Listeria monocytogenes invades the epithelial junctions at sites of cell extrusion

Protocol for characterizing cell motility and extracellular signal-regulated kinase dynamics in epithelial monolayers using FRET imaging data

Extracellular signal-regulated kinase (ERK) activity waves regulate critical processes like wound healing and bacterial pathogen dissemination by altering host cell motility and biomechanics. Here, we present a protocol for fluorescence resonance energy transfer (FRET) imaging of ERK activity in the nuclei of epithelial cells in a monolayer using an ERK biosensor. Moreover, we outline all image processing steps for the spatiotemporal quantification of single-cell ERK oscillations and wave propagation. Modifications, especially with respect to FRET imaging, may be necessary if different ERK biosensors are used. For complete details on the use and execution of this protocol, please refer to Hundsdorfer et al.1.

Sphingosine-1-phosphate signaling mediates shedding of measles virus-infected respiratory epithelial cells

Measles virus (MeV) is an extremely infectious respiratory virus and a major cause of childhood morbidity and mortality worldwide. MeV infection of the respiratory epithelium induces shedding of multinucleate epithelial cells from the apical surface of the epithelium without compromising epithelial barrier integrity. To study the mechanisms driving the apical extrusion of MeV-infected respiratory epithelial cells, we used primary differentiated tracheal epithelial cell cultures (rhTECs) and respiratory samples from rhesus macaques infected with wild-type MeV (WT MeV) or live-attenuated MeV (LAMV). We show that sphingosine-1-phosphate (S1P) signaling, rather than cell death or inflammasome activation, plays a key role in WT MeV and LAMV-induced cell shedding. Inhibiting S1P signaling resulted in delayed shedding of clusters of infected cells and higher viral titers within the epithelium, suggesting that cell extrusion impacts viral dynamics within the respiratory tract. We also found that shedding of individual infected cells began early after apical infection, prior to the formation of infected cell clusters within the epithelium. These findings offer new insights into MeV biology and pathogenesis within the respiratory tract.

IMPORTANCE

Despite the availability of a safe and effective vaccine, measles virus (MeV) still has a significant global impact, and in 2022 alone led to over 136,000 deaths. MeV is one of the most contagious known viruses and spreads via the respiratory route. When respiratory epithelial cells are infected, they are shed into the lumen of the respiratory tract, but this process is poorly understood. Here, we use primary differentiated respiratory epithelial cells from rhesus macaques to show that sphingosine-1-phosphate (S1P) signaling, and not cell death or inflammasome activation, plays a role in cell shedding during both wild-type and live-attenuated MeV infection. Through this mechanism, MeV-infected cells are extruded without disrupting the integrity of the respiratory epithelium. Inhibiting S1P signaling resulted in delayed shedding of infected cells and higher viral titers in the epithelium. These findings indicate that host cellular responses play an important role in MeV infectivity.

Clostridioides difficile major toxins remodel the intestinal epithelia, affecting spore adherence/internalization into intestinal tissue and their association with gut vitronectin

The most common cause of healthcare-associated diarrhea and colitis in the U.S., is Clostridioides difficile, a spore-forming pathogen. Two toxins, TcdA and TcdB, are major virulence factors essential for disease manifestations, while C. difficile spores are essential for disease transmission and recurrence. Both toxins cause major damage to the epithelial barrier, trigger massive inflammation, and reshape the microbiome and metabolic composition, facilitating C. difficile colonization. C. difficile spores, essential for transmission and recurrence of the disease, persist adhered and internalized in the intestinal epithelia. Studies have suggested that toxin-neutralization in combination with antibiotic during CDI treatment in humans significantly reduces disease recurrence, suggesting a link between toxin-mediated damage and spore persistence. Here, we show that TcdA/TcdB-intoxication of intestinal epithelial Caco-2 cells leads to remodeling of accessible levels of fibronectin (Fn) and vitronectin (Vn) and their cognate alpha-integrin subunits. While TcdB-intoxication of intestinal tissue had no impact in accessible levels of Fn and Vn, but significantly increased levels of intracellular Vn. We observed that Fn and Vn released to the supernatant readily bind to C. difficile spores in vitro, while TcdB-intoxication of intestinal tissue led to increased association of C. difficile spores with gut Vn. Toxin-intoxication of the intestinal tissue also contributes to increased adherence and internalization of C. difficile spores. However, TcdB-intoxicated ligated loops infected of mice treated with Bezlotoxumanb (monoclonal anti-TcdB antibodies) did not prevent TcdB-mediated increased spore adherence and internalization into intestinal tissue. This study highlights the importance of studying the impact of C. difficile toxins of host tissues has in C. difficile interaction with host surfaces that may contribute to increased persistence and disease recurrence.

Lysteria Monocytogenes Infection during Monochorionic Twin Pregnancy: Case Report and Review of the Literature

Listeriosis is a rare but severe foodborne disease caused by Listeria Monocytogenes (LM), a small facultative intracellular bacillus. When occurring in pregnant women, it can be vertically transmitted to the fetus and the newborn. Infected women usually display aspecific and mild symptoms, and rarely develop the severe forms of the disease (such as neurolisteriosis). On the contrary, fetal and neonatal listeriosis can lead to complications such as fetal loss, preterm birth, neonatal sepsis, and respiratory distress syndrome (RDS). Prompt diagnosis is one of the main challenges because of the aspecific presentation of the disease; therapy relies on antibiotics that reach high intracellular concentration and can penetrate and pass the placenta reaching the fetus. Herein we report an infrequent case of LM infection involving a woman with monochorionic diamniotic twin pregnancy, followed by a comprehensive review of the available literature on listeriosis in pregnancy.

Current methodologies available to evaluate the virulence potential among Listeria monocytogenes clonal complexes

Listeria monocytogenes is a foodborne pathogen that causes listeriosis in humans, the severity of which depends on multiple factors, including intrinsic characteristics of the affected individuals and the pathogen itself. Additionally, emerging evidence suggests that epigenetic modifications may also modulate host susceptibility to infection. Therefore, different clinical outcomes can be expected, ranging from self-limiting gastroenteritis to severe central nervous system and maternal-neonatal infections, and bacteremia. Furthermore, L. monocytogenes is a genetically and phenotypically diverse species, resulting in a large variation in virulence potential between strains. Multilocus sequence typing (MLST) has been widely used to categorize the clonal structure of bacterial species and to define clonal complexes (CCs) of genetically related isolates. The combination of MLST and epidemiological data allows to distinguish hypervirulent CCs, which are notably more prevalent in clinical cases and typically associated with severe forms of the disease. Conversely, other CCs, termed hypovirulent, are predominantly isolated from food and food processing environments and are associated with the occurrence of listeriosis in immunosuppressed individuals. Reports of genetic traits associated with this diversity have been described. The Food and Agriculture Organization (FAO) is encouraging the search for virulence biomarkers to rapidly identify the main strains of concern to reduce food waste and economical losses. The aim of this review is to comprehensively collect, describe and discuss the methodologies used to discriminate the virulence potential of L. monocytogenes CCs. From the exploration of in vitro and in vivo models to the study of expression of virulence genes, each approach is critically explored to better understand its applicability and efficiency in distinguishing the virulence potential of the pathogen.

