INDUCTION OF BETA-LACTAMASE BY CEFOXITIN IN ANAEROBIC INTESTINAL MICROFLORA

Beta-lactamases produced by two anaerobic bacterial strains, Bacteroid es ovatus Ax34:1 and Clostridium butyricum NBL3, were shown to be sign ificantly inducible under anaerobic conditions in subinhibitory concen trations of cefoxitin, The induction ratio of beta-lactamase productio n for Bacteroides ovatus was 2.6 and for Clostridium butyricum 1.6. in cubation of faecal samples with different concentrations of cefoxitin did not result in any induction of beta-lactamase production. When add ing a highly inducible aerobic strain (Citrobacter freundii F72:6, ind uction ratio of 26.5 in broth culture) to faecal samples, an induction ratio of 4.5 was reached. Faeces seem to inhibit beta-lactamase induc tion in aerobic and anaerobic bacteria. The inducible enzymes produced by the anaerobic strains did not have the same properties as beta-lac tamases from aerobic inducible strains, according to substrate profile s and inhibition studies. The results of the present study indicate th at increased levels of beta-lactamases in the normal intestinal microf lora, which often are observed after administration of beta-lactam age nts, are probably due to selection of stably derepressed mutants rathe r than to induction of beta-lactamase production.