A Novel Negative Regulation Mechanism of Bacterial Outer Membrane Proteins in Response to Antibiotic Resistance

Although some outer membrane (OM) proteins involved in antibiotic resistance have been previously reported, the OM proteins regulating chlortetracycline (CTC) resistance are largely unknown. In this study, we employed a subproteomics approach to identify altered OM proteins of Escherichia coli in response to CTC exposure. Upregulation of ToIC and downregulation of LamB, FadL, OmpC, OmpT, and OmpW were found in E. coli strains exposed to CTC at a high concentration that was increased suddenly and at a half-minimum inhibitory concentration (MIC) that was kept constant in the culture medium. These changes in the level of protein expression were validated using Western blotting. In addition, the possible roles of these altered proteins and their regulation mechanisms in response to CTC exposure were investigated using genetically modified strains with gene deletion of these altered proteins. It was found that deletion of tolC, fadL, ompC, ompT, or ompW resulted in a decrease in the MICs and survival capabilities of the gene-deleted strains, whereas the absence of lamB led to an improvement of the two abilities. The downregulation of LamB expression in the CTC-resistant E. coli strain and the increased antibiotic resistance in its gene-deleted strain suggested a negative regulation mechanism in E. coli in response to CTC exposure. Meanwhile, the direction of the regulation pattern in response to CTC exposure was different from that in E. coli in response to exposure to other antibiotics. These findings uncover a novel antibiotic-resistant mechanism in which bacteria respond to exposure to antibiotics through alteration of the direction of regulation of OM proteins.