Listeria monocytogenes Sigma B Contributes to the Tolerance of Bile Stress in the Gastrointestinal Tract

The ability of Listeria monocytogenes resisting bile stress is very important for its successful infection and colonization in the human gastrointestinal tract. Some studies have indicated that the alternative sigma factor Sigma B (σ^B), encoded by sigB, is at least partly involved in bacteria resisting many adverse conditions. In order to investigate the role of L. monocytogenes Sigma B in bile tolerance in the gastrointestinal tract, sigB mutant strain EGD_ΔsigB was constructed, and in parallel with wild type strain EGD, exposed to 3% bile salt (sublethal concentration for bacteria) with or without pre-exposure to acid (pH 4.5), alkali (pH 9.0), or osmotic (0.3M NaCl) conditions by mimicking the human gastrointestinal juice for 30 min. Their growth and survival rates were examined after 5 hours cultivation. The results were shown (i) wild type strain EGD was more resistant to 3% bile salt stress than sigB mutant strain EGD_ΔsigB; (ii) Both alkali- and osmotic-adapted EGD and EGD_ΔsigB were more resistant to 3% bile salt than nonadapted strains; (iii) acid-adapted EGD and EGD_ΔsigB didn´t increase their ability to resist 3% bile salt. These data indicate that L. monocytogenes σ^B is required for bile tolerance in the gastrointestinal tract, and also reveal some σ^B-dependent cross-protection mechanisms involving in tolerance of alkaline (or osmolality) and bile salt stress.