Bacterial aggregation facilitates internalin-mediated invasion of Listeria monocytogenes

Dissemination of food-borne L. monocytogenes in the host relies on internalin-mediated invasion, but the underlying invasion strategies remain elusive. Here we use live-cell microscopy to follow single cell interactions between individual human cells and L. monocytogenes and elucidate mechanisms associated with internalin B (InlB)-mediated invasion. We demonstrate that whilst a replicative invasion of nonphagocytic cells is a rare event even at high multiplicities of invasion, L. monocytogenes overcomes this by utilising a strategy relaying on PrfA-mediated ActA-based aggregation. We show that L. monocytogenes forms aggregates in extracellular host cell environment, which promote approximately 5-fold more host cell adhesions than the non-aggregating actA-ΔC mutant (which lacks the C-terminus coding region), with the adhering bacteria inducing 3-fold more intracellular invasions. Aggregation is associated with robust MET tyrosine kinase receptor clustering in the host cells, a hallmark of InlB-mediated invasion, something not observed with the actA-ΔC mutant. Finally, we show via RNA-seq analyses that aggregation involves a global adaptive response to host cell environment (including iron depletion), resulting in metabolic changes in L. monocytogenes and upregulation of the PrfA virulence regulon. Overall, our analyses provide new mechanistic insights into internalin-mediated host-pathogen interactions of L. monocytogenes.

Listeria adhesion protein orchestrates caveolae-mediated apical junctional remodeling of epithelial barrier for Listeria monocytogenes translocation

The cellular junctional architecture remodeling by Listeria adhesion protein-heat shock protein 60 (LAP-Hsp60) interaction for Listeria monocytogenes (Lm) passage through the epithelial barrier is incompletely understood. Here, using the gerbil model, permissive to internalin (Inl) A/B-mediated pathways like in humans, we demonstrate that Lm crosses the intestinal villi at 48 h post-infection. In contrast, the single isogenic (lap- or ΔinlA) or double (lap-ΔinlA) mutant strains show significant defects. LAP promotes Lm translocation via endocytosis of cell-cell junctional complex in enterocytes that do not display luminal E-cadherin. In comparison, InlA facilitates Lm translocation at cells displaying apical E-cadherin during cell extrusion and mucus expulsion from goblet cells. LAP hijacks caveolar endocytosis to traffic integral junctional proteins to the early and recycling endosomes. Pharmacological inhibition in a cell line and genetic knockout of caveolin-1 in mice prevents LAP-induced intestinal permeability, junctional endocytosis, and Lm translocation. Furthermore, LAP-Hsp60-dependent tight junction remodeling is also necessary for InlA access to E-cadherin for Lm intestinal barrier crossing in InlA-permissive hosts.

IMPORTANCE

Listeria monocytogenes (Lm) is a foodborne pathogen with high mortality (20%-30%) and hospitalization rates (94%), particularly affecting vulnerable groups such as pregnant women, fetuses, newborns, seniors, and immunocompromised individuals. Invasive listeriosis involves Lm's internalin (InlA) protein binding to E-cadherin to breach the intestinal barrier. However, non-functional InlA variants have been identified in Lm isolates, suggesting InlA-independent pathways for translocation. Our study reveals that Listeria adhesion protein (LAP) and InlA cooperatively assist Lm entry into the gut lamina propria in a gerbil model, mimicking human listeriosis in early infection stages. LAP triggers caveolin-1-mediated endocytosis of critical junctional proteins, transporting them to early and recycling endosomes, facilitating Lm passage through enterocytes. Furthermore, LAP-Hsp60-mediated junctional protein endocytosis precedes InlA's interaction with basolateral E-cadherin, emphasizing LAP and InlA's cooperation in enhancing Lm intestinal translocation. This understanding is vital in combating the severe consequences of Lm infection, including sepsis, meningitis, encephalitis, and brain abscess.

Impact of vitamin B12 on rhamnose metabolism, stress defense and in-vitro virulence of Listeria monocytogenes

Listeria monocytogenes is a facultative anaerobe which can cause a severe food-borne infection known as listeriosis. L. monocytogenes is capable of utilizing various nutrient sources including rhamnose, a naturally occurring deoxy sugar abundant in foods. L. monocytogenes can degrade rhamnose into lactate, acetate and 1,2-propanediol. Our previous study showed that addition of vitamin B12 stimulated anaerobic growth of L. monocytogenes on rhamnose due to the activation of bacterial microcompartments for 1,2-propanediol utilization (pdu BMC) with concomitant production of propionate and propanol. Notably, anaerobic 1,2-propanediol metabolism has been linked to virulence of enteric pathogens including Salmonella spp. and L. monocytogenes. In this study we investigated the impact of B12 and BMC activation on i) aerobic and anerobic growth of L. monocytogenes on rhamnose and ii) the level of virulence. We observed B12-induced pdu BMC activation and growth stimulation only in anaerobically grown cells. Comparative Caco-2 virulence assays showed that these pdu BMC-induced cells have significantly higher translocation efficiency compared to non-induced cells (anaerobic growth without B12; aerobic growth with or without B12), while adhesion and invasion capacity is similar for all cells. Comparative proteome analysis showed specific and overlapping responses linked to metabolic shifts, activation of stress defense proteins and virulence factors, with RNA polymerase sigma factor SigL, teichoic acid export ATP-binding protein TagH, DNA repair and protection proteins, RadA and DPS, and glutathione synthase GshAB, previously linked to activation of virulence response in L. monocytogenes, uniquely upregulated in anaerobically rhamnose grown pdu-induced cells. Our results shed light on possible effects of B12 on L. monocytogenes competitive fitness and virulence activation when utilizing rhamnose in anaerobic conditions encountered during transmission and the human intestine.

Adenylate cyclase toxin of Bordetella parapertussis disrupts the epithelial barrier granting the bacterial access to the intracellular space of epithelial cells

B. parapertussis is one of the etiological agents of whooping cough. Once inhaled, the bacteria bind to the respiratory epithelium and start the infection. Little is known about this first step of host colonization and the role of the human airway epithelial barrier on B. parapertussis infection. We here investigated the outcome of the interaction of B. parapertussis with a polarized monolayer of respiratory epithelial cells. Our results show that B. parapertussis preferentially attaches to the intercellular boundaries, and causes the disruption of the tight junction integrity through the action of adenylate cyclase toxin (CyaA). We further found evidence indicating that this disruption enables the bacterial access to components of the basolateral membrane of epithelial cells to which B. parapertussis efficiently attaches and gains access to the intracellular location, where it can survive and eventually spread back into the extracellular environment. Altogether, these results suggest that the adenylate cyclase toxin enables B. parapertussis to overcome the epithelial barrier and eventually establish a niche of persistence within the respiratory epithelial cells.

