Population analysis of sensitivity to ampicillin and carbenicillin in Eschericia coli and Enterobacter cloacae

Abstract

Population analysis of sensitivity to ampicillin (A) and carbenicillin (Ca) was carried out with four strains of E. coli and five strains of Enterobacter cloacae. The two A-sensitive/Ca-sensitive (A-s/Ca-s) E. coli strains were homogeneous as regards A-sensitivity. From the A-resistant/Ca-s (A-r/Ca-s) E. coli strains, it was possible to select A-r/Ca-r mutants. Mutation frequency approx. 10-7.5. When the inoculum of A-s/Ca-s E. coli was raised, it was possible to select A-r/Ca-s/cephalothin-r variants with a frequency of approx. 10-9.7. In this laboratory 5% (10-1.3) of the received E. coli strains are A-r/Ca-s. These strains might consequently be mutants of the usual A-s/Ca-s E. coli, selected in an ampicillin environment. The two A-s/Ca-s E. cloacae strains developed mutants highly resistant to A, thereby differing from the A-s/Ca-s E. coli strains. The mutation frequency was approx. 10-7.2. Likewise it was possible to select A-r/Ca-r E. cloacae strains from the usual A-r/Ca-s ones. Mutation rate approx. 10-6.8.