Bacterial inhibition of Fas-mediated killing promotes neuroinvasion and persistence.

Infections of the central nervous system are among the most serious infections1,2, but the mechanisms by which pathogens access the brain remain poorly understood. The model microorganism Listeria monocytogenes (Lm) is a major foodborne pathogen that causes neurolisteriosis, one of the deadliest infections of the central nervous system3,4. Although immunosuppression is a well-established host risk factor for neurolisteriosis3,5, little is known about the bacterial factors that underlie the neuroinvasion of Lm. Here we develop a clinically relevant experimental model of neurolisteriosis, using hypervirulent neuroinvasive strains6 inoculated in a humanized mouse model of infection7, and we show that the bacterial surface protein InlB protects infected monocytes from Fas-mediated cell death by CD8+ T cells in a manner that depends on c-Met, PI3 kinase and FLIP. This blockade of specific anti-Lm cellular immune killing lengthens the lifespan of infected monocytes, and thereby favours the transfer of Lm from infected monocytes to the brain. The intracellular niche that is created by InlB-mediated cell-autonomous immune resistance also promotes Lm faecal shedding, which accounts for the selection of InlB as a core virulence gene of Lm. We have uncovered a specific mechanism by which a bacterial pathogen confers an increased lifespan to the cells it infects by rendering them resistant to cell-mediated immunity. This promotes the persistence of Lm within the host, its dissemination to the central nervous system and its transmission.

Microbiota-induced active translocation of peptidoglycan across the intestinal barrier dictates its within-host dissemination.

Peptidoglycan, the major structural polymer forming the cell wall of bacteria, is an important mediator of physiological and behavioral effects in mammalian hosts. These effects are frequently linked to its translocation from the intestinal lumen to host tissues. However, the modality and regulation of this translocation across the gut barrier has not been precisely addressed. In this study, we characterized the absorption of peptidoglycan across the intestine and its systemic dissemination. We report that peptidoglycan has a distinct tropism for host organs when absorbed via the gut, most notably by favoring access to the brain. We demonstrate that intestinal translocation of peptidoglycan occurs through a microbiota-induced active process. This process is regulated by the parasympathetic pathway via the muscarinic acetylcholine receptors. Together, this study reveals fundamental parameters concerning the uptake of a major microbiota molecular signal from the steady-state gut.

Liver-resident macrophage necroptosis orchestrates type 1 microbicidal inflammation and type-2-mediated tissue repair during bacterial infection.

Kupffer cells, the phagocytes of fetal origin that line the liver sinusoids, are key contributors of host defense against enteroinvasive bacteria. Here, we found that infection by Listeria monocytogenes induced the early necroptotic death of Kupffer cells, which was followed by monocyte recruitment and an anti-bacterial type 1 inflammatory response. Kupffer cell death also triggered a type 2 response that involved the hepatocyte-derived alarmin interleukin-33 (IL-33) and basophil-derived interleukin-4 (IL-4). This led to the alternative activation of the monocyte-derived macrophages recruited to the liver, which thereby replaced ablated Kupffer cells and restored liver homeostasis. Kupffer cell death is therefore a key signal orchestrating type 1 microbicidal inflammation and type-2-mediated liver repair upon infection. This indicates that beyond the classical dichotomy of type 1 and type 2 responses, these responses can develop sequentially in the context of a bacterial infection and act interdependently, orchestrating liver immune responses and return to homeostasis, respectively.

Outbreak of Listeriosis in South Africa Associated with Processed Meat.

BACKGROUND: An outbreak of listeriosis was identified in South Africa in 2017. The source was unknown. METHODS: We conducted epidemiologic, trace-back, and environmental investigations and used whole-genome sequencing to type Listeria monocytogenes isolates. A case was defined as laboratory-confirmed L. monocytogenes infection during the period from June 11, 2017, to April 7, 2018. RESULTS: A total of 937 cases were identified, of which 465 (50%) were associated with pregnancy; 406 of the pregnancy-associated cases (87%) occurred in neonates. Of the 937 cases, 229 (24%) occurred in patients 15 to 49 years of age (excluding those who were pregnant). Among the patients in whom human immunodeficiency virus (HIV) status was known, 38% of those with pregnancy-associated cases (77 of 204) and 46% of the remaining patients (97 of 211) were infected with HIV. Among 728 patients with a known outcome, 193 (27%) died. Clinical isolates from 609 patients were sequenced, and 567 (93%) were identified as sequence type 6 (ST6). In a case-control analysis, patients with ST6 infections were more likely to have eaten polony (a ready-to-eat processed meat) than those with non-ST6 infections (odds ratio, 8.55; 95% confidence interval, 1.66 to 43.35). Polony and environmental samples also yielded ST6 isolates, which, together with the isolates from the patients, belonged to the same core-genome multilocus sequence typing cluster with no more than 4 allelic differences; these findings showed that polony produced at a single facility was the outbreak source. A recall of ready-to-eat processed meat products from this facility was associated with a rapid decline in the incidence of L. monocytogenes ST6 infections. CONCLUSIONS: This investigation showed that in a middle-income country with a high prevalence of HIV infection, L. monocytogenes caused disproportionate illness among pregnant girls and women and HIV-infected persons. Whole-genome sequencing facilitated the detection of the outbreak and guided the trace-back investigations that led to the identification of the source.

