The Listeria monocytogenes Bile Stimulon under Acidic Conditions Is Characterized by Strain-Specific Patterns and the Upregulation of Motility, Cell Wall Modification Functions, and the PrfA Regulon

Abstract

Listeria monocytogenes uses a variety of transcriptional regulation strategies to adapt to the extra-host environment, the gastrointestinal tract, and the intracellular host environment. While the alternative sigma factor SigB has been proposed to be a key transcriptional regulator that facilitates L. monocytogenes adaptation to the gastrointestinal environment, the L. monocytogenes' transcriptional response to bile exposure is not well-understood. RNA-seq characterization of the bile stimulon was performed in two L. monocytogenes strains representing lineages I and II. Exposure to bile at pH 5.5 elicited a large transcriptomic response with ~16 and 23% of genes showing differential transcription in 10403S and H7858, respectively. The bile stimulon includes genes involved in motility and cell wall modification mechanisms, as well as genes in the PrfA regulon, which likely facilitate survival during the gastrointestinal stages of infection that follow bile exposure. The fact that bile exposure induced the PrfA regulon, but did not induce further upregulation of the SigB regulon (beyond that expected by exposure to pH 5.5), suggests a model where at the earlier stages of gastrointestinal infection (e.g., acid exposure in the stomach), SigB-dependent gene expression plays an important role. Subsequent exposure to bile induces the PrfA regulon, potentially priming L. monocytogenes for subsequent intracellular infection stages. Some members of the bile stimulon showed lineage- or strain-specific distribution when 27 Listeria genomes were analyzed. Even though sigB null mutants showed increased sensitivity to bile, the SigB regulon was not found to be upregulated in response to bile beyond levels expected by exposure to pH 5.5. Comparison of wildtype and corresponding ΔsigB strains newly identified 26 SigB-dependent genes, all with upstream putative SigB-dependent promoters.

Listeria monocytogenes σA Is Sufficient to Survive Gallbladder Bile Exposure

“…For instance, dltC encoding D-alanyl carrier protein and LMRG_02074 encoding teichoic acid D-Ala incorporation-associated protein DltX were induced under bile exposure. This finding supports that of others in that dltABCD operon is induced under bile exposure in both L. monocytogenes 10403S (lineage II) and H7858 (lineage I) (Guariglia-Oropeza et al, 2018) and Lactobacillus rhamnosus (Koskenniemi et al, 2011). DltABCD proteins are involved in D-alanylation of teichoic acid, which facilitates resistance to antimicrobial peptides (Revilla-Guarinos et al, 2014).…”

“…This finding is consistent with previous microarray analysis showing that the entire PrfA regulon including plcA , hly , mpI , actA , plcA , inlA , and inlB was repressed in response to bile (Quillin et al, 2011). However, it conflicted with previous RNA-seq data in which some PrfA regulon genes were induced under bile exposure (Guariglia-Oropeza et al, 2018). This could be explained by the pH difference of the bile used in these studies.…”

“…Approximately 2% of the total transcriptome was differentially expressed genes in L. monocytogenes 10403S wild type under bile exposure compared to BHI control. This finding showed less differentially expressed genes compared to a previous study (Guariglia-Oropeza et al, 2018) that showed approximately 16% of differentially expressed genes in L. monocytogenes 10403S exposed to bile pH 5.5. It has been shown that the toxicity of bile is pH-dependent; the lower the pH, the higher the toxicity (Begley et al, 2005b; White et al, 2015).…”

“…Previously, it was determined that bile salt hydrolase as well as bile exclusion system are regulated by σ B (Dussurget et al, 2002; Begley et al, 2005b). However, the bile stimulon defined recently by RNA-seq analyses was not identified as σ B -dependent (Guariglia-Oropeza et al, 2018). This is surprising, however, they suggested that it is possibly due to comparing transcriptional profiles of L. monocytogenes strains exposed to pH 5.5 with and without bile.…”

CAN ACID ADAPTATION OF Listeria monocytogenes INCREASE SURVIVAL IN SUCUK (A TURKISH DRY-FERMENTED SAUSAGE)?

“…It is known that the resistance of L. monocytogenes to acidic pH depends on the composition of the growing environment, bacterial strain, the phase of bacterial growth and the acid type used (Phan-Thanh et al, 2000;Cataldo et al, 2007). Acid-tolerant pathogenic bacteria are of importance in terms of both food industry and human health (Leyer et al, 1995;Gahan et al, 1996;Cheng et al, 2003;Öztürk and Halkman, 2015;Omori et al, 2017;Guariglia-Oropeza et al, 2018).…”