We have previously described strains of Escherichia coli K12 resistant to D, L-ampicillin concentrations of 50 μg/ml (Eriksson-Grennberg et al. 1965; Boman et al. 1967). Such strains were assumed to be double mutants carrying the genes ampA and ampB. We here describe genetic steps which produce strains carrying only the ampB gene. Determinations of resistance showed that ampA increased the resistance provided by ampA+ by a factor of 10–15. AmpB increased the resistance of both ampA and ampA+ by a factor of 2.
R-factors were introduced into two sets of strains with all combinations of ampA, ampB and their wild type alleles. AmpA and the R-factors gave additive effects on resistance, while ampB doubled the ampicillin resistance mediated by ampA as well as by the R-factors. AmpB also enhanced the chloramphenicol resistance of R-factor and of the wild type chromosomal genes. It is suggested that ampB resembles the modifying genes previously described and that R-factors can be useful for the identification of such genes.