Understanding how Listeria monocytogenes targets and crosses host barriers

Immunopeptidomics mapping of Listeria monocytogenes T cell epitopes in mice

Listeria monocytogenes is a foodborne intracellular bacterial model pathogen. Protective immunity against Listeria depends on an effective CD8+ T cell response, but very few T cell epitopes are known in mice as a common animal infection model for listeriosis. To identify epitopes we screened for Listeria immunopeptides presented in the spleen of infected mice by mass spectrometry-based immunopeptidomics. We mapped more than 6,000 mouse self-peptides presented on MHC Class I molecules, including 12 high confident Listeria peptides from 12 different bacterial proteins. Bacterial immunopeptides with confirmed fragmentation spectra were further tested for their potential to activate CD8+ T cells, revealing VTYNYINI from the putative cell wall surface anchor family protein LMON_0576 as a novel bona fide peptide epitope. The epitope showed high biological potency in a prime boost model and can be used as a research tool to probe CD8+ T cell responses in mouse models of Listeria infection. Together, our results demonstrate the power of immunopeptidomics for bacterial antigen identification.

Controlling of foodborne pathogen biofilms on stainless steel by bacteriophages: A systematic review and meta-analysis

This study investigates the potential of using bacteriophages to control foodborne pathogen biofilms on stainless steel surfaces in the food industry. Biofilm-forming bacteria can attach to stainless steel surfaces, rendering them difficult to eradicate even after a thorough cleaning and sanitizing procedures. Bacteriophages have been proposed as a possible solution, as they can penetrate biofilms and destroy bacterial cells within, reducing the number of viable bacteria and preventing the growth and spread of biofilms. This systematic review and meta-analysis evaluates the potential of bacteriophages against different biofilm-forming foodborne bacteria, including Cronobacter sakazakii, Escherichia coli, Staphylococcus aureus, Pseudomonas fluorescens, Pseudomonas aeruginosa and Listeria monocytogenes. Bacteriophage treatment generally causes a significant average reduction of 38 % in biofilm formation of foodborne pathogens on stainless steel. Subgroup analyses revealed that phages are more efficient in long-duration treatment. Also, applying a cocktail of phages is 1.26-fold more effective than applying individual phages. Phages at concentrations exceeding 107 PFU/ml are significantly more efficacious in eradicating bacteria within a biofilm. The antibacterial phage activity decreases substantially by 3.54-fold when applied at 4 °C compared to temperatures above 25 °C. This analysis suggests that bacteriophages can be a promising solution for controlling biofilms in the food industry.

Characterisation of the growth behaviour of Listeria monocytogenes in Listeria synthetic media

The foodborne pathogen Listeria monocytogenes can grow in a wide range of environmental conditions. For the study of the physiology of this organism, several chemically defined media have been developed over the past decades. Here, we examined the ability of L. monocytogenes wildtype strains EGD-e and 10403S to grow under salt and pH stress in Listeria synthetic medium (LSM). Furthermore, we determined that a wide range of carbon sources could support the growth of both wildtype strains in LSM. However, for hexose phosphate sugars such as glucose-1-phosphate, both L. monocytogenes strains need to be pre-grown under conditions, where the major virulence regulator PrfA is active. In addition, growth of both L. monocytogenes strains was observed when LSM was supplemented with the amino acid sugar N-acetylmannosamine (ManNAc). We were able to show that some of the proteins encoded in the operon lmo2795-nanE, such as the ManNAc-6-phosphate epimerase NanE, are required for growth in the presence of ManNAc. The first gene of the operon, lmo2795, encodes a transcriptional regulator of the RpiR family. Using electrophoretic mobility shift assays and quantitative real-time PCR analysis, we were able to show that Lmo2795 binds to the promoter region of the operon lmo2795-nanE and activates its expression.

Hacking the Immune Response to Solid Tumors: Harnessing the Anti-Cancer Capacities of Oncolytic Bacteria

Oncolytic bacteria are a classification of bacteria with a natural ability to specifically target solid tumors and, in the process, stimulate a potent immune response. Currently, these include species of Klebsiella, Listeria, Mycobacteria, Streptococcus/Serratia (Coley's Toxin), Proteus, Salmonella, and Clostridium. Advancements in techniques and methodology, including genetic engineering, create opportunities to "hijack" typical host-pathogen interactions and subsequently harness oncolytic capacities. Engineering, sometimes termed "domestication", of oncolytic bacterial species is especially beneficial when solid tumors are inaccessible or metastasize early in development. This review examines reported oncolytic bacteria-host immune interactions and details the known mechanisms of these interactions to the protein level. A synopsis of the presented membrane surface molecules that elicit particularly promising oncolytic capacities is paired with the stimulated localized and systemic immunogenic effects. In addition, oncolytic bacterial progression toward clinical translation through engineering efforts are discussed, with thorough attention given to strains that have accomplished Phase III clinical trial initiation. In addition to therapeutic mitigation after the tumor has formed, some bacterial species, referred to as "prophylactic", may even be able to prevent or "derail" tumor formation through anti-inflammatory capabilities. These promising species and their particularly favorable characteristics are summarized as well. A complete understanding of the bacteria-host interaction will likely be necessary to assess anti-cancer capacities and unlock the full cancer therapeutic potential of oncolytic bacteria.

La listeriosi neonatale e materno-fetale

Listeriosis is a foodborne condition associated with the ingestion of food contaminated by Listeria monocytogenes that can cause serious health consequences in neonates and pregnant women. In these groups the incidence of the disease is higher due to the relative immunocompromised state and the particular Listeria monocytogenes tropism for the placenta. The paediatrician and the neonatologist must therefore be able to recognize the signs and symptoms of the disease promptly so as to set up an adequate treatment according to the most recent international guidelines. This article provides epidemiological data and describes the pathophysiology and clinical manifestations of maternal-foetal and neonatal listeriosis, as well as the therapeutic indications according to very recent recommendations.

A subtractive proteomics and immunoinformatics approach towards designing a potential multi-epitope vaccine against pathogenic Listeriamonocytogenes

Listeria monocytogenes is the causative agent of listeriosis, which is dangerous for pregnant women, the elderly or individuals with a weakened immune system. Individuals with leukaemia, cancer, HIV/AIDS, kidney transplant and steroid therapy suffer from immunological damage are menaced. World Health Organization (WHO) reports that human listeriosis has a high mortality rate of 20-30% every year. To date, no vaccine is available to treat listeriosis. Thereby, it is high time to design novel vaccines against L. monocytogenes. Here, we present computational approaches to design an antigenic, stable and safe vaccine against the L. monocytogenes that could help to control the infections associated with the pathogen. Three vital pathogenic proteins of L. monocytogenes, such as Listeriolysin O (LLO), Phosphatidylinositol-specific phospholipase C (PI-PLC), and Actin polymerization protein (ActA), were selected using a subtractive proteomics approach to design the multi-epitope vaccine (MEV). A total of 5 Cytotoxic T-lymphocyte (CTL) and 9 Helper T-lymphocyte (HTL) epitopes were predicted from these selected proteins. To design the multi-epitope vaccine (MEV) from the selected proteins, CTL epitopes were joined with the AAY linker, and HTL epitopes were joined with the GPGPG linker. Additionally, a human β-defensin-3 (hBD-3) adjuvant was added to the N-terminal side of the final MEV construct to increase the immune response to the vaccine. The final MEV was predicted to be antigenic, non-allergen and non-toxic in nature. Physicochemical property analysis suggested that the MEV construct is stable and could be easily purified through the E. coli expression system. This in-silico study showed that MEV has a robust binding interaction with Toll-like receptor 2 (TLR2), a key player in the innate immune system. Current subtractive proteomics and immunoinformatics study provides a background for designing a suitable, safe and effective vaccine against pathogenic L. monocytogenes.