Phage resistance at the cost of virulence: Listeria monocytogenes serovar 4b requires galactosylated teichoic acids for InlB-mediated invasion.

The intracellular pathogen Listeria monocytogenes is distinguished by its ability to invade and replicate within mammalian cells. Remarkably, of the 15 serovars within the genus, strains belonging to serovar 4b cause the majority of listeriosis clinical cases and outbreaks. The Listeria O-antigens are defined by subtle structural differences amongst the peptidoglycan-associated wall-teichoic acids (WTAs), and their specific glycosylation patterns. Here, we outline the genetic determinants required for WTA decoration in serovar 4b L. monocytogenes, and demonstrate the exact nature of the 4b-specific antigen. We show that challenge by bacteriophages selects for surviving clones that feature mutations in genes involved in teichoic acid glycosylation, leading to a loss of galactose from both wall teichoic acid and lipoteichoic acid molecules, and a switch from serovar 4b to 4d. Surprisingly, loss of this galactose decoration not only prevents phage adsorption, but leads to a complete loss of surface-associated Internalin B (InlB),the inability to form actin tails, and a virulence attenuation in vivo. We show that InlB specifically recognizes and attaches to galactosylated teichoic acid polymers, and is secreted upon loss of this modification, leading to a drastically reduced cellular invasiveness. Consequently, these phage-insensitive bacteria are unable to interact with cMet and gC1q-R host cell receptors, which normally trigger cellular uptake upon interaction with InlB. Collectively, we provide detailed mechanistic insight into the dual role of a surface antigen crucial for both phage adsorption and cellular invasiveness, demonstrating a trade-off between phage resistance and virulence in this opportunistic pathogen.

Atypical Hemolytic Listeria innocua Isolates Are Virulent, albeit Less than Listeria monocytogenes.

Listeria innocua is considered a nonpathogenic Listeria species. Natural atypical hemolytic L. innocua isolates have been reported but have not been characterized in detail. Here, we report the genomic and functional characterization of representative isolates from the two known natural hemolytic L. innocua clades. Whole-genome sequencing confirmed the presence of Listeria pathogenicity islands (LIPI) characteristic of Listeria monocytogenes species. Functional assays showed that LIPI-1 and inlA genes are transcribed, and the corresponding gene products are expressed and functional. Using in vitro and in vivo assays, we show that atypical hemolytic L. innocua is virulent, can actively cross the intestinal epithelium, and spreads systemically to the liver and spleen, albeit to a lesser degree than the reference L. monocytogenes EGDe strain. Although human exposure to hemolytic L. innocua is likely rare, these findings are important for food safety and public health. The presence of virulence traits in some L. innocua clades supports the existence of a common virulent ancestor of L. monocytogenes and L. innocua.

Spontaneous Loss of Virulence in Natural Populations of Listeria monocytogenes.