Redistribution of the Novel Clostridioides difficile Spore Adherence Receptor E-Cadherin by TcdA and TcdB Increases Spore Binding to Adherens Junctions

Clostridioides difficile causes antibiotic-associated diseases in humans, ranging from mild diarrhea to severe pseudomembranous colitis and death. A major clinical challenge is the prevention of disease recurrence, which affects nearly ~20 to 30% of the patients with a primary C. difficile infection (CDI). During CDI, C. difficile forms metabolically dormant spores that are essential for recurrence of CDI (R-CDI). In prior studies, we have shown that C. difficile spores interact with intestinal epithelial cells (IECs), which contribute to R-CDI. However, this interaction remains poorly understood. Here, we provide evidence that C. difficile spores interact with E-cadherin, contributing to spore adherence and internalization into IECs. C. difficile toxins TcdA and TcdB lead to adherens junctions opening and increase spore adherence to IECs. Confocal micrographs demonstrate that C. difficile spores associate with accessible E-cadherin; spore-E-cadherin association increases upon TcdA and TcdB intoxication. The presence of anti-E-cadherin antibodies decreased spore adherence and entry into IECs. By enzyme-linked immunosorbent assay (ELISA), immunofluorescence, and immunogold labeling, we observed that E-cadherin binds to C. difficile spores, specifically to the hairlike projections of the spore, reducing spore adherence to IECs. Overall, these results expand our knowledge of how C. difficile spores bind to IECs by providing evidence that E-cadherin acts as a spore adherence receptor to IECs and by revealing how toxin-mediated damage affects spore interactions with IECs.

In through the Out Exit: the Role of the Exocyst in Listeria monocytogenes Cell Entry

The intracellular pathogen Listeria monocytogenes is one of the leading causes of death from foodborne illness in the United States. Internalin A is the key surface protein that drives Listeria uptake by epithelial cells expressing E-cadherin. G. C. Gyanwali, T. U. B. Herath, A. Gianfelice, and K. Ireton (Infect Immun 90:e00326-22, 2022, https://doi.org/10.1128/iai.00326-22) unravel the close relationship between internalin A and the exocyst, adding another layer of complexity to the bacterial internalization process.

Maternal sepsis caused by Listeria monocytogenes with a fatal fetal outcome

Improving maternal and child health is a global priority. Although infection with Listeria monocytogenes (LM), a small facultative anaerobic, gram-positive motile bacillus is rare, when it infects the maternal-fetoplacental unit, it can result in adverse fetal sequelae such as chorioamnionitis, preterm labour, neonatal sepsis, meningitis and neonatal death. Pregnancy-associated listeriosis may present with a plethora of diverse, non-specific symptoms such as fever, influenza-like or gastrointestinal symptoms, premature contractions and preterm labour. It has a predilection for the second and third trimester of pregnancy, occurring sporadically or as part of an outbreak, most of which have involved unpasteurised dairy products, long shelf life products, contaminated ready-to-eat food, deli meats and soft cheeses. Strains belonging to the clonal complexes 1, 4 and 6 are hypervigilant and are commonly associated with maternal-neonatal infections. Maternal listeriosis occurs as a direct consequence of LM-specific placental tropism, which is mediated by the conjugated action of internalin A and internalin B at the placental barrier. The diagnosis is established from placental culture. Penicillin, ampicillin and amoxicillin are the antimicrobials of choice. It has a high fetal morbidity of up to 30%. The authors present the case of a multiparous woman in her early 20s presenting with sepsis and preterm premature rupture of her membranes at 21 weeks gestation. A live baby was delivered spontaneously and died shortly after birth. Placental cultures and postmortem examination were consistent with the diagnosis of disseminated Listeria infection. Due to the increased susceptibility of pregnant women for LM, a high index of clinical suspicion is required to establish the diagnosis and initiate appropriate antimicrobial therapy to reduce adverse fetal outcomes.

Listeria monocytogenes Pathogenesis: The Role of Stress Adaptation

Adaptive stress tolerance responses are the driving force behind the survival ability of Listeria monocytogenes in different environmental niches, within foods, and ultimately, the ability to cause human infections. Although the bacterial stress adaptive responses are primarily a necessity for survival in foods and the environment, some aspects of the stress responses are linked to bacterial pathogenesis. Food stress-induced adaptive tolerance responses to acid and osmotic stresses can protect the pathogen against similar stresses in the gastrointestinal tract (GIT) and, thus, directly aid its virulence potential. Moreover, once in the GIT, the reprogramming of gene expression from the stress survival-related genes to virulence-related genes allows L. monocytogenes to switch from an avirulent to a virulent state. This transition is controlled by two overlapping and interlinked transcriptional networks for general stress response (regulated by Sigma factor B, (SigB)) and virulence (regulated by the positive regulatory factor A (PrfA)). This review explores the current knowledge on the molecular basis of the connection between stress tolerance responses and the pathogenesis of L. monocytogenes. The review gives a detailed background on the currently known mechanisms of pathogenesis and stress adaptation. Furthermore, the paper looks at the current literature and theories on the overlaps and connections between the regulatory networks for SigB and PrfA.

Mechanical Forces Govern Interactions of Host Cells with Intracellular Bacterial Pathogens

To combat infectious diseases, it is important to understand how host cells interact with bacterial pathogens. Signals conveyed from pathogen to host, and vice versa, may be either chemical or mechanical. While the molecular and biochemical basis of host-pathogen interactions has been extensively explored, relatively less is known about mechanical signals and responses in the context of those interactions. Nevertheless, a wide variety of bacterial pathogens appear to have developed mechanisms to alter the cellular biomechanics of their hosts in order to promote their survival and dissemination, and in turn many host responses to infection rely on mechanical alterations in host cells and tissues to limit the spread of infection. In this review, we present recent findings on how mechanical forces generated by host cells can promote or obstruct the dissemination of intracellular bacterial pathogens. In addition, we discuss how in vivo extracellular mechanical signals influence interactions between host cells and intracellular bacterial pathogens. Examples of such signals include shear stresses caused by fluid flow over the surface of cells and variable stiffness of the extracellular matrix on which cells are anchored. We highlight bioengineering-inspired tools and techniques that can be used to measure host cell mechanics during infection. These allow for the interrogation of how mechanical signals can modulate infection alongside biochemical signals. We hope that this review will inspire the microbiology community to embrace those tools in future studies so that host cell biomechanics can be more readily explored in the context of infection studies.

Tight junction proteins occludin and ZO-1 as regulators of epithelial proliferation and survival

Epithelial cells are the first line of mucosal defense. In the intestine, a single layer of epithelial cells must establish a selectively permeable barrier that supports nutrient absorption and waste secretion while preventing the leakage of potentially harmful luminal materials. Key to this is the tight junction, which seals the paracellular space and prevents unrestricted leakage. The tight junction is a protein complex established by interactions between members of the claudin, zonula occludens, and tight junction-associated MARVEL protein (TAMP) families. Claudins form the characteristic tight junction strands seen by freeze-fracture microscopy and create paracellular channels, but the functions of ZO-1 and occludin, founding members of the zonula occludens and TAMP families, respectively, are less well defined. Recent studies have revealed that these proteins have essential noncanonical (nonbarrier) functions that allow them to regulate epithelial apoptosis and proliferation, facilitate viral entry, and organize specialized epithelial structures. Surprisingly, neither is required for intestinal barrier function or overall health in the absence of exogenous stressors. Here, we provide a brief overview of ZO-1 and occludin canonical (barrier-related) functions, and a more detailed examination of their noncanonical functions.

Ube2N is present and functions within listeria Actin-rich structures and lamellipodia: A localization and pharmacological inhibition study

The actin cytoskeleton forms much of the structure needed for the intracellular motility of an assortment of microbes as well as entire cells. The co-factor to the ubiquitin conjugating enzyme Ube2N (Ube2V1) has been implicated in both cancer cell metastasis and lysine-63 ubiquitylation of β actin. As this protein complexes with Ube2N, we sought to investigate whether Ube2N itself was involved in actin-based events occurring during the Listeria monocytogenes infections as well as within motile whole cells. Through examination of L. monocytogenes actin clouds, comet tails and membrane protrusions as well as lamellipodia in migrating cells, we show that Ube2N is recruited to actin-rich structures. When pharmacologically inhibited we demonstrate that Ube2N is crucial for the function of actin-rich structures when associated with the plasma membrane.

