Vancomycin-resistant Staphylococcus aureus screening

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Screening

Method

Laboratories that use automated methods that are not validated for VRSA detection should also include a vancomycin screen agar plate for enhanced detection of VRSA. If possible, laboratories should incorporate the vancomycin agar screen plate for testing all S. aureus. Alternatively, the screening may be limited to MRSA isolates, since nearly all VISA and all VRSA reported to date (i.e., April 2006) were also MRSA. Laboratories using disk diffusion to determine vancomycin susceptibility should consider adding a second method for VISA detection. The vancomycin screen plate is useful for detecting VISA (MIC = 8 µg/ml). Reliable detection of VISA (MIC = 4 µg/ml) may require a non-automated MIC method.

• Vancomycin agar screen test:

The vancomycin agar screen test uses commercially prepared agar plates to screen pure cultures of bacteria for vancomycin. These plates contain BHI agar and 6 µg/ml of vancomycin. A 10µl inoculum of a 0.5 McFarland suspension should be spotted on the agar using a micropipette (final concentration=106 CFU/ml). Alternatively, a swab may be dipped in the McFarland suspension, the excess liquid expressed, and used to inoculate the vancomycin agar screen plate. For quality control, laboratories should use Entercococcus faecalis ATCC 29212 as the susceptible control and E. faecalis ATCC 51299 as the resistant control. Up to eight isolates can be tested per plate; quality control should be performed each day of testing. Growth of more than one colony is considered a positive result. All staphylococci that grow on these plates should be inspected for purity, and the original clinical isolates should be tested using an FDA-cleared MIC method for confirmation. Plates prepared in-house using various lots of media performed inconsistently and were inferior to those obtained commercially (CDC unpublished data). Performance of commercially prepared plates varies by individual manufacturer.