The pathogenesis of Listeria monocytogenes depends on the ability of this bacterium to escape from the phagosome of the host cells via the action of the pore-forming toxin listeriolysin O (LLO). Expression of the LLO-encoding gene (hly) requires the transcriptional activator PrfA, and both hly and prfA genes are essential for L. monocytogenes virulence. Here, we used the hemolytic activity of LLO as a phenotypic marker to screen for spontaneous virulence-attenuating mutations in L. monocytogenes Sixty nonhemolytic isolates were identified among a collection of 57,820 confirmed L. monocytogenes strains isolated from a variety of sources (0.1%). In most cases (56/60; 93.3%), the nonhemolytic phenotype resulted from nonsense, missense, or frameshift mutations in prfA Five strains carried hly mutations leading to a single amino acid substitution (G299V) or a premature stop codon causing strong virulence attenuation in mice. In one strain, both hly and gshF (encoding a glutathione synthase required for full PrfA activity) were missing due to genomic rearrangements likely caused by a transposable element. The PrfA/LLO loss-of-function (PrfA-/LLO-) mutants belonged to phylogenetically diverse clades of L. monocytogenes, and most were identified among nonclinical strains (57/60). Consistent with the rare occurrence of loss-of-virulence mutations, we show that prfA and hly are under purifying selection. Although occurring at a low frequency, PrfA-/LLO- mutational events in L. monocytogenes lead to niche restriction and open an evolutionary path for obligate saprophytism in this facultative intracellular pathogen.

Translocation and dissemination to target neurons of botulinum neurotoxin type B in the mouse intestinal wall.

Botulinum neurotoxins (BoNTs) are responsible for severe flaccid paralysis (botulism), which in most cases enter the organism via the digestive tract and then disseminate into the blood or lymph circulation to target autonomic and motor nerve endings. The passage way of BoNTs alone or in complex forms with associated nontoxic proteins through the epithelial barrier of the digestive tract still remains unclear. Here, we show using an in vivo model of mouse ligated intestinal loop that BoNT/B alone or the BoNT/B C-terminal domain of the heavy chain (HCcB), which interacts with cell surface receptors, translocates across the intestinal barrier. The BoNT/B or HCcB translocation through the intestinal barrier occurred via an endocytosis-dependent mechanism within 10-20 min, because Dynasore, a potent endocytosis inhibitor, significantly prevented BoNT/B as well as HCcB translocation. We also show that HCcB or BoNT/B specifically targets neuronal cells and neuronal extensions in the intestinal submucosa and musculosa expressing synaptotagmin, preferentially cholinergic neurons and to a lower extent other neuronal cell types, notably serotonergic neurons. Interestingly, rare intestinal epithelial cells accumulated HCcB suggesting that distinct cell types of the intestinal epithelium, still undefined, might mediate efficient translocation of BoNT/B.

Murinization of internalin extends its receptor repertoire, altering Listeria monocytogenes cell tropism and host responses.

Listeria monocytogenes (Lm) is an invasive foodborne pathogen that leads to severe central nervous system and maternal-fetal infections. Lm ability to actively cross the intestinal barrier is one of its key pathogenic properties. Lm crosses the intestinal epithelium upon the interaction of its surface protein internalin (InlA) with its host receptor E-cadherin (Ecad). InlA-Ecad interaction is species-specific, does not occur in wild-type mice, but does in transgenic mice expressing human Ecad and knock-in mice expressing humanized mouse Ecad. To study listeriosis in wild-type mice, InlA has been "murinized" to interact with mouse Ecad. Here, we demonstrate that, unexpectedly, murinized InlA (InlA(m)) mediates not only Ecad-dependent internalization, but also N-cadherin-dependent internalization. Consequently, InlA(m)-expressing Lm targets not only goblet cells expressing luminally-accessible Ecad, as does Lm in humanized mice, but also targets villous M cells, which express luminally-accessible N-cadherin. This aberrant Lm portal of entry results in enhanced innate immune responses and intestinal barrier damage, both of which are not observed in wild-type Lm-infected humanized mice. Murinization of InlA therefore not only extends the host range of Lm, but also broadens its receptor repertoire, providing Lm with artifactual pathogenic properties. These results challenge the relevance of using InlA(m)-expressing Lm to study human listeriosis and in vivo host responses to this human pathogen.

ActA promotes Listeria monocytogenes aggregation, intestinal colonization and carriage.

Listeria monocytogenes (Lm) is a ubiquitous bacterium able to survive and thrive within the environment and readily colonizes a wide range of substrates, often as a biofilm. It is also a facultative intracellular pathogen, which actively invades diverse hosts and induces listeriosis. So far, these two complementary facets of Lm biology have been studied independently. Here we demonstrate that the major Lm virulence determinant ActA, a PrfA-regulated gene product enabling actin polymerization and thereby promoting its intracellular motility and cell-to-cell spread, is critical for bacterial aggregation and biofilm formation. We show that ActA mediates Lm aggregation via direct ActA-ActA interactions and that the ActA C-terminal region, which is not involved in actin polymerization, is essential for aggregation in vitro. In mice permissive to orally-acquired listeriosis, ActA-mediated Lm aggregation is not observed in infected tissues but occurs in the gut lumen. Strikingly, ActA-dependent aggregating bacteria exhibit an increased ability to persist within the cecum and colon lumen of mice, and are shed in the feces three order of magnitude more efficiently and for twice as long than bacteria unable to aggregate. In conclusion, this study identifies a novel function for ActA and illustrates that in addition to contributing to its dissemination within the host, ActA plays a key role in Lm persistence within the host and in transmission from the host back to the environment.