Impact of Epithelial Cell Shedding on Intestinal Homeostasis

The gut barrier acts as a first line of defense in the body, and plays a vital role in nutrition and immunoregulation. A layer of epithelial cells bound together via intercellular junction proteins maintains intestinal barrier integrity. Based on a tight equilibrium between cell extrusion and cell restitution, the renewal of the epithelium (epithelial turnover) permits the preservation of cell numbers. As the last step within the epithelial turnover, cell shedding occurs due to the pressure of cell division and migration from the base of the crypt. During this process, redistribution of tight junction proteins enables the sealing of the epithelial gap left by the extruded cell, and thereby maintains barrier function. Disturbance in cell shedding can create transient gaps (leaky gut) or cell accumulation in the epithelial layer. In fact, numerous studies have described the association between dysregulated cell shedding and infection, inflammation, and cancer; thus epithelial cell extrusion is considered a key defense mechanism. In the gastrointestinal tract, altered cell shedding has been observed in mouse models of intestinal inflammation and appears as a potential cause of barrier loss in human inflammatory bowel disease (IBD). Despite the relevance of this process, there are many unanswered questions regarding cell shedding. The investigation of those mechanisms controlling cell extrusion in the gut will definitely contribute to our understanding of intestinal homeostasis. In this review, we summarized the current knowledge about intestinal cell shedding under both physiological and pathological circumstances.

Analysis of localized cAMP perturbations within a tissue reveal the effects of a local, dynamic gap junction state on ERK signaling

Beyond natural stimuli such as growth factors and stresses, the ability to experimentally modulate at will the levels or activity of specific intracellular signaling molecule(s) in specified cells within a tissue can be a powerful tool for uncovering new regulation and tissue behaviors. Here we perturb the levels of cAMP within specific cells of an epithelial monolayer to probe the time-dynamic behavior of cell-cell communication protocols implemented by the cAMP/PKA pathway and its coupling to the ERK pathway. The time-dependent ERK responses we observe in the perturbed cells for spatially uniform cAMP perturbations (all cells) can be very different from those due to spatially localized perturbations (a few cells). Through a combination of pharmacological and genetic perturbations, signal analysis, and computational modeling, we infer how intracellular regulation and regulated cell-cell coupling each impact the intracellular ERK response in single cells. Our approach reveals how a dynamic gap junction state helps sculpt the intracellular ERK response over time in locally perturbed cells.

Listeria monocytogenes at the interface between ruminants and humans: A comparative pathology and pathogenesis review

The bacterium Listeria monocytogenes (Lm) is widely distributed in the environment as a saprophyte, but may turn into a lethal intracellular pathogen upon ingestion. Invasive infections occur in numerous species worldwide, but most commonly in humans and farmed ruminants, and manifest as distinct forms. Of those, neuroinfection is remarkably threatening due to its high mortality. Lm is widely studied not only as a pathogen but also as an essential model for intracellular infections and host-pathogen interactions. Many aspects of its ecology and pathogenesis, however, remain unclear and are rarely addressed in its natural hosts. This review highlights the heterogeneity and adaptability of Lm by summarizing its association with the environment, farm animals, and disease. It also provides current knowledge on key features of the pathology and (molecular) pathogenesis of various listeriosis forms in naturally susceptible species with a special focus on ruminants and on the neuroinvasive form of the disease. Moreover, knowledge gaps on pathomechanisms of listerial infections and relevant unexplored topics in Lm pathogenesis research are highlighted.

Campylobacter jejuni Triggers Signaling through Host Cell Focal Adhesions To Inhibit Cell Motility

Campylobacter jejuni is a major foodborne pathogen that exploits the focal adhesions of intestinal cells to promote invasion and cause severe gastritis. Focal adhesions are multiprotein complexes involved in bidirectional signaling between the actin cytoskeleton and the extracellular matrix. We investigated the dynamics of focal adhesion structure and function in C. jejuni-infected cells using a comprehensive set of approaches, including confocal microscopy of live and fixed cells, immunoblotting, and superresolution interferometric photoactivated localization microscopy (iPALM). We found that C. jejuni infection of epithelial cells results in increased focal adhesion size and altered topology. These changes resulted in a persistent modulatory effect on the host cell focal adhesion, evidenced by an increase in cell adhesion strength, a decrease in individual cell motility, and a reduction in collective cell migration. We discovered that C. jejuni infection causes an increase in phosphorylation of paxillin and an alteration of paxillin turnover at the focal adhesion, which together represent a potential mechanistic basis for altered cell motility. Finally, we observed that infection of epithelial cells with the C. jejuni wild-type strain in the presence of a protein synthesis inhibitor, a C. jejuni CadF and FlpA fibronectin-binding protein mutant, or a C. jejuni flagellar export mutant blunts paxillin phosphorylation and partially reestablishes individual host cell motility and collective cell migration. These findings provide a potential mechanism for the restricted intestinal repair observed in C. jejuni-infected animals and raise the possibility that bacteria targeting extracellular matrix components can alter cell behavior after binding and internalization by manipulating focal adhesions.

Volume measurement and biophysical characterization of mounds in epithelial monolayers after intracellular bacterial infection

Mechanical forces are important in (patho)physiological processes, including how host epithelial cells interact with intracellular bacterial pathogens. As these pathogens disseminate within host epithelial monolayers, large mounds of infected cells are formed due to the forceful action of surrounding uninfected cells, limiting bacterial spread across the basal cell monolayer. Here, we present a protocol for mound volume measurement and biophysical characterization of mound formation. Modifications to this protocol may be necessary for studying different host cell types or pathogenic organisms.

For complete details on the use and execution of this protocol, please refer to Bastounis et al. (2021).

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Protocol allows for formation of mounds of extruded infected cells in cell monolayers

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Confocal microscopy and image processing to calculate volume of extruded domains

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Laser wounding protocol for tension estimation built around mounds

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TFM incorporated to measure traction stresses of infected mounders and surrounders

Antilisterial Potential of Lactic Acid Bacteria in Eliminating Listeria monocytogenes in Host and Ready-to-Eat Food Application

Listeriosis is a severe food borne disease with a mortality rate of up to 30% caused by pathogenic Listeria monocytogenes via the production of several virulence factors including listeriolysin O (LLO), transcriptional activator (PrfA), actin (Act), internalin (Int), etc. It is a foodborne disease predominantly causing infections through consumption of contaminated food and is often associated with ready-to-eat food (RTE) and dairy products. Common medication for listeriosis such as antibiotics might cause an eagle effect and antibiotic resistance if it is overused. Therefore, exploration of the use of lactic acid bacteria (LAB) with probiotic characteristics and multiple antimicrobial properties is increasingly getting attention for their capability to treat listeriosis, vaccine development, and hurdle technologies. The antilisterial gene, a gene coding to produce antimicrobial peptide (AMP), one of the inhibitory substances found in LAB, is one of the potential key factors in listeriosis treatment, coupled with the vast array of functions and strategies; this review summarizes the various strategies by LAB against L. monocytogenes and the prospect in development of a ‘generally regarded as safe’ LAB for treatment of listeriosis.

