Chitin Attenuates Expression of Listeria monocytogenes Virulence Genes in vitro.

Miguel Villoria Recio,Eva Maria Sternkopf Lillebæk,Hanne Ingmer,Bo-Hyung Lee,Marianne Halberg Larsen,Birgitte H Kallipolitis,Cormac G M Gahan

doi:10.3389/fmicb.2020.588906

Abstract

External signals are crucial for bacteria to sense their immediate environment and fine-tune gene expression accordingly. The foodborne pathogen Listeria monocytogenes senses a range of environmental cues in order to activate or deactivate the virulence-inducing transcriptional factor PrfA during transition between infectious and saprophytic lifecycles. Chitin is an abundant biopolymer formed from linked β-(1–4)-N-acetyl-D-glucosamine residues associated with fungi, the exoskeleton of insects and often incorporated into foods as a thickener or stabilizer. L. monocytogenes evolved to hydrolyse chitin, presumably, to facilitate nutrient acquisition from competitive environments such as soil where the polymer is abundant. Since mammals do not produce chitin, we reasoned that the polymer could serve as an environmental signal contributing to repression of L. monocytogenes PrfA-dependent expression. This study shows a significant downregulation of the core PrfA-regulon during virulence-inducing conditions in vitro in the presence of chitin. Our data suggest this phenomenon occurs through a mechanism that differs from PTS-transport of oligosaccharides generated from either degradation or chitinase-mediated hydrolysis of the polymer. Importantly, an indication that chitin can repress virulence expression of a constitutively active PrfA∗ mutant is shown, possibly mediated via a post-translational modification inhibiting PrfA∗ activity. To our knowledge, this is the first time that chitin is reported as a molecule with anti-virulence properties against a pathogenic bacterium. Thus, our findings identify chitin as a signal which may downregulate the virulence potential of the pathogen and may provide an alternative approach toward reducing disease risk.

Highlights

Listeria monocytogenes is a ubiquitous foodborne pathogen and the causative agent of listeriosis, a serious disease that can be life-threatening in both humans and animals (Schlech and Acheson, 2000; Vázquez-Boland et al, 2001; de las Heras et al, 2011)
The transcriptional profile of L. monocytogenes grown under virulence inducing conditions was compared with/without chitin (CDMgly +chitin/CDMgly) using RNA Sequencing (RNA-Seq) to assess the effect of chitin on virulence expression
When deferentially expressed genes (DEGs) were classified by clusters of orthologs groups (COGs), upregulated genes mainly involved uncategorized functions and transport and metabolism, including carbohydrate, nucleotide, inorganic ion and lipid, and cell motility (Figure 1B and Supplementary Table S2)

Summary

Introduction

Listeria monocytogenes is a ubiquitous foodborne pathogen and the causative agent of listeriosis, a serious disease that can be life-threatening in both humans and animals (Schlech and Acheson, 2000; Vázquez-Boland et al, 2001; de las Heras et al, 2011). Further regulation involves the general stress response regulator σB, direct binding of the nutrient-response regulator CodY to the prfA sequence in low branched-chained amino-acids availability, the environmental concentration of L-glutamine and a positive feedback loop dependent on cytosolic amounts of PrfA (Mengaud et al, 1991; Johansson et al, 2002; Scortti et al, 2007; de las Heras et al, 2011; Lobel et al, 2012; Haber et al, 2017; Lebreton and Cossart, 2017). PrfA facilitates transformation from the saprophyte to pathogen by integrating environmental cues that help finetune virulence gene expression (Freitag et al, 2009; Toledo-Arana et al, 2009)

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Conclusion