An E-test strip for vancomycin against a Staphylococcus aureus isolate shows an elliptical inhibition zone intersecting the strip at MIC 6 μg/mL. According to CLSI breakpoints, this isolate is classified as:

• A MRSA (MIC ≤2 μg/mL)
• B VISA (vancomycin-intermediate S. aureus, MIC 4–8 μg/mL) ✓
• C VRSA (vancomycin-resistant S. aureus, MIC ≥16 μg/mL)
• D Susceptible (MIC ≤2 μg/mL)

Explanation

CLSI breakpoints for S. aureus and vancomycin: susceptible MIC ≤2 μg/mL, VISA (intermediate) MIC 4–8 μg/mL, VRSA (resistant) MIC ≥16 μg/mL. A MIC of 6 μg/mL falls in the VISA range. VISA strains have thickened cell walls that 'trap' vancomycin and require alternative agents (linezolid, daptomycin). VRSA carries vanA/vanB genes and has MIC ≥16 μg/mL.

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

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