The molecular mechanisms of listeriolysin O-induced lipid membrane damage

Listeria monocytogenes is an intracellular food-borne pathogen that causes listeriosis, a severe and potentially life-threatening disease. Listeria uses a number of virulence factors to proliferate and spread to various cells and tissues. In this process, three bacterial virulence factors, the pore-forming protein listeriolysin O and phospholipases PlcA and PlcB, play a crucial role. Listeriolysin O belongs to a family of cholesterol-dependent cytolysins that are mostly expressed by gram-positive bacteria. Its unique structural features in an otherwise conserved three-dimensional fold, such as the acidic triad and proline-glutamate-serine-threonine-like sequence, enable the regulation of its intracellular activity as well as distinct extracellular functions. The stability of listeriolysin O is pH- and temperature-dependent, and this provides another layer of control of its activity in cells. Moreover, many recent studies have demonstrated a unique mechanism of pore formation by listeriolysin O, i.e., the formation of arc-shaped oligomers that can subsequently fuse to form membrane defects of various shapes and sizes. During listerial invasion of host cells, these membrane defects can disrupt phagosome membranes, allowing bacteria to escape into the cytosol and rapidly multiply. The activity of listeriolysin O is profoundly dependent on the amount and accessibility of cholesterol in the lipid membrane, which can be modulated by the phospholipase PlcB. All these prominent features of listeriolysin O play a role during different stages of the L. monocytogenes life cycle by promoting the proliferation of the pathogen while mitigating excessive damage to its replicative niche in the cytosol of the host cell.

Quo vadis? Central Rules of Pathogen and Disease Tropism

Understanding why certain people get sick and die while others recover or never become ill is a fundamental question in biomedical research. A key determinant of this process is pathogen and disease tropism: the locations that become infected (pathogen tropism), and the locations that become damaged (disease tropism). Identifying the factors that regulate tropism is essential to understand disease processes, but also to drive the development of new interventions. This review intersects research from across infectious diseases to define the central mediators of disease and pathogen tropism. This review also highlights methods of study, and translational implications. Overall, tropism is a central but under-appreciated aspect of infection pathogenesis which should be at the forefront when considering the development of new methods of intervention.

Mechanical competition triggered by innate immune signaling drives the collective extrusion of bacterially infected epithelial cells

Intracellular pathogens alter their host cells' mechanics to promote dissemination through tissues. Conversely, host cells may respond to the presence of pathogens by altering their mechanics to limit infection. Here, we monitored epithelial cell monolayers infected with intracellular bacterial pathogens, Listeria monocytogenes or Rickettsia parkeri, over days. Under conditions in which these pathogens trigger innate immune signaling through NF-κB and use actin-based motility to spread non-lytically intercellularly, we found that infected cell domains formed three-dimensional mounds. These mounds resulted from uninfected cells moving toward the infection site, collectively squeezing the softer and less contractile infected cells upward and ejecting them from the monolayer. Bacteria in mounds were less able to spread laterally in the monolayer, limiting the growth of the infection focus, while extruded infected cells underwent cell death. Thus, the coordinated forceful action of uninfected cells actively eliminates large domains of infected cells, consistent with this collective cell response representing an innate immunity-driven process.

Entry of spores into intestinal epithelial cells contributes to recurrence of Clostridioides difficile infection

Clostridioides difficile spores produced during infection are important for the recurrence of the disease. Here, we show that C. difficile spores gain entry into the intestinal mucosa via pathways dependent on host fibronectin-α5β1 and vitronectin-αvβ1. The exosporium protein BclA3, on the spore surface, is required for both entry pathways. Deletion of the bclA3 gene in C. difficile, or pharmacological inhibition of endocytosis using nystatin, leads to reduced entry into the intestinal mucosa and reduced recurrence of the disease in a mouse model. Our findings indicate that C. difficile spore entry into the intestinal barrier can contribute to spore persistence and infection recurrence, and suggest potential avenues for new therapies.

Receptor-targeted engineered probiotics mitigate lethal Listeria infection

Probiotic bacteria reduce the intestinal colonization of pathogens. Yet, their use in preventing fatal infection caused by foodborne Listeria monocytogenes (Lm), is inconsistent. Here, we bioengineered Lactobacillus probiotics (BLP) to express the Listeria adhesion protein (LAP) from a non-pathogenic Listeria (L. innocua) and a pathogenic Listeria (Lm) on the surface of Lactobacillus casei. The BLP strains colonize the intestine, reduce Lm mucosal colonization and systemic dissemination, and protect mice from lethal infection. The BLP competitively excludes Lm by occupying the surface presented LAP receptor, heat shock protein 60 and ameliorates the Lm-induced intestinal barrier dysfunction by blocking the nuclear factor-κB and myosin light chain kinase-mediated redistribution of the major epithelial junctional proteins. Additionally, the BLP increases intestinal immunomodulatory functions by recruiting FOXP3+T cells, CD11c+ dendritic cells and natural killer cells. Engineering a probiotic strain with an adhesion protein from a non-pathogenic bacterium provides a new paradigm to exclude pathogens and amplify their inherent health benefits.

The Role of Gram-Positive Surface Proteins in Bacterial Niche- and Host-Specialization

Gram-positive bacterial pathogens have an array of proteins on their cell surface that mediate interactions with the host environment. In particular, bacterial cell wall-associated (CWA) proteins play key roles in both colonization and pathogenesis. Furthermore, some CWA proteins promote specialization for host-species or mediate colonization of specific anatomical niches within a host. In this mini review, we provide examples of the many ways by which major pathogens, such as Staphylococci, Streptococci and Listeria monocytogenes, utilize CWA proteins for both host- and niche-specialization. We describe different biological mechanisms mediated by CWA proteins including: the acquisition of iron from hemoglobin in the bloodstream, adherence to and invasion of host cells, and innate immune evasion through binding to the plasma proteins fibrinogen, immunoglobulin G, and complement. We also discuss the limitations of using animal models for understanding the role of specific CWA proteins in host-specialization and how transformative technologies, such as CRISPR-Cas, offer tremendous potential for developing transgenic models that simulate the host environment of interest. Improved understanding of the role of CWA proteins in niche- or host-specificity will allow the design of new therapeutic approaches which target key host–pathogen interactions underpinning Gram-positive bacterial infections.

Cell death in the gut epithelium and implications for chronic inflammation

The intestinal epithelium has one of the highest rates of cellular turnover in a process that is tightly regulated. As the transit-amplifying progenitors of the intestinal epithelium generate ~300 cells per crypt every day, regulated cell death and sloughing at the apical surface keeps the overall cell number in check. An aberrant increase in the rate of intestinal epithelial cell (IEC) death underlies instances of extensive epithelial erosion, which is characteristic of several intestinal diseases such as inflammatory bowel disease and infectious colitis. Emerging evidence points to a crucial role of necroptosis, autophagy and pyroptosis as important modes of programmed cell death in the intestine in addition to apoptosis. The mode of cell death affects tissue restitution responses and ultimately the long-term risks of intestinal fibrosis and colorectal cancer. A vicious cycle of intestinal barrier breach, misregulated cell death and subsequent inflammation is at the heart of chronic inflammatory and infectious gastrointestinal diseases. This Review discusses the underlying molecular and cellular underpinnings that control programmed cell death in IECs, which emerge during intestinal diseases. Translational aspects of cell death modulation for the development of novel therapeutic alternatives for inflammatory bowel diseases and colorectal cancer are also discussed.

