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

Previously, we demonstrated thatEscherichia coli tolC mutations reduce the high-level resistance to tetracycline afforded by the transposon Tn10 -encoded TetA pump from resistance at 200 μg/ml to resistance at 40 μg/ml. In this study, we found that the addition of ansbmA mutation to atolC ::Tn10 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 ofsbmA 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 Tn10 tetracycline resistance.