Infection of the murine placenta by Listeria monocytogenes induces sex-specific responses in the fetal brain

BACKGROUND

Epidemiological data indicate that prenatal infection is associated with an increased risk of several neurodevelopmental disorders in the progeny. These disorders display sex differences in presentation. The role of the placenta in the sex-specificity of infection-induced neurodevelopmental abnormalities is not well-defined. We used an imaging-based animal model of the bacterial pathogen Listeria monocytogenes to identify sex-specific effects of placental infection on neurodevelopment of the fetus.

METHODS

Pregnant CD1 mice were infected with a bioluminescent strain of Listeria on embryonic day 14.5 (E14.5). Excised fetuses were imaged on E18.5 to identify the infected placentas. The associated fetal brains were analyzed for gene expression and altered brain structure due to infection. The behavior of adult offspring affected by prenatal Listeria infection was analyzed.

RESULTS

Placental infection induced sex-specific alteration of gene expression patterns in the fetal brain and resulted in abnormal cortical development correlated with placental infection levels. Furthermore, male offspring exhibited abnormal social interaction, whereas females exhibited elevated anxiety.

CONCLUSION

Placental infection by Listeria induced sex-specific abnormalities in neurodevelopment of the fetus. Prenatal infection also affected the behavior of the offspring in a sex-specific manner.

IMPACT

Placental infection with Listeria monocytogenes induces sexually dichotomous gene expression patterns in the fetal brains of mice. Abnormal cortical lamination is correlated with placental infection levels. Placental infection results in autism-related behavior in male offspring and heightened anxiety levels in female offspring.

The combination of thymol and cinnamaldehyde reduces the survival and virulence of Listeria monocytogenes on autoclaved chicken breast

AIMS

To reveal the antibacterial mechanism of the combination of thymol and cinnamaldehyde to Listeria monocytogenes ATCC 19115 on autoclaved chicken breast.

METHODS AND RESULTS

In this study, Listeria monocytogenes ATCC 19115 on autoclaved chicken breast was exposed to the stress of 125 μg/mL thymol and 125 μg/mL cinnamaldehyde, and transcriptome analysis was used to reveal the crucial antibacterial mechanism. According to the results, 1303 significantly differentially expressed genes (DEGs) were identified. Treated by thymol and cinnamaldehyde in combination, pyrimidine and branched-chain amino acids biosynthesis of L. monocytogenes were thwarted which impairs its nucleic acid biosynthesis and intracellular metabolism. The up-regulated DEGs involved in membrane composition and function contributed to membrane repair. Besides, pyruvate catabolism and TCA cycle were restrained which brought about the disturbance of amino acid metabolism. ABC transporters were also perturbed, for instance, the uptake of cysteine, D-methionine and betaine was activated, while the uptake of vitamin, iron and carnitine was repressed. Thus, L. monocytogenes tended to activate PTS, glycolysis, glycerol catabolism, and pentose phosphate pathways to obtain energy to adapt to the hostile condition. Noticeably, DEGs involved in virulence factors were totally down-regulated, including genes devoted to encoding flagella, chemotaxis, biofilm formation, internalin as well as virulence gene clusters.

CONCLUSIONS

The combination of thymol and cinnamaldehyde is effective to reduce the survival and potential virulence of L. monocytogenes on autoclaved chicken breast.

SIGNIFICANCE AND IMPACT OF STUDY

This work contributes to providing theoretical information for the application and optimization of thymol and cinnamaldehyde in ready-to-eat meat products to inhibit L. monocytogenes.

Acute Exposure to the Food-Borne Pathogen Listeria monocytogenes Does Not Induce α-Synuclein Pathology in the Colonic ENS of Nonhuman Primates

INTRODUCTION

Gastrointestinal (GI) inflammation elicited by environmental factors is proposed to trigger Parkinson's disease (PD) by stimulating accumulation of pathological α-synuclein (α-syn) in the enteric nervous system (ENS), which then propagates to the central nervous system via the vagus nerve. The goal of this study was to model, in nonhuman primates, an acute exposure to a common food-borne pathogen in order to assess whether the related acute GI inflammation could initiate persistent α-syn pathology in the ENS, ultimately leading to PD.

METHODS

Adult female cynomolgus macaques were inoculated by oral gavage with 1×108 colony-forming units (CFUs) Listeria monocytogenes (LM, n=10) or vehicle (mock, n=3) and euthanized 2 weeks later. Evaluations included clinical monitoring, blood and fecal shedding of LM, and postmortem pathological analysis of colonic and cecal tissues.

RESULTS

LM inoculation of healthy adult cynomolgus macaques induced minimal to mild clinical signs of infection; LM shedding in feces was not seen in any of the animals nor was bacteremia detected. Colitis varied from none to moderate in LM-treated subjects and none to minimal in mock-treated subjects. Expression of inflammatory markers (HLA-DR, CD3, CD20), oxidative stress (8-OHDG), α-syn, and phosphorylated-α-syn in the enteric ganglia was not significantly different between treatment groups.

DISCUSSION

Our results demonstrate that cynomolgus macaques orally inoculated with LM present with a clinical response that resembles human LM exposure. They also suggest that acute exposure to food-borne pathogens is not sufficient to induce significant and persistent α-syn changes in healthy adult female subjects. Based on the results of this limited experimental setting, we propose that, if LM has a role in PD pathology, other underlying factors or conditions, such as male sex, inflammatory bowel disease, exposure to toxins, dysbiosis, and/or aging, are needed to be present.

Diversity of Listeria monocytogenes Strains Isolated from Food Products in the Central European Part of Russia in 2000-2005 and 2019-2020

Totally, 45 L. monocytogenes strains isolated from meat, poultry, dairy, and fish products in the Central European part of Russia in 2001–2005 and 2019–2020 were typed using a combined MLST and internalin profile (IP) scheme. Strains belonged to 14 clonal complexes (CCs) of the phylogenetic lineages I and II. Almost half of the strains (20 of 45) belonged to six CCs previously recognized as epidemic clones (ECs). ECI and ECV strains were isolated during both studied periods, and ECII, ECIV, ECVI, and ECVII strains were isolated in 2001–2005, but not in 2019–2020. ECI, ECIV, ECV, and ECVII strains were isolated from products of animal origin. ECII and ECVI were isolated from fish. Testing of invasion efficiencies of 10 strains isolated in different years and from different sources and belonging to distinct CCs revealed a statistically significant difference between phylogenetic lineage I and II strains but not between ECs and non-EC CCs or strains differing by year and source of isolation. Strains isolated in 2001–2005 were characterized by higher phylogenetic diversity and greater presentation of ECs and CCs non-typical for natural and anthropogenic environments of the European part of Russia comparatively to isolates obtained in 2019–2020.Closing of the Russian market in 2019–2020 for imported food might be responsible for these differences.

