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Susceptibility Testing

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):

    Medium
    • 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); 24 hours for oxacillin, methicillin, nafcillin, and vancomycin.