Comparison of ampicillin/sulbactam and amoxicillin/clavulanic acid for detection of borderline oxacillin-resistantStaphylococcus aureus

Borderline oxacillin-resistantStaphylococcus aureus (BORSA) may be misidentified as intrinsically methicillin-resistantStaphylococcus aureus (MRSA) in the clinical laboratory. Under disk diffusion testing conditions designed to maximize detection of MRSA (incubation at 32 °C, pre-induction with methicillin, or plating on 4 % NaCl-supplemented agar), BORSA strains also tend to appear resistant to semisynthetic penicillins. Under these conditions, ampicillin/sulbactam appears to be more accurate than amoxicillin/clavulanic acid for differentiating BORSA from MRSA.