = resistant (VRSA). An MIC of 2 µg/mL is at the upper end of the susceptible range and is classified as susceptible, NOT intermediate — VISA requires MIC 4–8 µg/mL. However, this question as written has MIC 2 µg/mL; per current CLSI 2023 guidelines, MIC ≤2 is susceptible, making the correct answer A (VSSA). Choice B (VISA) is incorrect for this MIC value. The correct answer is A.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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