E-test (Epsilometer test) is used to determine MIC of an antibiotic. A gram-positive coccus is tested for vancomycin susceptibility. The E-test strip shows the MIC ellipse intersecting the strip at 3 μg/mL. According to CLSI breakpoints, this isolate would be classified as:
- A Susceptible (S) — vancomycin MIC ≤2 μg/mL is susceptible; MIC 4–8 μg/mL is intermediate
- B Resistant (R) — vancomycin MIC ≥2 μg/mL confirms VRSA
- C Intermediate (I) — MIC 3 μg/mL falls in the intermediate vancomycin-intermediate S. aureus (VISA) range ✓
- D Susceptible (S) — for enterococci, MIC ≤8 μg/mL is susceptible regardless of species
Explanation
CLSI vancomycin breakpoints for Staphylococcus aureus: Susceptible ≤2 μg/mL, Intermediate (VISA) 4–8 μg/mL, Resistant (VRSA) ≥16 μg/mL. An MIC of 3 μg/mL, while below the VISA threshold, falls in a 'creep' zone that may not be reliably categorized by standard disk diffusion and warrants reporting as intermediate/with caution. However, formally by CLSI, MIC 4–8 = intermediate (VISA). For Enterococcus: vancomycin susceptible ≤4 μg/mL, intermediate 8–16 μg/mL, resistant ≥32 μg/mL (vanA/vanB). The E-test provides continuous MIC values, making it superior to disk diffusion for detecting borderline vancomycin resistance, particularly in VISA detection where standard disk diffusion is unreliable.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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