Antagonism of the antibacterial action of some penicillins by other penicillins and cephalosporins

Abstract

There are many examples of 2 penicillins acting synergistically, usually by one competitively inhibiting β lactamase, thus protecting the other from inactivation. There are few reports on penicillins antagonizing each other. Eight strains of 3 genera (Proteus, Escherichia, Pseudomonas) showed antagonism of carbenicillin or ampicillin by cephaloridine, cloxacillin, or 6 aminopenicillanic acid. Broth dilution tests showed that with 7 of the 8 strains the minimum inhibiting concentration (MIC) of the more active antibiotic was increased by 800-6,400% by low concentrations (often 1/10th of the MIC) of the antagonist, whereas higher concentrations of 'antagonist' were not as antagonistic. This suggested that 2 or more receptor sites of action for penicillins were present; the antagonist thus blocks the antibacterial action at the more sensitive site but acts additively with the antagonized antibiotic at the less sensitive site. The possibility that the mechanism of antagonism was induction of inactivating enzymes (β lactamase, penicillin acylase) was studied in 2 strains (one Escherichia coli and one Proteus rettgeri), and 2 antagonists were studied in detail. With E. coli cephaloridine was a poorer inducer of β lactamase than were the antagonized antibiotic and 6 aminopenicillanic acid. From these organisms, the good inducers of a β lactamase that acted on benzylpenicillin did not induce enzymes that inactivated carbenicillin. Thus, the mechanism of antagonism was not due to β lactamase induction.