A Combination of sbmA and tolC Mutations in Escherichia coli K-12 Tn 10 -Carrying Strains Results in Hypersusceptibility to Tetracycline

Abstract

ABSTRACT Previously, we demonstrated that Escherichia coli tolC mutations reduce the high-level resistance to tetracycline afforded by the transposon Tn 10 -encoded TetA pump from resistance at 200 μg/ml to resistance at 40 μg/ml. In this study, we found that the addition of an sbmA mutation to a tolC ::Tn 10 mutant exacerbates this phenotype: the double mutant did not form colonies, even in the presence of tetracycline at a concentration as low as 5 μg/ml. Inactivation of sbmA alone partially inhibited high-level tetracycline resistance, from resistance at 200 μg/ml to resistance at 120 μg/ml. There thus appears to be an additive effect of the mutations, resulting in almost complete suppression of the phenotypic expression of Tn 10 tetracycline resistance.