AsnB Mediates Amidation of Meso-Diaminopimelic Acid Residues in the Peptidoglycan of Listeria monocytogenes and Affects Bacterial Surface Properties and Host Cell Invasion

A mutant of Listeria monocytogenes ScottA with a transposon in the 5' untranslated region of the asnB gene was identified to be hypersensitive to the antimicrobial t-cinnamaldehyde. Here, we report the functional characterization of AsnB in peptidoglycan (PG) modification and intracellular infection. While AsnB of Listeria is annotated as a glutamine-dependent asparagine synthase, sequence alignment showed that this protein is closely related to a subset of homologs that catalyze the amidation of meso-diaminopimelic acid (mDAP) residues in the peptidoglycan of other bacterial species. Structural analysis of peptidoglycan from an asnB mutant, compared to that of isogenic wild-type (WT) and complemented mutant strains, confirmed that AsnB mediates mDAP amidation in L. monocytogenes. Deficiency in mDAP amidation caused several peptidoglycan- and cell surface-related phenotypes in the asnB mutant, including formation of shorter but thicker cells, susceptibility to lysozyme, loss of flagellation and motility, and a strong reduction in biofilm formation. In addition, the mutant showed reduced invasion of human epithelial JEG-3 and Caco-2 cells. Analysis by immunofluorescence microscopy revealed that asnB inactivation abrogated the proper display at the listerial surface of the invasion protein InlA, which normally gets cross-linked to mDAP via its LPXTG motif. Together, this work shows that AsnB of L. monocytogenes, like several of its homologs in related Gram-positive bacteria, mediates the amidation of mDAP residues in the peptidoglycan and, in this way, affects several cell wall and cell surface-related properties. It also for the first time implicates the amidation of peptidoglycan mDAP residues in cell wall anchoring of InlA and in bacterial virulence.

PASTA kinase-dependent control of peptidoglycan synthesis via ReoM is required for cell wall stress responses, cytosolic survival, and virulence in Listeria monocytogenes

Pathogenic bacteria rely on protein phosphorylation to adapt quickly to stress, including that imposed by the host during infection. Penicillin-binding protein and serine/threonine-associated (PASTA) kinases are signal transduction systems that sense cell wall integrity and modulate multiple facets of bacterial physiology in response to cell envelope stress. The PASTA kinase in the cytosolic pathogen Listeria monocytogenes, PrkA, is required for cell wall stress responses, cytosolic survival, and virulence, yet its substrates and downstream signaling pathways remain incompletely defined. We combined orthogonal phosphoproteomic and genetic analyses in the presence of a β-lactam antibiotic to define PrkA phosphotargets and pathways modulated by PrkA. These analyses synergistically highlighted ReoM, which was recently identified as a PrkA target that influences peptidoglycan (PG) synthesis, as an important phosphosubstrate during cell wall stress. We find that deletion of reoM restores cell wall stress sensitivities and cytosolic survival defects of a ΔprkA mutant to nearly wild-type levels. While a ΔprkA mutant is defective for PG synthesis during cell wall stress, a double ΔreoM ΔprkA mutant synthesizes PG at rates similar to wild type. In a mouse model of systemic listeriosis, deletion of reoM in a ΔprkA background almost fully restored virulence to wild-type levels. However, loss of reoM alone also resulted in attenuated virulence, suggesting ReoM is critical at some points during pathogenesis. Finally, we demonstrate that the PASTA kinase/ReoM cell wall stress response pathway is conserved in a related pathogen, methicillin-resistant Staphylococcus aureus. Taken together, our phosphoproteomic analysis provides a comprehensive overview of the PASTA kinase targets of an important model pathogen and suggests that a critical role of PrkA in vivo is modulating PG synthesis through regulation of ReoM to facilitate cytosolic survival and virulence.

Many antibiotics target bacterial cell wall biosynthesis, justifying continued study of this process and the ways bacteria respond to cell wall insults during infection. Penicillin-binding protein and serine/threonine-associated (PASTA) kinases are master regulators of cell wall stress responses in bacteria and are conserved in several major pathogens, including Listeria monocytogenes, Staphylococcus aureus, and Mycobacterium tuberculosis. We previously showed that the PASTA kinase in L. monocytogenes, PrkA, is essential for the response to cell wall stress and for virulence. In this work, we combined proteomic and genetic approaches to identify PrkA substrates in L. monocytogenes. We show that regulation of one candidate from both screens, ReoM, increases synthesis of the cell wall component peptidoglycan and that this regulation is required for pathogenesis. We also demonstrate that the PASTA kinase-ReoM pathway regulates cell wall stress responses in another significant pathogen, methicillin-resistant S. aureus. Additionally, we uncover a PrkA-independent role for ReoM in vivo in L. monocytogenes, suggesting a need for nuanced modulation of peptidoglycan synthesis during infection. Cumulatively, this study provides new insight into how bacterial pathogens control cell wall synthesis during infection.

Human Placental Trophoblasts Infected by Listeria monocytogenes Undergo a Pro-Inflammatory Switch Associated With Poor Pregnancy Outcomes

The placenta controls the growth of the fetus and ensures its immune protection. Key to these functions, the syncytiotrophoblast (SYN) is a syncytium formed by fusion of underlying mononuclear trophoblasts. The SYN covers the placental surface and is bathed in maternal blood to mediate nutritional and waste exchanges between the mother and fetus. The bacterial pathogen Listeria monocytogenes breaches the trophoblast barrier and infects the placental/fetal unit resulting in poor pregnancy outcomes. In this work, we analyzed the L. monocytogenes intracellular lifecycle in primary human trophoblasts. In accordance with previous studies, we found that the SYN is 20-fold more resistant to infection compared to mononuclear trophoblasts, forming a protective barrier to infection at the maternal interface. We show for the first time that this is due to a significant reduction in L. monocytogenes uptake by the SYN rather than inhibition of the bacterial intracellular division or motility. We here report the first transcriptomic analysis of L. monocytogenes-infected trophoblasts (RNA sequencing). Pathway analysis showed that infection upregulated TLR2, NOD-like, and cytosolic DNA sensing pathways, as well as downstream pro-inflammatory circuitry (NF-κB, AP-1, IRF4, IRF7) leading to the production of mediators known to elicit the recruitment and activation of maternal leukocytes (IL8, IL6, TNFα, MIP-1). Signature genes associated with poor pregnancy outcomes were also upregulated upon infection. Measuring the release of 54 inflammatory mediators confirmed the transcriptomic data and revealed sustained production of tolerogenic factors (IL-27, IL-10, IL-1RA, TSLP) despite infection. Both the SYN and mononuclear trophoblasts produced cytokines, but surprisingly, some cytokines were predominantly produced by the SYN (IL-8, IL-6) or by non-fused trophoblasts (TNFα). Collectively, our data support that trophoblasts act as placental gatekeepers that limit and detect L. monocytogenes infection resulting in a pro-inflammatory response, which may contribute to the poor pregnancy outcomes if the pathogen persists.

Active Lysogeny in Listeria Monocytogenes Is a Bacteria-Phage Adaptive Response in the Mammalian Environment

Some Listeria monocytogenes (Lm) strains harbor a prophage within the comK gene, which renders it inactive. During Lm infection of macrophage cells, the prophage turns into a molecular switch, promoting comK gene expression and therefore Lm intracellular growth. During this process, the prophage does not produce infective phages or cause bacterial lysis, suggesting it has acquired an adaptive behavior suited to the pathogenic lifestyle of its host. In this study, we demonstrate that this non-classical phage behavior, named active lysogeny, relies on a transcriptional response that is specific to the intracellular niche. While the prophage undergoes lytic induction, the process is arrested midway, preventing the transcription of the late genes. Further, we demonstrate key phage factors, such as LlgA transcription regulator and a DNA replicase, that support the phage adaptive behavior. This study provides molecular insights into the adaptation of phages to their pathogenic hosts, uncovering unusual cooperative interactions.