Molecular Mechanisms of Intercellular Dissemination of Bacterial Pathogens

Several intracellular bacterial pathogens, including Listeria monocytogenes, Shigella flexerni, and Rickettsia spp. use an actin-based motility process to spread in mammalian cell monolayers. Cell-to-cell spread is mediated by protrusive structures that contain bacteria encased in the host cell plasma membrane. These protrusions, which form in infected host cells, are internalized by neighboring cells. In this review, we summarize key findings on cell-to-cell spread, focusing on recent work on mechanisms of protrusion formation and internalization. We also discuss the dynamic behavior of bacterial populations during spread, and highlight recent findings showing that intercellular spread by an extracellular bacterial pathogen.

Pregnancy Complicated by Listeria Monocytogenes: A Case Report and Review of the Literature

Listeria Monocytogenes, a small facultative anaerobic, gram positive, motile bacillus is a rare, but consequential etiologic agent of food borne illness which inordinately impacts immunocompromised individuals. The organism infects many types of animals and contaminates a multitude of foodstuffs such as milk, chicken, beef and vegetables. This microbe additionally has a distinct proclivity to infect the maternal-fetoplacental unit with resultant adverse perinatal outcomes inclusive of spontaneous abortion, preterm delivery, chorioamnionitis, neonatal meningitis and death. We present a case of Listeriosis complicating pregnancy with a subsequent comprehensive review of the literature.

Controlling Epithelial Polarity: A Human Enteroid Model for Host-Pathogen Interactions

Human enteroids-epithelial spheroids derived from primary gastrointestinal tissue-are a promising model to study pathogen-epithelial interactions. However, accessing the apical enteroid surface is challenging because it is enclosed within the spheroid. We developed a technique to reverse enteroid polarity such that the apical surface everts to face the media. Apical-out enteroids maintain proper polarity and barrier function, differentiate into the major intestinal epithelial cell (IEC) types, and exhibit polarized absorption of nutrients. We used this model to study host-pathogen interactions and identified distinct polarity-specific patterns of infection by invasive enteropathogens. Salmonella enterica serovar Typhimurium targets IEC apical surfaces for invasion via cytoskeletal rearrangements, and Listeria monocytogenes, which binds to basolateral receptors, invade apical surfaces at sites of cell extrusion. Despite different modes of entry, both pathogens exit the epithelium within apically extruding enteroid cells. This model will enable further examination of IECs in health and disease.

Taking Control: Campylobacter jejuni Binding to Fibronectin Sets the Stage for Cellular Adherence and Invasion

Campylobacter jejuni, a foodborne pathogen, is one of the most common bacterial causes of gastroenteritis in the world. Undercooked poultry, raw (unpasteurized) dairy products, untreated water, and contaminated produce are the most common sources associated with infection. C. jejuni establishes a niche in the gut by adhering to and invading epithelial cells, which results in diarrhea with blood and mucus in the stool. The process of colonization is mediated, in part, by surface-exposed molecules (adhesins) that bind directly to host cell ligands or the extracellular matrix (ECM) surrounding cells. In this review, we introduce the known and putative adhesins of the foodborne pathogen C. jejuni. We then focus our discussion on two C. jejuni Microbial Surface Components Recognizing Adhesive Matrix Molecule(s) (MSCRAMMs), termed CadF and FlpA, which have been demonstrated to contribute to C. jejuni colonization and pathogenesis. In vitro studies have determined that these two surface-exposed proteins bind to the ECM glycoprotein fibronectin (FN). In vivo studies have shown that cadF and flpA mutants exhibit impaired colonization of chickens compared to the wild-type strain. Additional studies have revealed that CadF and FlpA stimulate epithelial cell signaling pathways necessary for cell invasion. Interestingly, CadF and FlpA have distinct FN-binding domains, suggesting that the functions of these proteins are non-redundant. In summary, the binding of FN by C. jejuni CadF and FlpA adhesins has been demonstrated to contribute to adherence, invasion, and cell signaling.

Characterization of internalin genes in Listeria monocytogenes from food and humans, and their association with the invasion of Caco-2 cells

Background

Internalins are surface proteins that are utilized by Listeria monocytogenes to facilitate its invasion into human intestinal epithelial cells. The expression of a full-length InlA is one of essential virulence factors for L. monocytogenes to cross the intestinal barrier in order to invade epithelial cells.

Results

In this study, the gene sequences of inlA in 120 L. monocytogenes isolates from food (n = 107) and humans (n = 13) were analyzed. Premature stop codon (PMSC) mutations in inlA were identified in 51 isolates (50 from food and 1 from human). Six mutation types of PMSCs were identified. Among the 51 isolates with PMSCs in inlA, there were 44 serogroup 1/2c, 3c isolates from food, of which seven belonged to serogroups 1/2a, 3a. A total of 153,382 SNPs in 2247 core genes from 42 genomes were identified and used to construct a phylogenetic tree. Serotype 1/2c isolates with inlA PMSC mutations were grouped together. Cell culture studies on 21 isolates showed that the invasion to Caco-2 cells was significantly reduced among isolates with inlA PMSC mutations compared to those without PMSC mutations (P < 0.01). The PMSC mutations in inlA correlated with the inability of the L. monocytogenes isolates to invade Caco-2 cells (Pearson’s coefficient 0.927, P < 0.01).

Conclusion

Overall, the study has revealed the reduced ability of L. monocytogenes to invade human intestinal epithelial cells in vitro was linked to the presence of PMSC mutations in inlA. Isolates with PMSC mutations shared the same genomic characteristics indicating the genetic basis on the potential virulence of L. monocytogenes invasion.

Electronic supplementary material

The online version of this article (10.1186/s13099-019-0307-8) contains supplementary material, which is available to authorized users.

Crossing the Intestinal Barrier via Listeria Adhesion Protein and Internalin A

The intestinal epithelial cell lining provides the first line of defense, yet foodborne pathogens such as Listeria monocytogenes can overcome this barrier; however, the underlying mechanism is not well understood. Though the host M cells in Peyer's patch and the bacterial invasion protein internalin A (InlA) are involved, L. monocytogenes can cross the gut barrier in their absence. The interaction of Listeria adhesion protein (LAP) with the host cell receptor (heat shock protein 60) disrupts the epithelial barrier, promoting bacterial translocation. InlA aids L. monocytogenes transcytosis via interaction with the E-cadherin receptor, which is facilitated by epithelial cell extrusion and goblet cell exocytosis; however, LAP-induced cell junction opening may be an alternative bacterial strategy for InlA access to E-cadherin and its translocation. Here, we summarize the strategies that L. monocytogenes employs to circumvent the intestinal epithelial barrier and compare and contrast these strategies with other enteric bacterial pathogens. Additionally, we provide implications of recent findings for food safety regulations.

