Role of beta-lactamase and different testing conditions in oxacillin-borderline-susceptible staphylococci

A group of staphylococcal isolates for which oxacillin MICs were intermediate (1 to 4 micrograms/ml) were studied to establish the role of beta-lactamase in this phenomenon. MICs and MBCs of oxacillin and penicillin with and without clavulanic acid or sulbactam (4 or 16 micrograms/ml, respectively) were determined for 11 Staphylococcus aureus and 2 coagulase-negative Staphylococcus isolates for which oxacillin MICs were 1 to 4 micrograms/ml. The susceptibility studies were done with incubation at 35 and 30 degrees C, and the MICs were read at 24 and 48 h. Of the 13 isolates, 4 became resistant when longer incubation or 30 degrees C incubation was used, and the MICs for 9 remained in the intermediate range. Only three of these strains were susceptible to penicillin, and beta-lactamase was not detected. For 6 of 10 beta-lactamase-positive strains, there was a greater-than-twofold-dilution reduction in oxacillin MICs with the addition of clavulanic acid or sulbactam. Of the four strains that became resistant with incubation at the lower temperature, a clavulanic acid effect was observed in three but only at 35 degrees C. The oxacillin MIC for one of the beta-lactamase-negative strains was also reduced with clavulanic acid; however, this strain was inhibited by 1 microgram of clavulanic acid per ml alone. Bactericidal activity was observed with two or four times the oxacillin MIC in eight strains tested at both temperatures, and the combination with clavulanic acid was bactericidal at higher than four times the MIC in five of the strains at 30 degrees C. Our results suggest that oxacillin intermediate MICs for staphylococcal isolates are due not only to beta-lactamase hyperproduction but also some other unidentified factor. The reduction in oxacillin MIC observed when clavulanic acid was added to one strain was probably due to the intrinsic inhibitory activity of clavulanic acid.