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SUMMARY : Cephalosporin C, a hydrophilic, penicillin-like antibiotic which is insensitive to penicillinase, is much less active than benzylpenicillin against the Oxford strain of Staphylococcus aureus but much more active than benzylpenicillin against a penicillinase-producing strain of S. aureus (D3R). When used together, cephalosporin C and benzylpenicillin act synergistically against S. aureus strain D3R in Witro, presumably because cephalosporin C is a competitive inhibitor of penicillinase. When grown under certain conditions in the presence of cephalosporin C, benzyl- penicillin, or bacitracin, the Oxford staphylococcus gives rise during a single culture to a resistant bacterial population consisting largely or partly of Gram-negative bacilli. In the presence of bacitracin a similar change also occurs with S. aureus strain D3R. Two hydrophilic antibiotics, cephalosporin N and cephalosporin C, have been isolated from the culture fluid of a species of Cephalosporium (Abraham, Newton & Hale, 1954; Newton & Abraham, 1956). Cephalosporin N is a new type of penicillin with a side-chain derived from D-a-aminoadipic acid (Newton & Abraham, 1954), and appears to be identical with synnematin B (Abraham et al. 1955). Cephalosporin C is chemically related to cephalo- sporin N, but is not a true penicillin (Abraham & Newton, 1956~). It is not sensitive to penicillinase, but is a competitive inhibitor of the action of this enzyme on penicillin (Abraham & Newton, 1956b), and it induces the forma- tion of penicillinase by Bacillus cereus (Pollock, 1957). It has a low anti- bacterial activity, but no significant toxicity to mice, and it affords protection to mice infected with Streptococcus pyogenes or with a penicillin-resistant strain of Staphylococcus aureus (Florey, 1955, 1957). The finding that cephalosporin C was a competitive inhibitor of penicillinase raised the question whether it would act synergistically with penicillin against penicillin-resistant staphylococci which produce this enzyme. The work described in the present paper was begun with the object of obtaining an answer to this question. During the work it was observed that staphylococci underwent profound morphological and biochemical changes when grown under certain conditions in media containing cephalosporin C, benzyl- penicillin, or bacitracin. METHODS Materials. Benzylpenicillin (1650 units/mg., Glaxo Laboratories Ltd.) and cephalosporin C (Newton & Abraham, 1956) were used as the crystalline sodium salts. The bacitracin was a commercial sample (A/S Apothekernes

The Synergistic Action of Cephalosporin C and Benzyl-penicillin against a Penicillinase-producing Strain of Staphylococcus aureus