Beta-Lactamase-Directed Barrier for Penicillins of Escherichia coli Carrying R Plasmids

Abstract

Strains of Escherichia coli and Salmonella typhimurium carrying R plasmids, which were obtained from ampicillin-resistant clinical isolates of E. coli and Klebsiella spp. and specified either the type IIIa (TEM-type) or type Va (oxacillin-hydrolyzing) β-lactamase, are resistant not only to ampicillin but also to carbenicillin and sulbenicillin. The latter two derivatives, however, are poorly hydrolyzed in vitro by the β-lactamases. Although values of K_m of the enzymes are lower for sulbenicillin and carbenicillin than for ampicillin, the ratios of V_max to K_m for sulbenicillin and carbenicillin are not high enough to explain the high resistance in E. coli bearing the R plasmid. Two mutants of the plasmids conferring a temperature-sensitive ampicillin resistance were induced by nitrosoguanidine treatment. It was confirmed that E. coli CSH2, harboring the mutant plasmid, produces a temperature-sensitive β-lactamase and is resistant only at low temperatures (below 33°C), but not at 42°C, to ampicillin, sulbenicillin, and carbenicillin simultaneously. It is thus concluded that β-lactamase itself is responsible for the mechanism of resistance not only to ampicillin but also to sulbenicillin and carbenicillin, even though the enzyme as determined in cell-free extracts hydrolyzes the latter two drugs poorly. An unknown barrier for sulbenicillin and carbenicillin directed by β-lactamase in E. coli strains carrying R (bla) plasmids is postulated.