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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.

genetical research

Resistance of<i>Escherichia coli</i>to penicillins: III. AmpB, a locus affecting episomally and chromosomally mediated resistance to ampicillin and chloramphenicol

Resistance ofEscherichia colito penicillins: III. AmpB, a locus affecting episomally and chromosomally mediated resistance to ampicillin and chloramphenicol