Hydrolytic rate at low drug concentration as a limiting factor in resistance to newer cephalosporins.

Hydrolysis kinetics of two cephalosporinases from Citrobacter freundii and Proteus vulgaris having different affinities for cefotaxime and ceftazidime was assessed in studies with cefotaxime, ceftazidime, BMY-28142, and imipenem. The two cephalosporinase genes were cloned into strains of Escherichia coli. The production of these cephalosporinases in strains of E. coli, as well as in the derepressed mutants of C. freundii and P. vulgaris, caused a decrease in susceptibility to the newer cephalosporins. The difference in the rate of hydrolysis at a 0.1 microM concentration of substrate adequately explains the difference in antibacterial activity between cefotaxime and BMY-28142 against E. coli strains with the two cephalosporinases. The results indicate that hydrolysis rate at low substrate concentration, rather than binding with beta-lactams, would be a limiting factor in resistance. The low affinity of cephalosporinases for BMY-28142 means high stability of the agent to the enzymes at low concentration. Furthermore, outer membrane permeability affects the susceptibility of E. coli to cephalosporins synergistically with hydrolysis by cephalosporinases.