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

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 (sigma) 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 sigma factor mutants, six double mutants, four triple mutants, and one quadruple mutant). To further investigate the role of sigma(L) in bile response, RNA-seq was conducted to compare the transcriptional profiles among L. monocytogenes 10403S Delta BCH triple mutant (lacking sigB, sigC, and sigH genes; expressing housekeeping sigma(A) and sigma(L)) and Delta BCHL quadruple mutant (lacking all alternative sigma factor genes; expressing only sigma(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 sigma(L). Interestingly, more than 80% of DEGs were found to be negatively regulated in the presence of sigma(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 sigma(L). This result suggests the potential negative role of sigma(L) on bile survival, and the roles of sigma(L) and sigma(B) might be in a seesaw model prior to host cell invasion.</p>