SigB plays a major role in Listeria monocytogenes tolerance to bile stress

The ability of Listeria monocytogenes to tolerate high levels of bile stress is critical to its successful infection and colonization in the human gastrointestinal tract. L. monocytogenes encodes bile salt hydrolase by a bsh gene which plays a significant role in hydrolyzing high concentrations of bile salt when L. monocytogenes grows under hypoxemic condition. As the bsh promoter contains consensus SigB and PrfA binding sites, we investigated the role of SigB (σB) and PrfA in L. monocytogenes tolerance against bile stress by comparing the survival of isogenic deletion mutants of L. monocytogenes EGDΔsigB, EGDΔprfA and EGDΔprfAΔsigB with their parent strain EGD at high levels of bile salt. Our results show that the sigB deletion significantly reduced the MICs of bile salt for EGDΔsigB and EGDΔprfAΔsigB (2.6% and 2.2% vs 3.5% in wild type strain EGD), while the growth rates of these two sigB deletion mutants (EGDΔsigB and EGDΔprfAΔsigB) were affected the most in the presence of 3% bile salt. Pre-exposure to alkali (pH 9.0) and osmotic (0.3 M NaCl) stresses for a short period of time (30 min) resulted in improved growth of L. monocytogenes as well as its prfA-sigB isogenic mutants even under sublethal concentrations of bile salt, while pre-exposure to acid pH (pH 4.5) failed to provide cross-protection against subsequent bile stress. Furthermore, the sigB gene had more remarkable influence than that of prfA on bsh expression, as much lower levels of bsh transciption were observed in EGDΔsigB and EGDΔprfAΔsigB. Meanwhile, bsh expression in the deletion mutants did not respond to elevated levels of bile salt. These data indicate that σB might play a crucial role in Listeria survival under bile salt environment in the gastrointestinal tract before its successful colonization, invasion and intracellular propagation.

Research Highlights
► σB seems to play more important role than PrfA in tolerance against bile stress.
► The bsh gene expression was affected the most by σB.
► Other PrfA- and (or) Sigma B-regulated loci could play a role in bile tolerance.