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

Abstract

We have previously described strains ofEscherichia coliK12 resistant to D, L-ampicillin concentrations of 50 μg/ml (Eriksson-Grennberget al.1965; Bomanet al.1967). Such strains were assumed to be double mutants carrying the genesampAandampB. We here describe genetic steps which produce strains carrying only theampBgene. Determinations of resistance showed thatampAincreased the resistance provided byampA⁺by a factor of 10–15.AmpBincreased the resistance of bothampAandampA⁺by a factor of 2.R-factors were introduced into two sets of strains with all combinations ofampA,ampBand their wild type alleles.AmpAand the R-factors gave additive effects on resistance, whileampBdoubled the ampicillin resistance mediated byampAas well as by the R-factors.AmpBalso enhanced the chloramphenicol resistance of R-factor and of the wild type chromosomal genes. It is suggested thatampBresembles the modifying genes previously described and that R-factors can be useful for the identification of such genes.