Biofilm Formation by Listeria monocytogenes 15G01, a Persistent Isolate from a Seafood-Processing Plant, Is Influenced by Inactivation of Multiple Genes Belonging to Different Functional Groups

Listeria monocytogenes is a ubiquitous foodborne pathogen that results in a high rate of mortality in sensitive and immunocompromised people. Contamination of food with L. monocytogenes is thought to occur during food processing, most often as a result of the pathogen producing a biofilm that persists in the environment and acting as the source for subsequent dispersal of cells onto food. A survey of seafood-processing plants in New Zealand identified the persistent strain 15G01, which has a high capacity to form biofilms. In this study, a transposon library of L. monocytogenes 15G01 was screened for mutants with altered biofilm formation, assessed by a crystal violet assay, to identify genes involved in biofilm formation. This screen identified 36 transposants that showed a significant change in biofilm formation compared to the wild type. The insertion sites were in 27 genes, 20 of which led to decreased biofilm formation and seven to an increase. Two insertions were in intergenic regions. Annotation of the genes suggested that they are involved in diverse cellular processes, including stress response, autolysis, transporter systems, and cell wall/membrane synthesis. Analysis of the biofilms produced by the transposants using scanning electron microscopy and fluorescence microscopy showed notable differences in the structure of the biofilms compared to the wild type. In particular, inactivation of uvrB and mltD produced coccoid-shaped cells and elongated cells in long chains, respectively, and the mgtB mutant produced a unique biofilm with a sandwich structure which was reversed to the wild-type level upon magnesium addition. The mltD transposant was successfully complemented with the wild-type gene, whereas the phenotypes were not or only partially restored for the remaining mutants.IMPORTANCE The major source of contamination of food with Listeria monocytogenes is thought to be due to biofilm formation and/or persistence in food-processing plants. By establishing as a biofilm, L. monocytogenes cells become harder to eradicate due to their increased resistance to environmental threats. Understanding the genes involved in biofilm formation and their influence on biofilm structure will help identify new ways to eliminate harmful biofilms in food processing environments. To date, multiple genes have been identified as being involved in biofilm formation by L. monocytogenes; however, the exact mechanism remains unclear. This study identified four genes associated with biofilm formation by a persistent strain. Extensive microscopic analysis illustrated the effect of the disruption of mgtB, clsA, uvrB, and mltD and the influence of magnesium on the biofilm structure. The results strongly suggest an involvement in biofilm formation for the four genes and provide a basis for further studies to analyze gene regulation to assess the specific role of these biofilm-associated genes.

Identification of Listeria monocytogenes Genes Contributing to Oxidative Stress Resistance under Conditions Relevant to Host Infection

The Gram-positive bacterium Listeria monocytogenes survives in environments ranging from the soil to the cytosol of infected host cells. Key to L. monocytogenes intracellular survival is the activation of PrfA, a transcriptional regulator that is required for the expression of multiple bacterial virulence factors. Mutations that constitutively activate prfA (prfA* mutations) result in high-level expression of multiple bacterial virulence factors as well as the physiological adaptation of L. monocytogenes for optimal replication within host cells. Here, we demonstrate that L. monocytogenes prfA* mutants exhibit significantly enhanced resistance to oxidative stress in comparison to that of wild-type strains. Transposon mutagenesis of L. monocytogenes prfA* strains resulted in the identification of three novel gene targets required for full oxidative stress resistance only in the context of PrfA activation. One gene, lmo0779, predicted to encode an uncharacterized protein, and two additional genes known as cbpA and ygbB, encoding a cyclic di-AMP binding protein and a 2-C-methyl-d-erythritol 2,4-cyclodiphosphate synthase, respectively, contribute to the enhanced oxidative stress resistance of prfA* strains while exhibiting no significant contribution in wild-type L. monocytogenes Transposon inactivation of cbpA and lmo0779 in a prfA* background led to reduced virulence in the liver of infected mice. These results indicate that L. monocytogenes calls upon specific bacterial factors for stress resistance in the context of PrfA activation and thus under conditions favorable for bacterial replication within infected mammalian cells.

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.

Listeria monocytogenes Infection of Bat Pipistrellus nathusii Epithelial cells Depends on the Invasion Factors InlA and InlB

L. monocytogenes is a widespread facultative intracellular pathogen. The range of natural hosts that supporting L. monocytogenes persistence in the environment has not been fully established yet. In this study, we were interested in the potential of L. monocytogenes to infect cells of bats, which are being increasingly recognized as a reservoir for microorganisms that are pathogenic to humans and domestic animals. A stable epithelial cell line was developed from the kidneys of Pipistrellus nathusii, a small bat widely distributed across Europe. The wild-type L. monocytogenes strain EGDe infected this cell line with an invasion efficiency of 0.0078 ± 0.0009%. Once it entered bat cells, L. monocytogenes doubled within about 70 min. When L. monocytogenes lacked either of the major invasion factors, InlA and InlB, invasion efficiency decreased by a factor of 10 and 25 respectively (p < 0.000001). The obtained results suggest that bat epithelial cells are susceptible to L. monocytogenes infection and that L. monocytogenes invasion of bat cells depends on the major invasion factors InlA and InlB. These results constitute the first report on in vitro studies of L. monocytogenes infection in bats.

Innate Immune Mechanisms to Protect Against Infection at the Human Decidual-Placental Interface

During pregnancy, the placenta forms the anatomical barrier between the mother and developing fetus. Infectious agents can potentially breach the placental barrier resulting in pathogenic transmission from mother to fetus. Innate immune responses, orchestrated by maternal and fetal cells at the decidual-placental interface, are the first line of defense to avoid vertical transmission. Here, we outline the anatomy of the human placenta and uterine lining, the decidua, and discuss the potential capacity of pathogen pattern recognition and other host defense strategies present in the innate immune cells at the placental-decidual interface. We consider major congenital infections that access the placenta from hematogenous or decidual route. Finally, we highlight the challenges in studying human placental responses to pathogens and vertical transmission using current experimental models and identify gaps in knowledge that need to be addressed. We further propose novel experimental strategies to address such limitations.

Neurolisteriosis in a previously asymptomatic patient with serum IgM deficiency: a case report

BACKGROUND

Listeria monocytogenes is an opportunistic pathogen of the central nervous system commonly associated with impaired cell-mediated immunity. We hereby present a case of adult neurolisteriosis where the only immunological feature persistently present was serum IgM deficiency, suggesting that non-specific humoral immunity may also play a central role in the control of neuroinvasion by Listeria monocytogenes.

CASE PRESENTATION

A 62-year-old male who had never experienced severe infections presented with headache, nuchal rigidity and confusion. Neuroimaging was normal and lumbar puncture revealed pleiocytosis (760 leukocytes/mm3) and hypoglycorrhachia (34 mg/dL). The patient was treated empirically for bacterial meningitis. Indeed, further analysis of the CSF showed infection by Listeria monocytogenes, which was accompanied by reduced serum IgM levels that persisted well beyond the period of acute bacterial infection. Levels of IgG and IgA isotypes, along with peripheral blood counts of major leukocyte subsets, were at the same time largely preserved. Intriguingly, the absence of membrane-bound IgM on B cells was essentially complete in the acute post-infection period leading to a remarkable recovery after 12 months, suggesting that mechanisms other than defective membrane expression are underlying serum deficiency.

CONCLUSIONS

As far as we know, this is the first reported case of neurolisteriosis associated with IgM deficiency in an adult individual without a history of severe infections or other underlying conditions. A possible role of circulating IgM against invasive disease caused by Listeria monocytogenes, particularly in the early course of host-pathogen interaction, is discussed.

