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Abstract
Listeria monocytogenes is a foodborne Gram-positive bacterium causing listeriosis in both animals and humans. It can persist and grow in various environments including conditions countered during saprophytic or intra-host lifestyles. Sigma (σ) subunit of RNA polymerase is a transcriptional factor responsible for guiding the core RNA polymerase and initiating gene expression under normal growth or physiological changes. In L. monocytogenes, there is one housekeeping sigma factor, σA, and four alternative sigma factors σB, σC, σH, and σL. Generally, σA directs expression of genes required for normal growth while alternative σ factors alter gene expression in response to specific conditions (e.g., stress). In this study, we aimed to determine the exclusive role of σA in L. monocytogenes by comparing a wild type strain with its isogenic mutant lacking genes encoding all alternative sigma factors (i.e., sigB, sigC, sigH, and sigL). We further investigated their survival abilities in 6% porcine bile (pH 8.2) mimicking gallbladder bile and their transcriptomics profiles in rich medium (i.e., BHI) and 1% porcine bile. Surprisingly, the results showed that survival abilities of wild type and ΔsigBΔsigCΔsigHΔsigL (or ΔsigBCHL) quadruple mutant strains in 6% bile were similar suggesting a compensatory role for σA. RNA-seq results revealed that bile stimulon of L. monocytogenes wild type contained 66 genes (43 and 23 genes were up- and down-regulated, respectively); however, only 29 genes (five up- and 24 down-regulated by bile) were differentially expressed in ΔsigBCHL. We have shown that bile exposure mediates increased transcription levels of dlt and ilv operons and decreased transcription levels of prfA and heat shock genes in wild type. Furthermore, we identified σA-dependent bile inducible genes that are involved in phosphotransferase systems, chaperones, and transporter systems; these genes appear to contribute to L. monocytogenes cellular homeostasis. As a result, σA seemingly plays a compensatory role in the absence of alternative sigma factors under bile exposure. Our data support that the bile stimulon is prone to facilitate resistance to bile prior to initiated infection.
Highlights
The foodborne pathogen Listeria monocytogenes is a facultative Gram-positive intracellular bacterium that is able to adapt to a broad range of environments such as soil and waste water
Viability of each culture was determined at 10 and 20 min after bile treatment; two log CFU/ml reductions were observed in both wild type and sigBCHL mutant when compared to untreated controls (p < 0.05) (Figure 1)
Sixty-Six Significantly Differentially Expressed Genes Were Identified in Wild Type L. monocytogenes Exposed to Bile
Summary
The foodborne pathogen Listeria monocytogenes is a facultative Gram-positive intracellular bacterium that is able to adapt to a broad range of environments such as soil and waste water. It can survive and grow in a variety of temperature ranging from −1.5◦C to up to 45◦C, a wide range of pH and hypertonic conditions (Hudson et al, 1994; Membre et al, 2005; Goulet et al, 2013). While L. monocytogenes infections are rare, the mortality rate is 15.6% (EFSA, 2015) It ranks third among foodborne pathogens causing death in the United States (Scallan et al, 2011) and results in approximately 23,000 cases of listeriosis each year worldwide (de Noordhout et al, 2014)
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