Disk with High Oxacillin Content Discriminates between Methicillin-Resistant and Borderline Methicillin-Susceptible Staphylococcus aureus Strains in Disk Diffusion Assays Using a Low Salt Concentration

A separation between mecA(+) strains of Staphylococcus aureus and strains lacking mecA was achieved by the disk diffusion assay and the agar dilution method, utilizing disks containing 5 μg of oxacillin and inocula of approximately 5 × 10(5) CFU/spot, respectively, provided that agar with 0 to 0.5% NaCl and incubation at 30°C were employed. The 5-μg oxacillin disks clearly discriminated between borderline methicillin-susceptible and mecA(+) strains. The oxacillin MICs were more affected by the inoculum density and salt concentration than were the methicillin MICs, and oxacillin MICs of 4 to 16 μg/ml were obtained for strains lacking mecA. Significantly higher levels of β-lactamase production and reduced oxacillin susceptibilities were recorded for strains lacking mecA, in particular strains of phage group V, when agar with ≥2% NaCl was used than when agar with 0 to 0.5% NaCl was employed. The results indicate that the borderline methicillin-susceptible phenotype is a salt-dependent in vitro phenomenon of questionable clinical relevance.