Regulatory Role of the Two-Component System BasSR in the Expression of the EmrD Multidrug Efflux in Escherichia coli

Due to excessive use of antimicrobial agents in the treatment of infectious diseases, bacteria have developed resistance to antibacterial drugs and toxic compounds. The development of multidrug efflux pumps is one of the important mechanisms of bacterial drug resistance. A multidrug efflux pump, EmrD, belonging to the major facilitator superfamily of transporters, confers resistance to many antimicrobial agents. BasSR, a typical two-component signal transduction system (TCS), regulates susceptibility to the cationic antimicrobial peptide, polymyxin B, and the anionic bile detergent, deoxycholic acid, in Escherichia coli. However, whether or not the BasSR TCS affects susceptibility or resistance to other antimicrobial agents and transcription of emrD has not been reported in E. coli. In the present study, we constructed the basSR mutants of wild-type MG1655 and clinical strain APECX40 and performed antimicrobial susceptibility testing, antibacterial activity assays, real-time reverse transcription-PCR experiments and electrophoretic mobility shift assays (EMSA) to investigate the molecular mechanism by which BasSR regulates the EmrD multidrug efflux pump. Results showed that the basSR mutation increased cell susceptibility to eight antimicrobial agents, including ciprofloxacin, norfloxacin, doxycycline, tetracycline, clindamycin, lincomycin, erythromycin, and sodium dodecyl sulfate, by downregulating the transcriptional levels of emrD. Furthermore, EMSA indicated that BasR could directly bind to the emrD promoter. Therefore, this study was the first to demonstrate that BasSR activates transcription of emrD by binding directly to its promoter region, and then decreases susceptibility to various antimicrobial agents in E. coli strains, APECX40 and MG1655.