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We showed that most Staphylococcus aureus strains that have borderline or intermediate susceptibility to the penicillinase-resistant penicillins (PRPs) react this way because of the activity of their beta-lactamase on these antimicrobial agents. These strains produced large amounts of staphylococcal beta-lactamase that rapidly hydrolyzed penicillin and partially hydrolyzed the PRPs. Susceptibility to hydrolysis was penicillin greater than oxacillin greater than cephalothin greater than methicillin. The borderline results and the hydrolysis could be prevented by the beta-lactamase inhibitors clavulanic acid and sulbactam. For intrinsically methicillin-resistant (heteroresistant) S. aureus, the inhibitors reduced the penicillin MICs, but the strains remained resistant to all the beta-lactam antimicrobial agents, including penicillin. We conclude that the borderline in vitro susceptibility or resistance to PRPs in most of these S. aureus strains is mediated by beta-lactamase and they are not heteroresistant or intrinsically resistant. We do not know whether this in vitro resistance is expressed clinically.
McDougal et al. (Thu,) studied this question.