Germline CDH1 mutations are a significant contributor to the high frequency of early-onset diffuse gastric cancer cases in New Zealand Māori

New Zealand Māori have a considerably higher incidence of gastric cancer compared to non-Māori, and are one of the few populations worldwide with a higher prevalence of diffuse-type disease. Pathogenic germline CDH1 mutations are causative of hereditary diffuse gastric cancer, a cancer predisposition syndrome primarily characterised by an extreme lifetime risk of developing diffuse gastric cancer. Pathogenic CDH1 mutations are well described in Māori families in New Zealand. However, the contribution of these mutations to the high incidence of gastric cancer is unknown. We have used next-generation sequencing, Sanger sequencing, and Multiplex Ligation-dependent Probe Amplification to examine germline CDH1 in an unselected series of 94 Māori gastric cancer patients and 200 healthy matched controls. Overall, 18% of all cases, 34% of cases diagnosed with diffuse-type gastric cancer, and 67% of cases diagnosed aged less than 45 years carried pathogenic CDH1 mutations. After adjusting for the effect of screening known HDGC families, we estimate that 6% of all advanced gastric cancers and 13% of all advanced diffuse-type gastric cancers would carry germline CDH1 mutations. Our results demonstrate that germline CDH1 mutations are a significant contributor to the high frequency of diffuse gastric cancer in New Zealand Māori.

Listeria monocytogenes cell-to-cell spread in epithelia is heterogeneous and dominated by rare pioneer bacteria

Listeria monocytogenes hijacks host actin to promote its intracellular motility and intercellular spread. While L. monocytogenes virulence hinges on cell-to-cell spread, little is known about the dynamics of bacterial spread in epithelia at a population level. Here, we use live microscopy and statistical modeling to demonstrate that L. monocytogenes cell-to-cell spread proceeds anisotropically in an epithelial monolayer in culture. We show that boundaries of infection foci are irregular and dominated by rare pioneer bacteria that spread farther than the rest. We extend our quantitative model for bacterial spread to show that heterogeneous spreading behavior can improve the chances of creating a persistent L. monocytogenes infection in an actively extruding epithelium. Thus, our results indicate that L. monocytogenes cell-to-cell spread is heterogeneous, and that rare pioneer bacteria determine the frontier of infection foci and may promote bacterial infection persistence in dynamic epithelia.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Eating food that has been contaminated with bacteria called Listeria monocytogenes can result in life-threatening infections. The bacteria first invade the epithelial cells that line the small intestine. After this, L. monocytogenes can move from one host cell to another, which allows the infection to reach other organs.

Most studies into how L. monocytogenes infections spread have focused either on how single bacterial cells move from one host cell to the next, or on how millions of bacteria damage host tissues. Little was known about the intermediate steps of an infection, where the bacteria start to colonize the small intestine.

To investigate, Ortega et al. recorded videos of L. monocytogenes spreading between epithelial cells grown on a glass coverslip, and developed computer simulations to try to reproduce how the bacteria spread. This revealed that the bacteria do not all move in the same way. Instead, less than 1% of the bacteria move in ‘steps’ that are up to 10 times longer than those taken by the others. Ortega et al. named these bacteria ‘pioneers’.

Ortega et al. propose that the pioneers form long protrusions that allow them to spread directly from an infected cell to a non-neighboring cell. By taking these large steps, the pioneers may increase the chances that the bacteria will cause a long-lasting infection.

Future research will be needed to answer further questions about the pioneers. For example, how do the pioneer bacteria differ from the majority of bacterial cells? Would targeting anti-bacterial treatments at pioneers make it easier to treat infections? It also remains to be seen if other types of bacteria also show this pioneer behavior.

Tropism for Spasmolytic Polypeptide-Expressing Metaplasia Allows Helicobacter pylori to Expand Its Intragastric Niche

BACKGROUND & AIMS

In patients with chronic Helicobacter pylori (H pylori) infection, parietal and chief cell atrophy in the gastric corpus, a process known as spasmolytic polypeptide-expressing metaplasia (SPEM), increases the risk for progression to cancer. The relation between H pylori and these metaplastic changes is unclear. We investigated whether H pylori localizes to regions of SPEM.

METHODS

We developed an in situ adherence assay in which we incubated H pylori with free-floating tissue sections from the gastric corpora of mice; we assessed H pylori distribution along the gastric unit by immunofluorescence. We analyzed the interactions of H pylori with tissue collected from mice with acute SPEM, induced by high-dose tamoxifen. We also evaluated how adhesin-deficient H pylori strains, chemical competition assays, and epithelial glycosylation affected H pylori adhesion to SPEM glands. Mice colonized with the mouse-adapted PMSS1 strain were analyzed for H pylori colonization in vivo during tamoxifen-induced SPEM or after decrease of stomach acid with omeprazole.

RESULTS

Compared with uninjured glands, H pylori penetrated deep within SPEM glands, in situ, through interaction of its adhesin, SabA, with sialyl-Lewis X, which expanded in SPEM. H pylori markedly increased gastric corpus colonization when SPEM was induced, but this proximal spread reversed in mice allowed to recover from SPEM. Decreasing corpus acidity also promoted proximal spread. However, H pylori penetrated deep within corpus glands in vivo only when sialyl-Lewis X expanded during SPEM.

CONCLUSIONS

Helicobacter pylori differentially binds SPEM glands in situ and in mice, in large part by interacting with sialyl-Lewis X. Our findings indicate that H pylori expands its niche into the gastric corpus by promoting and exploiting epithelial metaplastic changes that can lead to tumorigenesis.

Listeria monocytogenes: cell biology of invasion and intracellular growth

The Gram-positive pathogen Listeria monocytogenes is able to promote its entry into a diverse range of mammalian host cells by triggering plasma membrane remodeling, leading to bacterial engulfment. Upon cell invasion, L. monocytogenes disrupts its internalization vacuole and translocates to the cytoplasm, where bacterial replication takes place. Subsequently, L. monocytogenes uses an actin-based motility system that allows bacterial cytoplasmic movement and cell-to-cell spread. L. monocytogenes therefore subverts host cell receptors, organelles and the cytoskeleton at different infection steps, manipulating diverse cellular functions that include ion transport, membrane trafficking, post-translational modifications, phosphoinositide production, innate immune responses as well as gene expression and DNA stability.

Hydrogen peroxide production by lactobacilli promotes epithelial restitution during colitis

Inflammatory bowel disease (IBD) is a multifactorial chronic inflammatory disease of the gastrointestinal tract, characterized by cycles of acute flares, recovery and remission phases. Treatments for accelerating tissue restitution and prolonging remission are scarce, but altering the microbiota composition to promote intestinal homeostasis is considered a safe, economic and promising approach. Although probiotic bacteria have not yet fulfilled fully their promise in clinical trials, understanding the mechanism of how they exert beneficial effects will permit devising improved therapeutic strategies. Here we probe if one of the defining features of lactobacilli, the ability to generate nanomolar H2O2, contributes to their beneficial role in colitis. H2O2 generation by wild type L. johnsonii was modified by either deleting or overexpressing the enzymatic H2O2 source(s) followed by orally administering the bacteria before and during DSS colitis. Boosting luminal H2O2 concentrations within a physiological range accelerated recovery from colitis, while significantly exceeding this H2O2 level triggered bacteraemia. This study supports a role for increasing H2O2 within the physiological range at the epithelial barrier, independently of the enzymatic source and/or delivery mechanism, for inducing recovery and remission in IBD.

