INFLUENCE OF BETA-LACTAMASE INHIBITORS ON THE ACTIVITY OF OXACILLIN AGAINST METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS

We studied the in vitro susceptibility to oxacillin of 46 isolates of methicillin-resistant Staphylococcus aureus (MRSA) isolates with a minimum inhibitory concentration (MIC) >8 mu g/ml of oxacillin, with and without adding clavulanic acid, sulbactam, or tazobactam in three different concentrations (2, 4, and 8 mu g/ml). All 46 strains were found by the vapid chromogenic cephalosporin method to be beta-lactamase producers. For those strains with low-level resistance (MIC of 16 or 32 mu g/ml), the MICs of oxacillin decreased four- to 32-fold and two- to 32-fold after adding sulbactam and tazobactam, respectively. For those with high-level resistance (MIG of greater than or equal to 64 mu g/ml), the MICs either did nor change or decreased only two-fold after we added one of three beta-lactamase inhibitors. The results suggest that beta-lactamase production probably plays a role in resistance to oxacillin in those MRSA strains of low-level oxacillin resistance.