The MarR Family Regulator BmrR Is Involved in Bile Tolerance of BBMN68 via Controlling the Expression of an ABC Transporter.

In order to colonize the human gastrointestinal tract and exert their beneficial effects, bifidobacteria must effectively cope with toxic bile salts in the intestine; however, the molecular mechanism underlying bile tolerance is poorly understood. In this study, heterologous expression of a MarR family transcriptional regulator, BmrR, significantly reduced the ox bile resistance of NZ9000, suggesting that BmrR might play a role in the bile stress response. analysis combined with reverse transcription-PCR assays demonstrated that was cotranscribed with and , which encoded multidrug resistance (MDR) ABC transporters. Promoter prediction and electrophoretic mobility shift assays revealed that BmrR could autoregulate the operon by binding to the box (ATTGTTG-6nt-CAACAAT) in the promoter region. Moreover, heterologous expression of and in yielded 20.77-fold higher tolerance to 0.10% ox bile, compared to the wild-type strain. In addition, ox bile could disrupt the DNA binding activity of BmrR as a ligand. Taken together, our findings indicate that the operon is autoregulated by the transcriptional regulator BmrR and ox bile serves as an inducer to activate the bile efflux transporter BmrAB in response to bile stress in BBMN68. Bifidobacteria are natural inhabitants of the human intestinal tract. Some bifidobacterial strains are used as probiotics in fermented dairy production because of their health-promoting effects. Following consumption, bifidobacteria colonize the lower intestinal tract, where the concentrations of bile salts remain nearly 0.05% to 2.0%. Bile salts, as detergent-like antimicrobial compounds, can cause cellular membrane disruption, protein misfolding, and DNA damage. Therefore, tolerance to physiological bile stress is indeed essential for bifidobacteria to survive and to exert probiotic effects in the gastrointestinal tract. In BBMN68, the MarR-type regulator BmrR was involved in the bile stress response by autoregulating the operon, and ox bile as an inducer could increase the expression of the BmrAB transporter to enhance the bile tolerance of BBMN68. Our study represents a functional analysis of the operon in the bile stress response, which will provide new insights into bile tolerance mechanisms in and other bacteria.