Conjugated action of two species-specific invasion proteins for fetoplacental listeriosis.

The ability to cross host barriers is an essential virulence determinant of invasive microbial pathogens. Listeria monocytogenes is a model microorganism that crosses human intestinal and placental barriers, and causes severe maternofetal infections by an unknown mechanism. Several studies have helped to characterize the bacterial invasion proteins InlA and InlB. However, their respective species specificity has complicated investigations on their in vivo role. Here we describe two novel and complementary animal models for human listeriosis: the gerbil, a natural host for L. monocytogenes, and a knock-in mouse line ubiquitously expressing humanized E-cadherin. Using these two models, we uncover the essential and interdependent roles of InlA and InlB in fetoplacental listeriosis, and thereby decipher the molecular mechanism underlying the ability of a microbe to target and cross the placental barrier.

Protocol for live imaging of bacteria-cell interactions in genetically modified mouse small intestinal organoids.

Here, we present a protocol for microinjection of bacteria into mouse small intestinal organoids that recapitulates the natural route of infection of intestinal epithelial cells from the intestinal lumen. We describe steps for visualizing bacteria-cell interactions by live imaging of infected organoids using light sheet microscopy. We then detail procedures for generating doxycycline-inducible expression of mutant proteins in organoids to study essential gene functions. The different techniques described in this protocol can be used independently as required. For complete details on the use and execution of this protocol, please refer to Kim et al. (2021).1.

Fatal neonatal listeriosis following L. monocytogenes horizontal transmission highlights neonatal susceptibility to orally acquired listeriosis.

We report a case of fulminant fatal neonatal listeriosis due to horizontal transmission of Listeria monocytogenes (Lm) in a neonatal double room. Genomic analyses reveal a close genetic relationship between clinical isolates, supporting cross-contamination. Oral inoculation experiments in adult and neonatal mice show that neonates are susceptible to a low Lm inoculum and that this susceptibility results from the immaturity of the neonatal gut microbiota. Infected neonates should therefore be isolated for as long as they shed Lm in their feces to avoid horizontal transmission and its dire consequences.

Listeriosis, a model infection to study host-pathogen interactions in vivo.

Listeria monocytogenes (Lm) is a foodborne pathogen and the etiological agent of listeriosis. This facultative intracellular Gram-positive bacterium has the ability to colonize the intestinal lumen, cross the intestinal, blood-brain and placental barriers, leading to bacteremia, neurolisteriosis and maternal-fetal listeriosis. Lm is a model microorganism for the study of the interplay between a pathogenic microbe, host tissues and microbiota in vivo. Here we review how animal models permissive to Lm-host interactions allow deciphering some of the key steps of the infectious process, from the intestinal lumen to the crossing of host barriers and dissemination within the host. We also highlight recent investigations using tagged Lm and clinically relevant strains that have shed light on within-host dynamics and the purifying selection of Lm virulence factors. Studying Lm infection in vivo is a way forward to explore host biology and unveil the mechanisms that have selected its capacity to closely associate with its vertebrate hosts.

Listeria-Associated Lymphadenitis: A Series of 11 Consecutive Cases and Review of the Literature.

We studied 11 cases of culture-proven Listeria-associated lymphadenitis reported to the French National Reference Center for Listeria from 1994 to 2019 and 8 additional published cases. Listeria-associated lymphadenitis is rare, but it is associated with a mortality as high as for invasive listeriosis, and it is frequently diagnosed with concomitant neoplasia.

Trained ILC3 responses promote intestinal defense.

Group 3 innate lymphoid cells (ILC3s) are innate immune effectors that contribute to host defense. Whether ILC3 functions are stably modified after pathogen encounter is unknown. Here, we assess the impact of a time-restricted enterobacterial challenge to long-term ILC3 activation in mice. We found that intestinal ILC3s persist for months in an activated state after exposure to Citrobacter rodentium. Upon rechallenge, these "trained" ILC3s proliferate, display enhanced interleukin-22 (IL-22) responses, and have a superior capacity to control infection compared with naïve ILC3s. Metabolic changes occur in C. rodentium-exposed ILC3s, but only trained ILC3s have an enhanced proliferative capacity that contributes to increased IL-22 production. Accordingly, a limited encounter with a pathogen can promote durable phenotypic and functional changes in intestinal ILC3s that contribute to long-term mucosal defense.

