ABSTRACT: Avian pathogenic Escherichia coli (APEC) is a specific group of extraintestinal pathogenic E. coli that causes a variety of extraintestinal diseases in chickens, ducks, pigeons, turkeys, and other avian species. These diseases lead to significant economic losses in the poultry industry worldwide. However, owing to excessive use of antibiotics in the treatment of infectious diseases, bacteria have developed antibiotic resistance. The development of multidrug efflux pumps is one important bacterial antibiotic resistance mechanism. A multidrug efflux pump, MdtH, which belongs to the major facilitator superfamily of transporters, confers resistance to quinolone antibiotics such as norfloxacin and enoxacin. LsrR regulates hundreds of genes that participate in myriad biological processes, including mobility, biofilm formation, and antibiotic susceptibility. However, whether LsrR regulates mdtH transcription and then affects bacterial resistance to various antibiotics in APEC has not been reported. In the present study, the lsrR mutant was constructed from its parent strain APECX40 (WT), and high-throughput sequencing was performed to analyze the transcriptional profile of the WT and mutant XY10 strains. The results showed that lsrR gene deletion upregulated the mdtH transcript level. Furthermore, we also constructed the lsrR- and mdtH-overexpressing strains and performed antimicrobial susceptibility testing, antibacterial activity assays, real-time reverse transcription PCR, and electrophoretic mobility shift assays to investigate the molecular regulatory mechanism of LsrR on the MdtH multidrug efflux pump. The lsrR mutation and the mdtH-overexpressing strain decreased cell susceptibility to norfloxacin, ofloxacin, ciprofloxacin, and tetracycline by upregulating mdtH transcript levels. In addition, the lsrR-overexpressing strain increased cell susceptibility to norfloxacin, ofloxacin, ciprofloxacin, and tetracycline by downregulating mdtH transcript levels. Electrophoretic mobility shift assays indicated that LsrR directly binds to the mdtH promoter. Therefore, this study is the first to demonstrate that LsrR inhibits mdtH transcription by directly binding to its promoter region. This action subsequently increases susceptibility to the aforementioned four antibiotics in APECX40.
