Contribution of Alternative Sigma Factors on Listeria monocytogenes Survival in Synthetic Bile

Introduction:  Listeria monocytogenes can persist and grow under extreme conditions, such as low pH in the stomach and bile in the intestinal fluid. The ability to survive exposure to bile is important for subsequent colonization and infection. Listeria monocytogenes regulates gene expression patterns in response to stressors, during gastrointestinal passage, through use of alternative sigma factors (σ).

Purpose: The purpose of this study is to determine the role of alternative sigma factor(s) (σ^B, σ^C, σ^H, σ^L) in L. monocytogenes stress response and survival in synthetic bile juice. We hypothesized that coregulation of sigma factors allowed L. monocytogenes to survive in bile stress and, subsequently, establish infection.

Methods:  Listeria monocytogenes parent strain 10403S and isogenic single, double, triple, and quadruple deletion mutants were grown in brain heart infusion (BHI) broth at 37°C overnight. Cultures were transferred to fresh BHI broth and grown to midlog phase (OD₆₀₀= 0.4), then exposed to synthetic bile juice (pH 8.2) for 10 and 20 min. Survival was determined by serial dilution and plating on BHI agar. Experiments were conducted in triplicates. Differences in survival abilities were determined using the t-test.

Results: Δσ^B and Δσ^C mutants showed significant decreases in survival in comparison to wildtype after 10 min exposure (P=0.010 and 0.036, respectively). Δσ^H and Δσ^L mutants survived significantly better than wildtype after 10 min (P=0.005 and 0.002, respectively). Moreover, among the alternative sigma factors, σ^L showed the greatest significant effect on L. monocytogenes survival abilities in synthetic bile.

Significance:  The data suggested that σ^B and σ^C have positive effects, while σ^H and σ^L have negative effects on L. monocytogenes survival in bile. Expression of genes under σ^B and σ^C regulation could potentially promote L. monocytogenes adaptation during gastrointestinal passage and, therefore, may be important control strategy targets.