Atomic Force Microscopy Investigation of the Contributions of Listeria monocytogenes Cell-Wall Biomacromolecules to Their Adherence and Mechanics

In this work, the contributions of the pathogenic Listeria monocytogenes cell-wall biomacromolecules to the bacterial mechanics and adhesion to a model inert surface of silicon nitride in water were investigated by atomic force microscopy. Chemical ethylenediaminetetraacetic acid (EDTA) and biological enzymatic trypsin treatments of cells were performed to partially or totally remove the bacterial cell-wall proteins and carbohydrates. Removal of 48.2% proteins and 29.2% of carbohydrates from the cell-wall of the bacterium by the EDTA treatment resulted in a significant decrease in the length of the bacterial cell-wall biomacromolecules and an increase in the rigidity of the bacterial cells as predicted from fitting a model of steric repulsion to the force-distance approach data and classic Hertz model to the indentation-force data, respectively. In comparison, removal of almost all the cell-wall proteins (99.5% removal) and 8.6% of cell-wall carbohydrates by the trypsin treatment resulted in an increase in the elasticity of the bacterial cells, an increase in the extension of the cell-wall biomacromolecules, and a significant decrease in their apparent grafting densities. In addition, adhesion strength of native-untreated L. monocytogenes to silicon nitride in water decreased by 30% on average after the EDTA treatment and further decreased by 60% on average after the trypsin treatment, showing a positive correlation with the% removal of cell-wall proteins by the EDTA and trypsin treatments, respectively.

Acute neonatal Listeria monocytogenes infection causes long-term, organ-specific changes in immune cell subset composition

Listeria monocytogenes (Lm) is a food-borne pathogen with a high chance of infecting neonates, pregnant women, elderly and immunocompromised individuals. Lm infection in neonates can cause neonatal meningitis and sepsis with a high risk of severe neurological and developmental sequelae and high mortality rates. However, whether an acute neonatal Lm infection causes long-term effects on the immune system persisting until adulthood has not been fully elucidated. Here, we established a neonatal Lm infection model and monitored the composition of major immune cell subsets at defined time points post infection (p.i.) in secondary lymphoid organs and the intestine. Twelve weeks p.i., the CD8⁺ T cell population was decreased in colon and mesenteric lymph nodes (mLNs) with an opposing increase in the spleen. In the colon, we observed an accumulation of CD4⁺ and CD8⁺ effector/memory T cells with an increase of T-bet⁺ T helper 1 (Th1) cells. In addition, 12 weeks p.i. an altered composition of innate lymphoid cell (ILC) and dendritic cell (DC) subsets was still observed in colon and mLNs, respectively. Together, these findings highlight organ-specific long-term consequences of an acute neonatal Lm infection on both the adaptive and innate immune system.

Listeria monocytogenes Response to Anaerobic Environments

Listeria monocytogenes is a Gram-positive facultative anaerobic bacterium that is responsible for the disease, listeriosis. It is particularly lethal in pregnant women, the fetus, the elderly and the immunocompromised. The pathogen survives and replicates over a wide range of temperatures (4 to 42 °C), pH, salt and oxygen concentrations. Because it can withstand various environments, L. monocytogenes is a major concern in food processing industries, especially in dairy products and ready-to-eat fruits, vegetables and deli meats. The environment in which the pathogen is exposed can influence the expression of virulence genes. For instance, studies have shown that variations in oxygen availability can impact resistance to stressors. Further investigation is needed to understand the essential genes required for the growth of L. monocytogenes in anaerobic conditions. Therefore, the purpose of this review is to highlight the data on L. monocytogenes under known environmental stresses in anaerobic environments and to focus on gaps in knowledge that may be advantageous to study in order to better understand the pathogenicity of the bacterium.

Susceptibility of aging mice to listeriosis: Role of anti-inflammatory responses with enhanced Treg-cell expression of CD39/CD73 and Th-17 cells

Foodborne Listeria monocytogenes (Lm) causes serious illness and death in immunosuppressed hosts, including the elderly population. We investigated Lm susceptibility and inflammatory cytokines in geriatric mice. Young-adult and old mice were gavaged with a Lm strain Lmo-InlA^m. Tissues were assayed for Lm burden and splenocytes were analyzed for Th1/Th2/Th17/Treg responses and expression of CD39 and CD73. Old Lm-infected mice lost body-weight dose-dependently, had higher Lm colonization, and showed higher inflammatory responses than Lm-infected young-adult mice. After infection, IL-17 levels increased significantly in old mice whereas IFN-γ levels were unchanged. Levels of IL-10 and Treg cells were increased in infected old mice as compared to infected young-adult mice. Age-dependent enhanced expression of CD39/CD73 was observed in purified Treg prior to infection, suggesting increased baseline adenosine production in old mice. Lm lysate-treated splenocytes from older mice produced significantly higher levels of IL-10, IL17, and IL-1β, produced less IFN-γ and IL-2, and proliferated less than splenocytes from young-adult mice. Data suggests that older mice maybe more susceptible to Lm infection due to an imbalance of Th cell responses with disproportionate and persistent anti-inflammatory responses. Lm infection enhanced differentiation of proinflammatory Th17 cells, which may also exacerbate pathological responses during listeriosis.

Listeriosis in Pregnancy: A Review

Importance

Listeriosis is a rare foodborne condition that can cause serious health consequences in neonates and pregnant women. Listeria monocytogenes can be vertically transmitted to the fetus, resulting in adverse maternal, fetal, and neonatal outcomes. In this review, we summarize what is currently known about listeriosis in pregnancy and review the current management and treatment recommendations.

Objective

To summarize the clinical and relevant evidence available regarding listeriosis in pregnancy and educate providers on common clinical symptoms, sequelae, and appropriate treatment guidelines.

Evidence Acquisition

A PubMed review was conducted using search terms "pregnancy" OR "Listeria" OR "Maternal Listeriosis," and "Neonatal Listeriosis." The search included review articles, original research articles, and guidelines on diagnosis and management of listeriosis in pregnancy. The search was limited to the English language and publications between 1988 and July 2018.

Conclusions

Listeriosis in pregnancy can result in severe adverse maternal, fetal, and neonatal outcomes including miscarriage, preterm labor, fetal death, and neonatal meningitis and sepsis. Early treatment has been shown to improve fetal and neonatal outcomes; therefore, prevention with education and early diagnosis prompting treatment will improve overall outcomes.

Relevance

The incidence of listeriosis is expected to rise in the coming years due to changes in the US population, with increasing numbers of older Americans and Hispanic individuals, both of whom are at higher risk. Pregnant women contract listeriosis at a rate that is 16- to 18-fold greater than the general population. Given the expected increased rise in incidence and increased susceptibility of pregnant women, understanding the common clinical symptoms, maternal and fetal sequelae, and appropriate treatment guidelines is essential.

Coordination of cohabiting phage elements supports bacteria-phage cooperation

Bacterial pathogens often carry multiple prophages and other phage-derived elements within their genome, some of which can produce viral particles in response to stress. Listeria monocytogenes 10403S harbors two phage elements in its chromosome, both of which can trigger bacterial lysis under stress: an active prophage (ϕ10403S) that promotes the virulence of its host and can produce infective virions, and a locus encoding phage tail-like bacteriocins. Here, we show that the two phage elements are co-regulated, with the bacteriocin locus controlling the induction of the prophage and thus its activity as a virulence-associated molecular switch. More specifically, a metalloprotease encoded in the bacteriocin locus is upregulated in response to stress and acts as an anti-repressor for CI-like repressors encoded in each phage element. Our results provide molecular insight into the phenomenon of polylysogeny and its intricate adaptation to complex environments.