To Be Cytosolic or Vacuolar: The Double Life of Listeria monocytogenes

Intracellular bacterial pathogens are generally classified into two types: those that exploit host membrane trafficking to construct specific niches in vacuoles (i.e., "vacuolar pathogens"), and those that escape from vacuoles into the cytosol, where they proliferate and often spread to neighboring cells (i.e., "cytosolic pathogens"). However, the boundary between these distinct intracellular phenotypes is tenuous and may depend on the timing of infection and on the host cell type. Here, we discuss recent progress highlighting this phenotypic duality in Listeria monocytogenes, which has long been a model for cytosolic pathogens, but now emerges as a bacterium also capable of residing in vacuoles, in a slow/non-growing state. The ability of L. monocytogenes to enter a persistence stage in vacuoles might play a role during the asymptomatic incubation period of listeriosis and/or the carriage of this pathogen in asymptomatic hosts. Moreover, persistent vacuolar Listeria could be less susceptible to antibiotics and more difficult to detect by routine techniques of clinical biology. These hypotheses deserve to be explored in order to better manage the risks related to this food-borne pathogen.

Small Rho GTPases and the Effector VipA Mediate the Invasion of Epithelial Cells by Filamentous Legionella pneumophila

Legionella pneumophila (Lp) exhibits different morphologies with varying degrees of virulence. Despite their detection in environmental sources of outbreaks and in respiratory tract secretions and lung autopsies from patients, the filamentous morphotype of Lp remains poorly studied. We previously demonstrated that filamentous Lp invades lung epithelial cells (LECs) and replicates intracellularly in a Legionella containing vacuole. Filamentous Lp activates β1integrin and E-cadherin receptors at the surface of LECs leading to the formation of actin-rich cell membrane structures we termed hooks and membrane wraps. These structures entrap segments of an Lp filament on host cell surface and mediate bacterial internalization. Here we investigated the molecular mechanisms responsible for the actin rearrangements needed for the formation and elongation of these membrane wraps and bacterial internalization. We combined genetic and pharmacological approaches to assess the contribution of signaling downstream of β1integrin and E-cadherin receptors, and Lp Dot/Icm secretion system- translocated effectors toward the invasion process. Our studies demonstrate a multi-stage mechanism of LEC invasion by filamentous Lp. Bacterial attachment to host cells depends on signaling downstream of β1integrin and E-cadherin activation, leading to Rho GTPases-dependent activation of cellular actin nucleating proteins, Arp2/3 and mDia. This mediates the formation of primordial membrane wraps that entrap the filamentous bacteria on the cell surface. Following this, in a second phase of the invasion process the Dot/Icm translocated effector VipA mediates rapid membrane wrap elongation, leading to the engulfment of the filamentous bacteria by the LECs. Our findings provide the first description of Rho GTPases and a Dot/Icm effector VipA regulating the actin dynamics needed for the invasion of epithelial cells by Lp.

Listeria monocytogenes InlP interacts with afadin and facilitates basement membrane crossing

During pregnancy, the placenta protects the fetus against the maternal immune response, as well as bacterial and viral pathogens. Bacterial pathogens that have evolved specific mechanisms of breaching this barrier, such as Listeria monocytogenes, present a unique opportunity for learning how the placenta carries out its protective function. We previously identified the L. monocytogenes protein Internalin P (InlP) as a secreted virulence factor critical for placental infection. Here, we show that InlP, but not the highly similar L. monocytogenes internalin Lmo2027, binds to human afadin (encoded by AF-6), a protein associated with cell-cell junctions. A crystal structure of InlP reveals several unique features, including an extended leucine-rich repeat (LRR) domain with a distinctive Ca2+-binding site. Despite afadin's involvement in the formation of cell-cell junctions, MDCK epithelial cells expressing InlP displayed a decrease in the magnitude of the traction stresses they could exert on deformable substrates, similar to the decrease in traction exhibited by AF-6 knock-out MDCK cells. L. monocytogenes ΔinlP mutants were deficient in their ability to form actin-rich protrusions from the basal face of polarized epithelial monolayers, a necessary step in the crossing of such monolayers (transcytosis). A similar phenotype was observed for bacteria expressing an internal in-frame deletion in inlP (inlP ΔLRR5) that specifically disrupts its interaction with afadin. However, afadin deletion in the host cells did not rescue the transcytosis defect. We conclude that secreted InlP targets cytosolic afadin to specifically promote L. monocytogenes transcytosis across the basal face of epithelial monolayers, which may contribute to the crossing of the basement membrane during placental infection.

Listeria Adhesion Protein Induces Intestinal Epithelial Barrier Dysfunction for Bacterial Translocation

Intestinal epithelial cells are the first line of defense against enteric pathogens, yet bacterial pathogens, such as Listeria monocytogenes, can breach this barrier. We show that Listeria adhesion protein (LAP) induces intestinal epithelial barrier dysfunction to promote bacterial translocation. These disruptions are attributed to the production of pro-inflammatory cytokines TNF-α and IL-6, which is observed in mice challenged with WT and isogenic strains lacking the surface invasion protein Internalin A (ΔinlA), but not a lap- mutant. Additionally, upon engagement of its surface receptor Hsp60, LAP activates canonical NF-κB signaling, facilitating myosin light-chain kinase (MLCK)-mediated opening of the epithelial barrier via cellular redistribution of the epithelial junctional proteins claudin-1, occludin, and E-cadherin. Pharmacological inhibition of MLCK or NF-κB in cells or genetic ablation of MLCK in mice prevents mislocalization of junctional proteins and L. monocytogenes translocation. Thus, L. monocytogenes uses LAP to exploit epithelial defenses and cross the intestinal epithelial barrier.

Cell Extrusion: A Stress-Responsive Force for Good or Evil in Epithelial Homeostasis

Epithelial tissues robustly respond to internal and external stressors via dynamic cellular rearrangements. Cell extrusion acts as a key regulator of epithelial homeostasis by removing apoptotic cells, orchestrating morphogenesis, and mediating competitive cellular battles during tumorigenesis. Here, we delineate the diverse functions of cell extrusion during development and disease. We emphasize the expanding role for apoptotic cell extrusion in exerting morphogenetic forces, as well as the strong intersection of cell extrusion with cell competition, a homeostatic mechanism that eliminates aberrant or unfit cells. While cell competition and extrusion can exert potent, tumor-suppressive effects, dysregulation of either critical homeostatic program can fuel cancer progression.

Modelling persistent Mycoplasma pneumoniae infection of human airway epithelium

Mycoplasma pneumoniae is a human respiratory tract pathogen causing acute and chronic airway disease states that can include long-term carriage and extrapulmonary spread. The mechanisms of persistence and migration beyond the conducting airways, however, remain poorly understood. We previously described an acute exposure model using normal human bronchial epithelium (NHBE) in air-liquid interface culture, showing that M. pneumoniae gliding motility is essential for initial colonisation and subsequent spread, including localisation to epithelial cell junctions. We extended those observations here, characterizing M. pneumoniae infection of NHBE for up to 4 weeks. Colonisation of the apical surface was followed by pericellular invasion of the basolateral compartment and migration across the underlying transwell membrane. Despite fluctuations in transepithelial electrical resistance and increased NHBE cell desquamation, barrier function remained largely intact. Desquamation was accompanied by epithelial remodelling that included cytoskeletal reorganisation and development of deep furrows in the epithelium. Finally, M. pneumoniae strains S1 and M129 differed with respect to invasion and histopathology, consistent with contrasting virulence in experimentally infected mice. In summary, this study reports pericellular invasion, NHBE cytoskeletal reorganisation, and tissue remodelling with persistent infection in a human airway epithelium model, providing clear insight into the likely route for extrapulmonary spread.