Listeria monocytogenes is responsible for gastroenteritis in healthy individuals and for a severe invasive disease in immunocompromised patients. Among the three identified L. monocytogenes evolutionary lineages, lineage I strains are overrepresented in epidemic listeriosis outbreaks, but the mechanisms underlying the higher virulence potential of strains of this lineage remain elusive. Here, we demonstrate that Listeriolysin S (LLS), a virulence factor only present in a subset of lineage I strains, is a bacteriocin highly expressed in the intestine of orally infected mice that alters the host intestinal microbiota and promotes intestinal colonization by L. monocytogenes, as well as deeper organ infection. To our knowledge, these results therefore identify LLS as the first bacteriocin described in L. monocytogenes and associate modulation of host microbiota by L. monocytogenes epidemic strains to increased virulence.T he gram-positive bacterium Listeria monocytogenes is a facultative intracellular pathogen that causes foodborne infections in humans and animals. Upon consumption of contaminated food, L. monocytogenes reaches the intestinal lumen, crosses the intestinal barrier, and disseminates within the host. The clinical manifestations of listeriosis vary from a mild, self-limiting gastroenteritis to severe intestinal and systemic infections, with a fatality rate estimated to 20-30% of infected individuals (1). Host gut microbiota plays a critical role in resistance against colonization by invading pathogens within the intestine (2). The mechanisms of L. monocytogenes to compete with the host microbiota to survive in the intestine remain unknown.During the last decades, the majority of Listeria studies in bacterial pathophysiology, cell biology, and immunology compared three pathogenic strains from lineage II: EGD, EGD-e, and 10403S (3). Interestingly, major listeriosis epidemics have been preferentially associated to L. monocytogenes clonal groups belonging to the evolutionary lineage I and, more specifically, to serotype 4b (4, 5), but the molecular mechanisms that contribute to the higher virulence potential of these bacterial strains have not been identified yet.Bacteriocins are bacterially synthesized proteinaceous substances that target and inhibit the growth of closely related bacteria, allowing competition in diverse ecological niches, including the digestive tract (6, 7). Production of these antimicrobial peptides is widespread among bacterial species, and such production is made possible by biosynthetic machineries present in the genome, plasmids, or conjugative transposons (7). A conserved biosynthetic gene cluster for the production of bacteriocins displaying thiazole and oxazole heterocycles was discovered in 2008 in six microbial phyla (8). These gene clusters encode a toxin precursor and all indispensable proteins for toxin maturation in a mode similar to that associated with the bacteriocin microcin B17 (8). This gene cluster in L .monocytogenes was only present in a subset of lineage I strains...

ISG15 is an interferon-stimulated, linear di-ubiquitin-like protein, with anti-viral activity. The role of ISG15 during bacterial infection remains elusive. We show that ISG15 expression in nonphagocytic cells is dramatically induced upon Listeria infection. Surprisingly this induction can be type I interferon independent and depends on the cytosolic surveillance pathway, which senses bacterial DNA and signals through STING, TBK1, IRF3 and IRF7. Most importantly, we observed that ISG15 expression restricts Listeria infection in vitro and in vivo. We made use of stable isotope labeling in tissue culture (SILAC) to identify ISGylated proteins that could be responsible for the protective effect. Strikingly, infection or overexpression of ISG15 leads to ISGylation of ER and Golgi proteins, which correlates with increased secretion of cytokines known to counteract infection. Together, our data reveal a previously uncharacterized ISG15-dependent restriction of Listeria infection, reinforcing the view that ISG15 is a key component of the innate immune response.DOI: http://dx.doi.org/10.7554/eLife.06848.001

Listeria monocytogenes is a saprophytic gram-positive bacterium, and an opportunistic foodborne pathogen that can produce listeriosis in humans and animals. It has evolved an exceptional ability to adapt to stress conditions encountered in different environments, resulting in a ubiquitous distribution. Because some food preservation methods and disinfection protocols in food-processing environments cannot efficiently prevent contaminations, L. monocytogenes constitutes a threat to human health and a challenge to food safety. In the host, Listeria colonizes the gastrointestinal tract, crosses the intestinal barrier, and disseminates through the blood to target organs. In immunocompromised individuals, the elderly, and pregnant women, the pathogen can cross the blood-brain and placental barriers, leading to neurolisteriosis and materno-fetal listeriosis. Molecular and cell biology studies of infection have proven L. monocytogenes to be a versatile pathogen that deploys unique strategies to invade different cell types, survive and move inside the eukaryotic host cell, and spread from cell to cell. Here, we present the multifaceted Listeria life cycle from a comprehensive perspective. We discuss genetic features of pathogenic Listeria species, analyze factors involved in food contamination, and review bacterial strategies to tolerate stresses encountered both during food processing and along the host’s gastrointestinal tract. Then we dissect host–pathogen interactions underlying listerial pathogenesis in mammals from a cell biology and systemic point of view. Finally, we summarize the epidemiology, pathophysiology, and clinical features of listeriosis in humans and animals. This work aims to gather information from different fields crucial for a comprehensive understanding of the pathogenesis of L. monocytogenes.

