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

Strains ofEscherichia coli andSalmonella typhimurium carrying R plasmids, which were obtained from ampicillin-resistant clinical isolates ofE. coli andKlebsiella 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 ofKm of the enzymes are lower for sulbenicillin and carbenicillin than for ampicillin, the ratios ofVmax toKm for sulbenicillin and carbenicillin are not high enough to explain the high resistance inE. coli bearing the R plasmid. Two mutants of the plasmids conferring a temperature-sensitive ampicillin resistance were induced by nitrosoguanidine treatment. It was confirmed thatE. 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 inE. coli strains carrying R (bla ) plasmids is postulated.