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Testing for Vancomycin Susceptibility

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 vancomycin-resistance gene (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 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 (MIC's > 8 µg/mL) can be performed utilizing a vancomycin agar screening plate – BHI (brain heart infusion) agar containing 6 mg/mL vancomycin. However testing on BHI screening agar does not reliably detect all vancomycin intermediate S. aureus strains.