Streptolysin S (SLS)-like virulence factors from clinically relevant Gram-positive pathogens have been proposed to behave as potent cytotoxins, playing key roles in tissue infection. Listeriolysin S (LLS) is an SLS-like hemolysin/bacteriocin present among Listeria monocytogenes strains responsible for human listeriosis outbreaks. As LLS cytotoxic activity has been associated with virulence, we investigated the LLS-specific contribution to host tissue infection. Surprisingly, we first show that LLS causes only weak red blood cell (RBC) hemolysis in vitro and neither confers resistance to phagocytic killing nor favors survival of L. monocytogenes within the blood cells or in the extracellular space (in the plasma). We reveal that LLS does not elicit specific immune responses, is not cytotoxic for eukaryotic cells, and does not impact cell infection by L. monocytogenes. Using in vitro cell infection systems and a murine intravenous infection model, we actually demonstrate that LLS expression is undetectable during infection of cells and murine inner organs. Importantly, upon intravenous animal inoculation, L. monocytogenes is found in the gastrointestinal system, and only in this environment LLS expression is detected in vivo. Finally, we confirm that LLS production is associated with destruction of target bacteria. Our results demonstrate therefore that LLS does not contribute to L. monocytogenes tissue injury and virulence in inner host organs as previously reported. Moreover, we describe that LlsB, a putative posttranslational modification enzyme encoded in the LLS operon, is necessary for murine inner organ colonization. Overall, we demonstrate that LLS is the first SLS-like virulence factor targeting exclusively prokaryotic cells during in vivo infections.

In the context of a study on the occurrence of Listeria species in an animal farm environment in Valencia, Spain, six Listeria -like isolates could not be assigned to any known species. Phylogenetic analysis based on the 16S rRNA gene and on 231 Listeria core genes grouped these isolates in a monophyletic clade within the genus Listeria , with highest similarity to Listeria thailandensis . Whole-genome sequence analyses based on in silico DNA–DNA hybridization, the average nucleotide blast and the pairwise amino acid identities against all currently known Listeria species confirmed that these isolates constituted a new taxon within the genus Listeria . Phenotypically, these isolates differed from other Listeria species mainly by the production of acid from inositol, the absence of acidification in presence of methyl α-d-glucoside, and the absence of α-mannosidase and nitrate reductase activities. The name Listeria valentina sp. nov. is proposed for this novel species, and the type strain is CLIP 2019/00642T (=CIP 111799T=DSM 110544T).

Listeria monocytogenes is a Gram-positive food-borne pathogen that in humans may traverse the intestinal, placental and blood/brain barriers, causing gastroenteritis, abortions and meningitis. Crossing of these barriers is dependent on the bacterial ability to enter host cells, and several L. monocytogenes surface and secreted virulence factors are known to facilitate entry and the intracellular lifecycle. The study of L. monocytogenes strains associated to human listeriosis epidemics has revealed the presence of novel virulence factors. One such factor is Listeriolysin S, a thiazole/oxazole modified microcin that displays bactericidal activity and modifies the host microbiota during infection. Our recent results therefore highlight the interaction of L. monocytogenes with gut microbes as a crucial step in epidemic listeriosis. In this article, we will discuss novel implications for this family of toxins in the pathogenesis of diverse medically relevant microorganisms.

The vaginal microbiota plays an important role in the health of dairy cattle, and it could be manipulated for the prevention and treatment of reproduction-related infections. The present study profiles and compares the vaginal microbiota of healthy dairy heifers during the estrous cycle focusing the results in follicular (estrus) and luteal (diestrus) phases using 16S rRNA sequencing of the V3-V4 hypervariable region. Twenty 13-16-months-old virgin dairy heifers from a single farm were included in this study. Vaginal swabs and blood samples were obtained during estrus (6-8 h before artificial insemination) and diestrus (14 days after insemination). Estrus was evaluated by an activity monitoring system and confirmed with plasma progesterone immunoassay. Results showed that the taxonomic composition of the vaginal microbiota was different during the follicular and luteal phases. At the phylum level, the most abundant bacterial phyla were Tenericutes, Firmicutes, and Bacteroidetes which comprised more than 75% of the vaginal microbiota composition. The next more abundant phyla, in order of decreasing abundance, were Proteobacteria, Actinobacteria, Fusobacteria, Epsilonbacteraeota, and Patescibacteria. Together with Tenericutes, Firmicutes, and Bacteroidetes represented more than 96% of the bacterial composition. Ureaplasma, Histophilus, f_Corynebacteriaceae, Porphyromonas, Mycoplasma, Ruminococcaceae UCG-005, were the most abundant genera or families. The results also showed that the vaginal microbiota of dairy heifers was non-lactobacillus dominant. The genus Lactobacillus was always found at a low relative abundance during the estrous cycle being more abundant in the follicular than in the luteal phase. Despite more research is needed to explore the potential use of native vaginal microbiota members as probiotics in dairy heifers, this study represents an important step forward. Understanding how the microbiota behaves in healthy heifers will help to identify vaginal dysbiosis related to disease.

Noncoding RNAs (ncRNAs) regulating virulence have been identified in most pathogens. This review discusses RNA-mediated mechanisms exploited by bacterial pathogens to successfully infect and colonize their hosts. It discusses the most representative RNA-mediated regulatory mechanisms employed by two intracellular [Listeria monocytogenes and Salmonella enterica serovar Typhimurium (S. Typhimurium)] and two extracellular (Vibrio cholerae and Staphylococcus aureus) bacterial pathogens. We review the RNA-mediated regulators (e.g., thermosensors, riboswitches, cis- and trans-encoded RNAs) used for adaptation to the specific niches colonized by these bacteria (intestine, blood, or the intracellular environment, for example) in the framework of the specific pathophysiological aspects of the diseases caused by these microorganisms. A critical discussion of the newest findings in the field of bacterial ncRNAs shows how examples in model pathogens could pave the way for the discovery of new mechanisms in other medically important bacterial pathogens.