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Journal of Antimicrobial Chemotherapy (1987) 20, 15-22
© 1987 The British Society for Antimicrobial Chemotherapy

research-article

The effect of imipenem on strains of Enterobacteriaceae expressing Richmond & Sykes Class I ß-lactamases

J. Ashby, B. Kirkpatrick, L. J. V. Piddock and R. Wise

Department of Medical Microbiology, Dudley Road Hospital Birmingham B18 7QH, U.K.

accepted 18 February 1987

Fourteen strains of Enterobacteriaceae producing Richmond & Sykes Class I ß-lactamase were studied. The ability of cefoxitin and imipenem to induce ß-lactamase production (reversible derepression) and to select stably derepressed mutants in these strains was assessed. ß-Lactamase induction by cefoxitin and imipenem was demonstrated by the disc diffusion technique in all strains. Cefoxitin selected stably derepressed mutants for all strains in broth cultures, but in an identical experiment imipenem did not. The susceptibility of each strain and its stably derepressed mutant (selected with cefoxitin) to a range of ß-lactam antibiotics was then ascertained. The stably derepressed mutants exhibited decreased susceptibility to all antibiotics tested except imipenem. The decrease in susceptibility varied between strains and between antibiotics but reached a maximum of a 256-fold decrease. The ß-lactamase activity of selected stably derepressed mutant strains showed at least a 600-fold increase in activity.

Imipenem would therefore seem an appropriate choice for therapy of infections caused by this group of organisms, as it is active against derepressed mutants and unlikely to select any such strains during therapy.


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