Effect of beta-lactamase induction on susceptibility to cephalosporins in Enterobacter cloacae and Serratia marcescens.

For Enterobacter cloacae GN5797 and Serratia marcescens 72-2, which produce inducible beta-lactamases and respond antagonistically to many combinations of cephalosporins, the changes in beta-lactamase activity and sensitivity to cephalosporins were examined during semi-continuous cultivation in which the concentration of inducers was decreased by two-fold dilution of the culture every 30 min starting from 1 h after the addition of inducers. The levels of beta-lactamase, which peaked at 0.5-2 h after induction, decreased concomitantly with the disappearance of the inducers in the culture, and returned to the uninduced levels by 6 h. The ability of cefoxitin and cefmetazole to induce beta-lactamases was more potent than that of cefazolin and cefotiam. The sensitivity to some cephalosporins, judged by the MIC and bactericidal effect, was slightly decreased at an early stage of induction, and reverted to the uninduced levels within several hours after the inducers were depleted.