The current CLSI recommendation is that MIC tests should be performed to determine the susceptibility of staphylococci to vancomycin. The disk test does not differentiate vancomycin-susceptible isolates of S. aureus from vancomycin-intermediate strains.
Disk diffusion will detect S. aureus isolates containing the vanA-type Staphylococcus aureus strain (VRSA). These isolates will show no zone of inhibition around the disk (zone = 6mm); their identification should be confirmed. Isolates producing vancomycin zones ≥7mm should not be reported as susceptible without performing a vancomycin MIC test.
Recommended methods are CLSI Broth Microdilution, agar dilution, and Etest® with inoculum prepared to match McFarland 0.5 turbidity standard. The Etest® is considered the most discriminatory of these methods as it allows for visualization of small colonies around zones of inhibition. A pure culture must be used. Repeat the test for confirmation. The CLSI recommends that the inoculum should be prepared using the direct suspension method and plates incubated for a full 24 hours in ambient air at 35°C.
Screening for vancomycin resistance in staphylococci (MICs ≥8 µg/mL) can be performed utilizing a vancomycin agar screening plate—brain heart infusion (BHI) agar containing 6 mg/mL vancomycin. However, testing on BHI screening agar does not reliably detect all vancomycin-intermediate S. aureus strains.
