Detection of low-level oxacillin resistance in mecA-positive Staphylococcus aureus

ABSTRACTDetection of low-level oxacillin-resistant Staphylococcus aureus is a problem that needs special attention, particularly in relation to methicillin-resistant S. aureus (MRSA) strains in the community that belong to clonal lineage ST80. This study compared different phenotypic methods for the detection of 74 low-level oxacillin-resistant S. aureus strains (oxacillin MIC ≤1 mg/L), 46 MRSA strains (oxacillin MIC ≥2 mg/L) and 117 methicillin-susceptible S. aureus strains. Determination of microbroth dilution MICs for oxacillin was wholly unsatisfactory, and gave a limited specificity for cefoxitin. The sensitivity of disk-diffusion performed according to CLSI recommendations was 92% with an oxacillin 1-μg disk, and 96% with a cefoxitin 30-μg disk; use of a 10-μg cefoxitin disk and a semi-confluent inoculum (breakpoint for resistance <18 mm zone diameter) gave a sensitivity of 97%. When disk-diffusion was performed on IsoSensitest agar with a zone diameter breakpoint for resistance of <22 mm (as recommended by the Swedish Reference Group for Antibiotics), the sensitivity was 95%.