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Two unusual, heterogeneously-resistant, strains of Staphylococcus aureus appeared resistant to oxacillin, but susceptible to methicillin by disc diffusion methods. In agar dilution tests, both strains were oxacillin-resistant. One was susceptible to methicillin, and the other gave a paradoxical reaction, with growth only on plates containing low (0.5, 1 and 2 mg/l) and high (32 and 64 mg/l) concentrations of antibiotic. Induction of methicillin resistance was tested by inoculating each strain on to agar plates containing an inhibitory concentration of methicillin (8 mg/l), and then placing discs containing inducers (oxacillin, nafcillin, methicillin and CBAP 2-(2'-carboxyphenyl) benzoyl-6-aminopenicillanic acid) on the agar surface. Colonies grew only around discs containing effective inducers. Oxacillin and CBAP were much more potent inducers of methicillin resistance and beta-lactamase than was nafcillin or methicillin. These data suggest that the mechanism that regulates induction of the low-affinity penicillin binding protein (PBP-2') may be altered in these strains. Similar mechanisms appear to induce both beta-lactamase and methicillin resistance.
Boyce et al. (Mon,) studied this question.