Sensitivity of penicillinase-forming strains of Staphylococcus aureus and of their penicillinase-negative variants to cephaloridine, cephalothin, methicillin, and benzylpenicillin

Twenty-eight penicillinase-forming cultures of Staphylococcus aureus and their penicillinase-negative variants were examined for resistance to benzylpenicillin, methicillin, cephalothin, and cephaloridine. The results supported the view that cephaloridine was more easily destroyed by staphylococcal penicillinase than was cephalothin. In our tube-dilution tests, the minimum inhibitory concentration (M.I.C.) of cephaloridine for methicillin-sensitive cultures was never as high as some of the values reported by other workers who used apparently comparable methods. This was probably due to small differences in technique. The M.I.C. is an unsatisfactory measure of the antibiotic sensitivity of an organism which produces an enzyme which destroys the antibiotic.Methicillin-resistant strains of Staph. aureus have an intrinsic resistance of ;heterogenous' type also to benzylpenicillin, cephalothin, and cephaloridine.