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Listeria monocytogenes  is a Gram-positive bacterium causing listeriosis in animals and humans. To initiate a foodborne infection,  L. monocytogenes  has to pass through the host gastrointestinal tract (GIT). In this study, we evaluated survival abilities of  L. monocytogenes  10403S wild type (WT) and its isogenic mutants in alternative sigma (σ) factor genes (i.e.,  sigB ,  sigC ,  sigH , and  sigL ) under simulated gastric, duodenal, and bile fluids. Within 10min of exposures, only bile fluid was able to significantly reduce survival ability of  L. monocytogenes  WT by 2 logs CFU/ml. Loss of  sigL  showed the greatest bile resistance among 16 strains tested,  p <0.0001, (i.e., WT, four single alternative σ factor mutants, six double mutants, four triple mutants, and one quadruple mutant). To further investigate the role of σ L  in bile response, RNA-seq was conducted to compare the transcriptional profiles among  L. monocytogenes  10403S  ΔBCH  triple mutant (lacking  sigB ,  sigC , and  sigH  genes; expressing housekeeping σ A  and σ L ) and  ΔBCHL  quadruple mutant (lacking all alternative sigma factor genes; expressing only σ A ) strains under BHI and 1% bile conditions. A total of 216 and 176 differentially expressed genes (DEGs) were identified in BHI and bile, respectively. We confirmed that  mpt  operon was shown to be strongly activated by σ L . Interestingly, more than 80% of DEGs were found to be negatively regulated in the presence of σ L . This includes PrfA regulon and its mediated genes (i.e.,  hly ,  hpt ,  inlB ,  clpP ,  clpE ,  groL , and  inlC ) which were downregulated in response to bile in the presence of σ L . This result suggests the potential negative role of σ L  on bile survival, and the roles of σ L  and σ B  might be in a seesaw model prior to host cell invasion.

Listeria monocytogenes 10403S Alternative Sigma-54 Factor σL Has a Negative Role on Survival Ability Under Bile Exposure