When a clinical isolate is presumptively identified as S. aureus, susceptibility testing will be performed by either the standardized disk diffusion (Kirby-Bauer) or broth dilution (MIC) methods, using the following testing conditions as recommended by the Clinical and Laboratory Standards Institute (CLSI):
- Mueller Hinton agar (MHA) for disk diffusion
- Cation-adjusted Mueller-Hinton broth (CAMHB) + 2% NaCL for oxacillin, methicillin, and nafcillin
- CAMHB supplemented up to 50 µg/mL calcium for daptomycin
- Direct colony suspension -- Inoculum from an 18-24 hour non-selective agar plate used to prepare a direct inoculum equivalent to a 0.5 McFarland standard.
- 35° C (testing at temperatures above 35° C may not detect MRSA); 24 hours for oxacillin, methicillin, nafcillin, and vancomycin.