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 (minimum inhibitory concentration [MIC] test) methods, using the following testing conditions as recommended by the Clinical and Laboratory Standards Institute (CLSI):
Medium
- 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
Inoculum
- 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.
Incubation
- 35°C (testing at temperatures above 35°C may not detect MRSA) for 24 hours for oxacillin, methicillin, nafcillin, and vancomycin.
- Incubated in ambient air and not increased CO2.
