In vitro Activity of Penicillins Against Staphylococcus albus.

Abstract

Summary and conclusions Among 175 clinical isolates of Staph. albus (Staph. epidermidis)—most of them from blood cultures—resistance (MIC ≤ 12.5 μg/ml) was most frequent to penicillin G (50%) and ampicillin (41%), but 10% were resistant to methicillin, 5% to oxacillin and diphenicillin, less than 2% to nafcillin and none to cephalothin. A varying number of these strains were also tested against 3 additional penicillins, 6 other antibiotics and fusidic acid; resistance was most frequent to chloramphenicol (60%), tetracycline (48%) and erythromycin (29%), none of them were resistant to kanamycin and a varying proportion were resistant to the other agents. Penicillinase-like action was demonstrated by the Haight-Finland test with 119 of the 175 strains; 12 of these strains, including 5 that were methicillin-resistant, were studied further. The 12 strains all produced a penicillin G-beta-lactamase but cell preparations from the 5 methicillin-resistant strains failed to destroy methicillin in 24 hours. There was no correlation between the MIC of methicillin and enzyme activity or results of the Haight-Finland test. Inoculum size had a marked effect on the MIC of methicillin for methicillin-resistant strains, but not for methicillin-sensitive strains. The experimental data favor the hypothesis that the inoculum effect was due to presence in the cultures of a small number of cells with inherent resistance rather than to the action of a methicillin-destroying enzyme. No methicillin-resistant strains of Staph. aureus have been isolated at this hospital, but the finding of methicillin-resistant strains of Staph. albus may portend the appearance of such strains.