Mutation experiments with AcrB and MexB D408A mutants * indicate the efflux liability of antibiotics

BackgroundWe showed that exposure of an AcrB D408A mutant to efflux inhibitors applied evolutionary pressure to select bacteria with the wild type acrB sequence. This suggested that reversion to wild type can differentiate between efflux inhibitors. Thus, we hypothesized that this experiment could identify inhibitors of the primary RND pump, AcrB or its homologues in other species. ObjectivesTo construct three mutants, Escherichia coli AcrB D408A, Klebsiella pneumoniae AcrB D408A and Pseudomonas aeruginosa MexB D408A and expose the mutants to substrates and non-substrates of AcrB and/or MexB and determine the rate of reversion to wildtype acrB/mexB sequence. MethodsMutant Escherichia coli AcrB D408A, Klebsiella pneumoniae AcrB D408A and Pseudomonas aeruginosa MexB D408A were constructed with site-directed mutagenesis of the relevant nucleotide in the acrB/mexB gene. Mutants were exposed on agar to substrates and the mutation frequency and mutation rate determined. The MIC of antibiotics and the presence/absence of the D408A substitution was determined for mutants. ResultsExposure to the AcrB substrates chlorpromazine and minocycline reverted the D408A genotypes to wild type in a species-dependent manner. Exposure to a non-AcrB substrate, spectinomycin, did not select wild type acrB. Chlorpromazine selected for wild type acrB K. pneumoniae as it had for S. Typhimurium, whereas minocycline selected for wild type E. coli acrB. None of the antibiotics selected wild type mexB, including the tested MexB substrates. ConclusionsEvolutionary paths depend upon the genetic background of the species and availability of alternative routes/genetic pathways that can confer resistance/decreased susceptibility to antibiotics.