Listeria monocytogenes is a human intracellular pathogen that is able to survive in the gastrointestinal environment and replicate in macrophages, thus bypassing the early innate immune defenses. Peptidoglycan (PG) is an essential component of the bacterial cell wall readily exposed to the host and, thus, an important target for the innate immune system. Characterization of the PG from L. monocytogenes demonstrated deacetylation of N -acetylglucosamine residues. We identified a PG N-deacetylase gene, pgdA , in L. monocytogenes genome sequence. Inactivation of pgdA revealed the key role of this PG modification in bacterial virulence because the mutant was extremely sensitive to the bacteriolytic activity of lysozyme, and growth was severely impaired after oral and i.v. inoculations. Within macrophage vacuoles, the mutant was rapidly destroyed and induced a massive IFN-β response in a TLR2 and Nod1-dependent manner. Together, these results reveal that PG N-deacetylation is a highly efficient mechanism used by Listeria to evade innate host defenses. The presence of deacetylase genes in other pathogenic bacteria indicates that PG N-deacetylation could be a general mechanism used by bacteria to evade the host innate immune system.

A virulence factor secreted by Listeria monocytogenes induces expression of interferon-stimulated genes

Listeria monocytogenes is a human intracellular pathogen able to colonize host tissues after ingestion of contaminated food, causing severe invasive infections. In order to gain a better understanding of the nature of host–pathogen interactions, we studied the L. monocytogenes genome expression during mouse infection. In the spleen of infected mice, ≈20% of the Listeria genome is differentially expressed, essentially through gene activation, as compared to exponential growth in rich broth medium. Data presented here show that, during infection, Listeria is in an active multiplication phase, as revealed by the high expression of genes involved in replication, cell division and multiplication. In vivo bacterial growth requires increased expression of genes involved in adaptation of the bacterial metabolism and stress responses, in particular to oxidative stress. Listeria interaction with its host induces cell wall metabolism and surface expression of virulence factors. During infection, L. monocytogenes also activates subversion mechanisms of host defenses, including resistance to cationic peptides, peptidoglycan modifications and release of muramyl peptides. We show that the in vivo differential expression of the Listeria genome is coordinated by a complex regulatory network, with a central role for the PrfA-SigB interplay. In particular, L. monocytogenes up regulates in vivo the two major virulence regulators, PrfA and VirR, and their downstream effectors. Mutagenesis of in vivo induced genes allowed the identification of novel L. monocytogenes virulence factors, including an LPXTG surface protein, suggesting a role for S-layer glycoproteins and for cadmium efflux system in Listeria virulence. The facultative intracellular bacterial pathogen Listeria monocytogenes is the etiological agent of a severe foodborne disease. In humans it causes a variety of manifestations ranging from asymptomatic intestinal carriage and gastroenteritis to invasive and disseminated severe diseases. Septicemia, meningoencephalitis, and infection of the foetus in pregnant women are the most serious clinical features of listeriosis. Virulence is a trait that only manifests in a susceptible host, involving a highly coordinated interaction between bacterial factors and host components. This article reports the use of in vivo genome expression profiling as a powerful approach to gain a detailed understanding of the Listeria responses and the molecular cross-talk taking place in infected mice. We showed that, during infection, L. monocytogenes shifts the expression of its entire genome to promote virulence, subverting host defenses and adapting to host conditions. This first analysis of L. monocytogenes gene expression in vivo significantly enhances our understanding of the means by which intracellular pathogens promote infection.

Listeria monocytogenes is an intracellular Gram-positive pathogen and the etiological agent of listeriosis, a human food-borne disease potentially fatal for certain risk groups. The virulence of L. monocytogenes is supported by a highly complex and coordinated intracellular life cycle that comprises several crucial steps: host cell adhesion and invasion, intracellular multiplication and motility, and intercellular spread. The completion of each stage is dependent on the orchestrated activity of specialized bacterial factors, in turn tightly controlled by a specific set of regulators. Some virulence factors and modulators also assume an important role in bacterial resistance and evasion to host defense mechanisms. In the last years, the advent of genomics promoted an increasingly prolific identification and functional characterization of new Listeria virulence factors. In this review, we summarize the current knowledge on nearly 50 molecules deployed by L. monocytogenes to promote its cell infection cycle.