Stability of Cephaloridine and Cephalothin to Staphylococcal Penicillinase

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Abstract
The rates of destruction of benzylpenicillin, cephaloridine, cephalothin and (in some cases) cloxacillin by cocci from induced cultures of 17 strains of Staphylococcus aureus were measured. Rates of destruction of cephaloridine, relative in each case to those of benzylpenicillin, by the 10 methicillin-resistant strains were not significantly different from those by the 7 methicillin-sensitive strains. Vmax for cephaloridine and cephalothin were about 0.2% and 0.009%, respectively, that for benzylpenicillin; cloxacillin was destroyed still more slowly. The inactivation of the cephalosporins is almost certainly brought about by the action of penicillinase. Notwithstanding its greater stability to staphylococcal penicillinase, cephalothin is less effective than cephaloridine at suppressing the growth of the methicillin-resistant strains, although there is cross-resistance between methicillin, cloxacillin, cephalothin and cephaloridine.