TABLE 1.
Susceptibility test methods used in this study
| Method | Mediaa | Oxacillin concn | Inoculum | Incubation | Interpretive guidelines |
|---|---|---|---|---|---|
| Agar dilution | MH (Difco) with 0% NaCl | 1 and 2 μg/ml | 104 CFU/spot | 30 and 35°C for 24 and 48 h (used for all media) | Analysis 1: susceptible, ≤2 μg/ml; resistant, ≥4 μg/ml |
| MH (Difco) with 2% NaCl | Analysis 2: Susceptible, ≤1 μg/ml; Resistant, ≥2 μg/ml | ||||
| MH (Difco) with 4% NaCl | |||||
| Disk diffusion | MH (Difco) | 1 μg disk | Swab, a McFarland standard equal to 0.5 to 1 | 35°C for 24 and 48 h (used for both media) | Susceptible, zone diameter of ≥13 mm; Intermediate, zone diameter of 11 to 12 mm; resistant, zone diameter of ≤10 mm |
| MH II (BBL) | |||||
| Agar screen | |||||
| Swab | MH agar with 4% NaCl (Remel and Difco) | 6 μg/ml (0.6 μg/ml for Difco only) | Swab, McFarland standard of 0.5 to 1 | 35°C for 24 and 48 h | Analysis 1: Resistant, >1 colony growth at 6 μg/ml |
| Spot | MH (Difco) with 4% NaCl | 104 CFU/spot | 30 and 35°C for 24 and 48 h | Analysis 2: Resistant, >1 colony growth at 0.6 μg/ml (for Difco swab only) |
a
All media prepared in-house except Remel.