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Background  Mastitis is one of the most common infectious diseases in dairy cattle and causes significant financial losses in the dairy industry worldwide. Antibiotic therapy has been used as the most effective strategy for clinical mastitis treatment. However, due to the extensive use of antibacterial agents, antimicrobial resistance (AMR) is considered to be one of the reasons for low cure rates in bovine mastitis. In addition, biofilms could protect bacteria by restricting antibiotic access and shielding the bacterial pathogen from mammary gland immune defences. The functional mechanisms of quorum sensing  E. coli  regulators B an d C (QseBC) have been well studied in  E. coli  model strains; however, whether QseBC regulates antibiotic susceptibility and biofilm formation in clinical  E. coli  strain has not been reported.    Methods  In this study, we performed construction of the  qseBC  gene mutant, complementation of the  qseBC  mutant, antimicrobial susceptibility testing, antibacterial activity assays, biofilm formation assays, real-time reverse transcription PCR (RT-PCR) experiments and electrophoretic mobility shift assays (EMSAs) to investigate the role of  qseBC  in regulating biofilm formation and antibiotic susceptibility in the clinical  E. coli  strain ECDCM2.    Results  We reported that inactivation of QseBC led to a decrease in biofilm formation capacity and an increase in antibiotic susceptibility of an   E. coli  strain isolated from a dairy cow that suffered from mastitis. In addition, this study indicated that QseBC increased biofilm formation by upregulating the transcription of the biofilm-associated genes  bcsA, csgA, fliC, motA, wcaF  and  fimA  and decreased antibiotic susceptibility by upregulating the transcription of the efflux-pump-associated genes  marA ,  acrA ,  acrB ,  acrD ,  emrD  and  mdtH . We also performed EMSA assays, and the results showed that QseB can directly bind to the  marA  promoter.    Conclusions  The QseBC two-component system affects antibiotic sensitivity by regulating the transcription of efflux-pump-associated genes. Further, biofilm-formation-associated genes were also regulated by QseBC TCS in  E. coli  ECDCM2. Hence, this study might provide new clues to the prevention and treatment of infections caused by the clinical  E. coli  strains.

QseBC is involved in the biofilm formation and antibiotic resistance in Escherichia coli isolated from bovine mastitis