Abstract
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 σL 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 σA and σL) and ΔBCHL quadruple mutant (lacking all alternative sigma factor genes; expressing only σA) 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 σL. Interestingly, more than 80% of DEGs were found to be negatively regulated in the presence of σL. 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 σL. This result suggests the potential negative role of σL on bile survival, and the roles of σL and σB might be in a seesaw model prior to host cell invasion.
Highlights
The foodborne pathogen Listeria monocytogenes is a facultative Gram-positive intracellular bacterium that is able to adapt to a board range of habitats
To determine the ability of L. monocytogenes to survive under simulated GI fluids, we exposed the bacteria to simulated gastric, duodenal, or bile for 10 and 20 min
Following 10-min gastric fluid exposures, most of the deletion mutant strains survived similar to wild type, that is, similar levels of log reduction [i.e., log (CFU at T0) − log (CFU recovered at that of 10 min (T10))] when compared between WT and mutants (Figure 1A)
Summary
The foodborne pathogen Listeria monocytogenes is a facultative Gram-positive intracellular bacterium that is able to adapt to a board range of habitats. It can survive and grow in a variety of temperatures, and a wide range of pH, osmotic pressure, and high salt (Membre et al, 2005; Goulet et al, 2013; Bevilacqua et al, 2018; Axelsson et al, 2020). Following being ingested with contaminated food, L. monocytogenes goes through GI passage. It encounters the low pH in stomach and the bile salt and high osmolality in intestinal fluid. It has been reported that L. monocytogenes utilizes unique bile resistance mechanisms to survive in the gallbladder (Hardy et al, 2004)
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