STAT4 Directs a Protective Innate Lymphoid Cell Response to Gastrointestinal Infection

Innate lymphoid cells (ILCs) are strategically positioned at mucosal barrier surfaces where they respond quickly to infection or injury. Therefore, we hypothesized that ILCs are key contributors to the early immune response in the intestine against Listeria monocytogenes Using a modified strain of L. monocytogenes that mimics human gastrointestinal listeriosis in mice, we find ILCs to be essential for control of early replication of L. monocytogenes in the intestine as well as for restricted dissemination of bacteria to peripheral tissues. Specifically, group 1 ILCs (ILC1s) and group 3 ILCs (ILC3s) respond to infection with proliferation and IFN-γ and IL-22 production. Mechanistically, we show that the transcription factor STAT4 is required for the proliferative and IFN-γ effector response by ILC1s and ILC3s, and loss of STAT4 signaling in the innate immune compartment results in an inability to control bacterial growth and dissemination. Interestingly, STAT4 acts acutely as a transcription factor to promote IFN-γ production. Together, these data illustrate a critical role for ILCs in the early responses to gastrointestinal infection with L. monocytogenes and identify STAT4 as a central modulator of ILC-mediated protection.

Phylogenetically Defined Isoforms of Listeria monocytogenes Invasion Factor InlB Differently Activate Intracellular Signaling Pathways and Interact with the Receptor gC1q-R

The pathogenic Gram-positive bacterium Listeria monocytogenes has been evolving into a few phylogenetic lineages. Phylogenetically defined substitutions were described in the L. monocytogenes virulence factor InlB, which mediates active invasion into mammalian cells via interactions with surface receptors c-Met and gC1q-R. InlB internalin domain (idInlB) is central to interactions with c-Met. Here we compared activity of purified recombinant idInlB isoforms characteristic for L. monocytogenes phylogenetic lineage I and II. Size exclusion chromatography and intrinsic fluorescence were used to characterize idInlBs. Western blotting was used to study activation of c-Met-dependent MAPK- and PI3K/Akt-pathways. Solid-phase microplate binding and competition assay was used to quantify interactions with gCq1-R. Isogenic recombinant L. monocytogenes strains were used to elucidate the input of idInlB isoforms in HEp-2 cell invasion. Physicochemical parameters of idInlB isoforms were similar but not identical. Kinetics of Erk1/2 and Akt phosphorylation in response to purified idInlBs was lineage specific. Lineage I but not lineage II idInlB specifically bound gC1q-R. Antibody against gC1q-R amino acids 221–249 inhibited invasion of L. monocytogenes carrying lineage I but not lineage II idInlB. Taken together, obtained results suggested that phylogenetically defined substitutions in idInlB provide functional distinctions and might be involved in phylogenetically determined differences in virulence potential.

Comprehensive proteomic analysis and pathogenic role of membrane vesicles of Listeria monocytogenes serotype 4b reveals proteins associated with virulence and their possible interaction with host

Membrane vesicles (MVs) are produced by various Gram positive and Gram negative pathogenic bacteria and play an important role in virulence. In this study, the membrane vesicles (MVs) of L. monocytogenes were isolated from the culture supernatant. High-resolution electron microscopy and dynamic light scattering analysis revealed that L. monocytogenes MVs are spherical with a diameter of 200 to 300 nm in size. Further, comprehensive proteomic analyses of MVs and whole cells of L. monocytogenes were performed using LC/MS/MS. A total of 1355 and 312 proteins were identified in the L. monocytogenes cells and MVs, respectively. We identified that 296 proteins are found in both whole cells, and MV proteome and 16 proteins were identified only in the MVs. Also, we have identified the virulence factors such as listeriolysin O (LLO), internalin B (InlB), autolysin, p60, NLP/P60 family protein, UPF0356 protein, and PLC-A in MVs. Computational prediction of host-MV interactions revealed a total of 1841 possible interactions with the host involving 99 MV proteins and 1513 host proteins. We elucidated the possible pathway that mediates internalization of L. monocytogenes MV to host cells and the subsequent pathogenesis mechanisms. The in vitro infection assays showed that the purified MVs could induce cytotoxicity in Caco-2 cells. Using endocytosis inhibitors, we demonstrated that MVs are internalized via actin-mediated endocytosis. These results suggest that L. monocytogenes MVs can interact with host cell and contribute to the pathogenesis of L. monocytogenes during infection.

Listeria monocytogenes virulence factors, including listeriolysin O, are secreted in biologically active extracellular vesicles

Outer membrane vesicles produced by Gram-negative bacteria have been studied for half a century but the possibility that Gram-positive bacteria secrete extracellular vesicles (EVs) was not pursued until recently due to the assumption that the thick peptidoglycan cell wall would prevent their release to the environment. However, following their discovery in fungi, which also have cell walls, EVs have now been described for a variety of Gram-positive bacteria. EVs purified from Gram-positive bacteria are implicated in virulence, toxin release, and transference to host cells, eliciting immune responses, and spread of antibiotic resistance. Listeria monocytogenes is a Gram-positive bacterium that causes listeriosis. Here we report that L. monocytogenes produces EVs with diameters ranging from 20 to 200 nm, containing the pore-forming toxin listeriolysin O (LLO) and phosphatidylinositol-specific phospholipase C (PI-PLC). Cell-free EV preparations were toxic to mammalian cells, the murine macrophage cell line J774.16, in a LLO-dependent manner, evidencing EV biological activity. The deletion of plcA increased EV toxicity, suggesting PI-PLC reduced LLO activity. Using simultaneous metabolite, protein, and lipid extraction (MPLEx) multiomics we characterized protein, lipid, and metabolite composition of bacterial cells and secreted EVs and found that EVs carry the majority of listerial virulence proteins. Using immunogold EM we detected LLO at several organelles within infected human epithelial cells and with high-resolution fluorescence imaging we show that dynamic lipid structures are released from L. monocytogenes during infection. Our findings demonstrate that L. monocytogenes uses EVs for toxin release and implicate these structures in mammalian cytotoxicity.

Pregnancy-related listeriosis

About one in seven cases of listeriosis occurs in pregnant women, and, although listeriosis is rare, it is the third leading cause of death from food-borne infections. Pregnancy-related listeriosis increases the risk for fetal and neonatal mortality by approximately 21%. During pregnancy, infections are more likely to occur in the third trimester (66%) than the first trimester (3%). However, fetal and neonatal adverse effects are less common as gestational age increases or with older gestational age at birth. Pregnant women seem highly susceptible in some listeriosis outbreaks making up a large percentage of cases, whereas others contain very few. Whether this results from differences in strains of L. monocytogenes, exposures, or other factors remains to be determined. Food and Agriculture Organization of United Nations/World Health Organization (FAO/WHO) estimates the human lethal dose for 50% (LD₅₀ ) for fetal/neonatal loss is 1.9 × 10⁶ colony forming units (CFUs) L. monocytogenes. Animal models have been developed for pregnancy-related listeriosis showing similar susceptibility and clinical outcomes as in humans. Nonhuman primate and guinea pig animal models have similar (LD₅₀ ) values to the estimated human LD₅₀ . Additional animal studies are needed to understand the pathways leading to fetal and neonatal listeriosis in humans. More information is needed to understand dose response, to model risk for listeriosis at lower concentrations, and to determine why some pregnant women may be more susceptible than others. To better treat listeriosis during pregnancy, biomarkers for early diagnosis of listeriosis are also needed. Last, pregnant women need to be educated about avoiding high-risk foods, like Mexican-style cheese and ready-to-eat meats. Birth Defects Research 109:324-335, 2017.© 2017 Wiley Periodicals, Inc.

