Influence of imipenem on the serum resistance of enterobacteriaceae.

Following growth in a subinhibitory concentration of imipenem and additional incubation in a 20% dilution of normal human serum (NHS) for 90 minutes, five of 12 serum-resistant strains of enterobacteriaceae showed a decrease in colony-forming units of two or more logs of growth compared with the control. Two strains (of Escherichia coli and Enterobacter aerogenes) showed this phenomenon even with incubation in 5% NHS. Treatment with imipenem did not change the serum resistance of the other seven strains (two strains each of Enterobacter cloacae, Klebsiella pneumonia, and Serratia marcescens, and one strain of Proteus morganii). The phenomenon of induced serum susceptibility is dose dependent and reversible. Other beta-lactam antibiotics either caused only a slight decrease of resistance (cefsulodin, cefoxitin, cefuroxime, cefodizime-HR221) or did not influence the serum resistance at all (cefotaxime, mecillinam). Killing of the induced serum-sensitive strains appeared to be antibody dependent.