Usp22 is an intracellular regulator of systemic emergency hematopoiesis.

Emergency hematopoiesis is a concerted response aimed toward enhanced protection from infection, involving multiple cell types and developmental stages across the immune system. Despite its importance, the underlying molecular regulation remains poorly understood. The deubiquitinase USP22 regulates the levels of monoubiquitinated histone H2B (H2Bub1), which is associated with activation of interferon responses upon viral infection. Here, we show that in the absence of infection or inflammation, mice lacking Usp22 in all hematopoietic cells display profound systemic emergency hematopoiesis, evident by increased hematopoietic stem cell proliferation, myeloid bias, and extramedullary hematopoiesis. Functionally, loss of Usp22 results in elevated phagocytosis by neutrophilic granulocytes and enhanced innate protection against Listeria monocytogenes infection. At the molecular level, we found this state of emergency hematopoiesis associated with transcriptional signatures of myeloid priming, enhanced mitochondrial respiration, and innate and adaptive immunity and inflammation. Augmented expression of many inflammatory genes was linked to elevated locus-specific H2Bub1 levels. Collectively, these results demonstrate the existence of a tunable epigenetic state that promotes systemic emergency hematopoiesis in a cell-autonomous manner to enhance innate protection, identifying potential paths toward immune enhancement.

Making Sense of the Biodiversity and Virulence of Listeria monocytogenes.

Listeria monocytogenes is a foodborne pathogen responsible for listeriosis, an infection that can manifest in humans as bacteremia, meningoencephalitis in immunocompromised patients and the elderly, and fetal-placental infection in pregnant women. Reference strains from this facultative intracellular bacterium have been instrumental in the investigation of basic mechanisms in microbiology, immunology, and cell biology. The integration of bacterial population genomics with environmental, epidemiological, and clinical data allowed the uncovering of new factors involved in the virulence of L. monocytogenes and its adaptation to different environments. This review illustrates how these investigations have led to a better understanding of the bacterium's virulence and the driving forces that shaped it.

Innate immune responses to Listeria in vivo.

Listeria monocytogenes (Lm) is a foodborne bacterial pathogen that causes listeriosis, a severe infection that manifests as bacteremia and meningo-encephalitis mostly in immunocompromised individuals, and maternal-fetal infection. A critical pathogenic determinant of Lm relies on its ability to actively cross the intestinal barrier, disseminate systemically and cross the blood-brain and placental barriers. Here we illustrate how Lm both evades innate immunity, favoring its dissemination in host tissues, and triggers innate immune defenses that participate to its control.

Live Imaging Reveals Listeria Hijacking of E-Cadherin Recycling as It Crosses the Intestinal Barrier.

Listeria monocytogenes is a foodborne bacterial pathogen that causes human listeriosis, a severe systemic infection.1 Its translocation across the intestinal epithelium is mediated by the interaction of internalin (InlA), a Listeria surface protein, with its host-species-specific receptor E-cadherin (Ecad).2-5 It occurs through goblet cells, on which Ecad is luminally accessible,6 via an unknown mechanism. In the absence of cell lines recapitulating this phenotype in vitro, we developed an ex vivo experimental system, based on the intraluminal microinjection of Listeria in untreated, pharmacologically treated, and genetically modified intestinal organoids. Using both live light-sheet microscopy and confocal imaging, we show that Listeria translocates through goblet cells within a membrane vacuole in an InlA- and microtubule-dependent manner. As Ecad undergoes constant apical-basal recycling,7,8 we hypothesized that Lm may transit through goblet cells by hijacking Ecad recycling pathway. Indeed, Listeria is stuck at goblet cell apex when Ecad endocytosis is blocked and remains trapped intracellularly at the basolateral pole of goblet cells when Rab11-dependent Ecad recycling is compromised. Together, these results show that Listeria, upon docking onto its luminally accessible receptor Ecad, hijacks its recycling pathway to be transferred by transcytosis across goblet cells. Live imaging of host-pathogen interactions in organoids is a promising approach to dissect their underlying cell and molecular biology.