Loss of Actin-Based Motility Impairs Ectromelia Virus Release In Vitro but Is Not Critical to Spread In Vivo

Ectromelia virus (ECTV) is an orthopoxvirus and the causative agent of mousepox. Like other poxviruses such as variola virus (agent of smallpox), monkeypox virus and vaccinia virus (the live vaccine for smallpox), ECTV promotes actin-nucleation at the surface of infected cells during virus release. Homologs of the viral protein A36 mediate this function through phosphorylation of one or two tyrosine residues that ultimately recruit the cellular Arp2/3 actin-nucleating complex. A36 also functions in the intracellular trafficking of virus mediated by kinesin-1. Here, we describe the generation of a recombinant ECTV that is specifically disrupted in actin-based motility allowing us to examine the role of this transport step in vivo for the first time. We show that actin-based motility has a critical role in promoting the release of virus from infected cells in vitro but plays a minor role in virus spread in vivo. It is likely that loss of microtubule-dependent transport is a major factor for the attenuation observed when A36R is deleted.

Pregnancy and infection: using disease pathogenesis to inform vaccine strategy

Vaccination is the mainstay of preventative medicine for many infectious diseases. Pregnant women, unborn fetuses, and neonates represent three at-risk populations that can be simultaneously protected by strategic vaccination protocols. Because the pathogenesis of different infectious microbes varies based on tissue tropism, timing of infection, and host susceptibility, the goals of immunization are not uniform across all vaccines. Mechanistic understanding of infectious disease pathogenesis and immune responses is therefore essential to inform vaccine design and the implementation of appropriate immunization protocols that optimize protection of pregnant women, fetuses, and neonates.

Role of intestinal flora in colorectal cancer from the metabolite perspective: a systematic review

Colorectal cancer is one of the most common human malignant tumors. Recent research has shown that colorectal cancer is a dysbacteriosis-induced disease; however, the role of intestinal bacteria in colorectal cancer is unclear. This review explores the role of intestinal flora in colorectal cancer. In total, 57 articles were included after identification and screening. The pertinent literature on floral metabolites in colorectal cancer from three metabolic perspectives - including carbohydrate, lipid, and amino acid metabolism - was analyzed. An association network regarding the role of intestinal flora from a metabolic perspective was constructed by analyzing the previous literature to provide direction and insight for further research on intestinal flora in colorectal cancer.

Listeria monocytogenes contamination of ready-to-eat foods and the risk for human health in the EU

Food safety criteria for Listeria monocytogenes in ready-to-eat (RTE) foods have been applied from 2006 onwards (Commission Regulation (EC) 2073/2005). Still, human invasive listeriosis was reported to increase over the period 2009-2013 in the European Union and European Economic Area (EU/EEA). Time series analysis for the 2008-2015 period in the EU/EEA indicated an increasing trend of the monthly notified incidence rate of confirmed human invasive listeriosis of the over 75 age groups and female age group between 25 and 44 years old (probably related to pregnancies). A conceptual model was used to identify factors in the food chain as potential drivers for L. monocytogenes contamination of RTE foods and listeriosis. Factors were related to the host (i. population size of the elderly and/or susceptible people; ii. underlying condition rate), the food (iii. L. monocytogenes prevalence in RTE food at retail; iv. L. monocytogenes concentration in RTE food at retail; v. storage conditions after retail; vi. consumption), the national surveillance systems (vii. improved surveillance), and/or the bacterium (viii. virulence). Factors considered likely to be responsible for the increasing trend in cases are the increased population size of the elderly and susceptible population except for the 25-44 female age group. For the increased incidence rates and cases, the likely factor is the increased proportion of susceptible persons in the age groups over 45 years old for both genders. Quantitative modelling suggests that more than 90% of invasive listeriosis is caused by ingestion of RTE food containing > 2,000 colony forming units (CFU)/g, and that one-third of cases are due to growth in the consumer phase. Awareness should be increased among stakeholders, especially in relation to susceptible risk groups. Innovative methodologies including whole genome sequencing (WGS) for strain identification and monitoring of trends are recommended.

Route of Injection Affects the Impact of InlB Internalin Domain Variants on Severity of Listeria monocytogenes Infection in Mice

The facultative intracellular pathogen Listeria monocytogenes causes a severe food-borne infection in humans and animals. L. monocytogenes invasion factor InlB interacts with the tyrosine kinase c-Met via the N-terminal internalin domain. Previously, distinct variants of the InlB internalin domain (idInlB) have been described in L. monocytogenes field isolates. Three variants were used to restore full-length InlB expression in the L. monocytogenes strain EGDeΔinlB. Obtained isogenic L. monocytogenes strains were tested in the invasion assay and intravenous, intraperitoneal, and intragastric models of infection in mice. All idInlBs were functional, restored InlB activity as an invasion factor, and improved invasion of the parental strain EGDeΔinlB into human kidney HEK23 cells. Meanwhile, distinct idInlBs provided different mortality rates and bacterial loads in internal organs. When recombinant strains were compared, the variant designated idInlB14 decreased severity of disease caused by intravenous and intraperitoneal bacterial administration, whereas this variant improved intestine colonization and stimulated intragastric infection. Obtained results demonstrated that naturally occurring idInlBs differed in their impact on severity of L. monocytogenes infection in mice in dependence on the infection route.

Multifaceted Defense against Listeria monocytogenes in the Gastro-Intestinal Lumen

Listeria monocytogenes is a foodborne pathogen that can cause febrile gastroenteritis in healthy subjects and systemic infections in immunocompromised individuals. Despite the high prevalence of L. monocytogenes in the environment and frequent contamination of uncooked meat and poultry products, infections with this pathogen are relatively uncommon, suggesting that protective defenses in the general population are effective. In the mammalian gastrointestinal tract, a variety of defense mechanisms prevent L. monocytogenes growth, epithelial penetration and systemic dissemination. Among these defenses, colonization resistance mediated by the gut microbiota is crucial in protection against a range of intestinal pathogens, including L. monocytogenes. Here we review defined mechanisms of defense against L. monocytogenes in the lumen of the gastro-intestinal tract, with particular emphasis on protection conferred by the autochthonous microbiota. We suggest that selected probiotic species derived from the microbiota may be developed for eventual clinical use to enhance resistance against L. monocytogenes infections.

Real-Time Whole-Genome Sequencing for Surveillance of Listeria monocytogenes, France

During 2015–2016, we evaluated the performance of whole-genome sequencing (WGS) as a routine typing tool. Its added value for microbiological and epidemiologic surveillance of listeriosis was compared with that for pulsed-field gel electrophoresis (PFGE), the current standard method. A total of 2,743 Listeria monocytogenes isolates collected as part of routine surveillance were characterized in parallel by PFGE and core genome multilocus sequence typing (cgMLST) extracted from WGS. We investigated PFGE and cgMLST clusters containing human isolates. Discrimination of isolates was significantly higher by cgMLST than by PFGE (p<0.001). cgMLST discriminated unrelated isolates that shared identical PFGE profiles and phylogenetically closely related isolates with distinct PFGE profiles. This procedure also refined epidemiologic investigations to include only phylogenetically closely related isolates, improved source identification, and facilitated epidemiologic investigations, enabling identification of more outbreaks at earlier stages. WGS-based typing should replace PFGE as the primary typing method for L. monocytogenes.

Commensal microbes provide first line defense against Listeria monocytogenes infection

Becattini et al. provide evidence that a diverse gut microbiota antagonizes the foodborne pathogen Listeria monocytogenes in the intestinal lumen, thereby reducing bloodstream invasion. Microbiota perturbation by antibiotic treatment increases susceptibility to listeriosis, with dramatic effects in immunocompromised hosts.

Listeria monocytogenes is a foodborne pathogen that causes septicemia, meningitis and chorioamnionitis and is associated with high mortality. Immunocompetent humans and animals, however, can tolerate high doses of L. monocytogenes without developing systemic disease. The intestinal microbiota provides colonization resistance against many orally acquired pathogens, and antibiotic-mediated depletion of the microbiota reduces host resistance to infection. Here we show that a diverse microbiota markedly reduces Listeria monocytogenes colonization of the gut lumen and prevents systemic dissemination. Antibiotic administration to mice before low dose oral inoculation increases L. monocytogenes growth in the intestine. In immunodeficient or chemotherapy-treated mice, the intestinal microbiota provides nonredundant defense against lethal, disseminated infection. We have assembled a consortium of commensal bacteria belonging to the Clostridiales order, which exerts in vitro antilisterial activity and confers in vivo resistance upon transfer into germ free mice. Thus, we demonstrate a defensive role of the gut microbiota against Listeria monocytogenes infection and identify intestinal commensal species that, by enhancing resistance against this pathogen, represent potential probiotics.

Adhesion and Invasion Assay Procedure Using Caco-2 Cells for Listeria monocytogenes

Listeria monocytogenes is an important Gram-positive foodborne pathogen that is a particular problem in ready-to-eat food. It has an ability to survive in harsh conditions like refrigeration temperatures and high salt concentrations and is known to cross intestinal, placental and blood-brain barriers. Several cancerous cell lines like cervical, liver, dendritic, intestinal and macrophages have been used to study in vitro propagation and survival of listeria in human cells. Human intestinal epithelial cells have been used to study how listeria crosses the intestinal barrier and cause infection. The protocol in this articles describes the procedures to grow Caco-2 cells, maintain cells and use them for adhesion and invasion assays. During adhesion assay the cells are incubated with listeria for 30 min but in invasion assay the cell growth is arrested at several time points after infection to monitor the growth and survival rate of listeria in cells.

A critical assessment of the "sterile womb" and "in utero colonization" hypotheses: implications for research on the pioneer infant microbiome

After more than a century of active research, the notion that the human fetal environment is sterile and that the neonate's microbiome is acquired during and after birth was an accepted dogma. However, recent studies using molecular techniques suggest bacterial communities in the placenta, amniotic fluid, and meconium from healthy pregnancies. These findings have led many scientists to challenge the "sterile womb paradigm" and propose that microbiome acquisition instead begins in utero, an idea that would fundamentally change our understanding of gut microbiota acquisition and its role in human development. In this review, we provide a critical assessment of the evidence supporting these two opposing hypotheses, specifically as it relates to (i) anatomical, immunological, and physiological characteristics of the placenta and fetus; (ii) the research methods currently used to study microbial populations in the intrauterine environment; (iii) the fecal microbiome during the first days of life; and (iv) the generation of axenic animals and humans. Based on this analysis, we argue that the evidence in support of the "in utero colonization hypothesis" is extremely weak as it is founded almost entirely on studies that (i) used molecular approaches with an insufficient detection limit to study "low-biomass" microbial populations, (ii) lacked appropriate controls for contamination, and (iii) failed to provide evidence of bacterial viability. Most importantly, the ability to reliably derive axenic animals via cesarean sections strongly supports sterility of the fetal environment in mammals. We conclude that current scientific evidence does not support the existence of microbiomes within the healthy fetal milieu, which has implications for the development of clinical practices that prevent microbiome perturbations after birth and the establishment of future research priorities.

Iron acquisition through the bacterial transferrin receptor

Transferrin is one of the sources of iron that is most readily available to colonizing and invading pathogens. In this review, we look at iron uptake by the bacterial transferrin receptor that is found in the families Neisseriaceae, Pasteurellaceae and Moraxellaceae. This bipartite receptor consists of the TonB-dependent transporter, TbpA, and the surface lipoprotein, TbpB. In the past three decades, major advancements have been made in our understanding of the mechanism through which the Tbps take up iron. We summarize these findings and discuss how they relate to the diversity and specificity of the transferrin receptor. We also outline several of the remaining unanswered questions about iron uptake via the bacterial transferrin receptor and suggest directions for future research.

Natural products as mediators of disease

Covering: up to 2016Humans are walking microbial ecosystems, each harboring a complex microbiome with the genetic potential to produce a vast array of natural products. Recent sequencing data suggest that our microbial inhabitants are critical for maintaining overall health. Shifts in microbial communities have been correlated to a number of diseases including infections, inflammation, cancer, and neurological disorders. Some of these clinically and diagnostically relevant phenotypes are a result of the presence of small molecules, yet we know remarkably little about their contributions to the health of individuals. Here, we review microbe-derived natural products as mediators of human disease.

Predicting the kinetics of Listeria monocytogenes and Yersinia enterocolitica under dynamic growth/death-inducing conditions, in Italian style fresh sausage

Traditional Italian pork products can be consumed after variable drying periods, where the temporal decrease of water activity spans from optimal to inactivating values. This makes it necessary to A) consider the bias factor when applying culture-medium-based predictive models to sausage; B) apply the dynamic version (described by differential equations) of those models; C) combine growth and death models in a continuous way, including the highly uncertain growth/no growth range separating the two regions. This paper tests the applicability of published predictive models on the responses of Listeria monocytogenes and Yersinia enterocolitica to dynamic conditions in traditional Italian pork sausage, where the environment changes from growth-supporting to inhibitory conditions, so the growth and death models need to be combined. The effect of indigenous lactic acid bacteria was also taken into account in the predictions. Challenge tests were carried out using such sausages, inoculated separately with L. monocytogenes and Y. enterocolitica, stored for 480h at 8, 12, 18 and 20°C. The pH was fairly constant, while the water activity changed dynamically. The effects of the environment on the specific growth and death rate of the studied organisms were predicted using previously published predictive models and parameters. Microbial kinetics in many products with a long shelf-life and dynamic internal environment, could result in both growth and inactivation, making it difficult to estimate the bacterial concentration at the time of consumption by means of commonly available predictive software tools. Our prediction of the effect of the storage environment, where the water activity gradually decreases during a drying period, is designed to overcome these difficulties. The methodology can be used generally to predict and visualise bacterial kinetics under temporal variation of environments, which is vital when assessing the safety of many similar products.

RT-qPCR Analysis of 15 Genes Encoding Putative Surface Proteins Involved in Adherence of Listeria monocytogenes

L. monocytogenes adherence to food-associated abiotic surfaces and the development of biofilms as one of the underlying reasons for the contamination of ready-to-eat products is well known. The over-expression of internalins that improves adherence has been noted in cells growing as attached cells or at elevated incubation temperatures. However, the role of other internalin-independent surface proteins as adhesins has been uncharacterized to date. Using two strains each of weakly- and strongly-adherent L. monocytogenes as platforms for temperature-dependent adherence assays and targeted mRNA analyses, these observations (i.e., sessile- and/or temperature-dependent gene expression) were further investigated. Microplate fluorescence assays of both surface-adherent strains exhibited significant (P < 0.05) adherence at higher incubation temperature (42 °C). Of the 15 genes selected for RT-qPCR, at least ten gene transcripts recovered from cells (weakly-adherent strain CW35, strongly-adherent strain 99-38) subject to various growth conditions were over expressed [planktonic/30 °C (10), sessile/30 °C (12), planktonic/42 °C (10)] compared to their internal control (16SrRNA transcripts). Of four genes overexpressed in all three conditions tested, three and one were implicated as virulence factors and unknown function, respectively. PCR analysis of six unexpressed genes revealed that CW35 possessed an altered genome. The results suggest the presence of other internalin-independent adhesins (induced by growth temperature and/or substratum) and that a group of suspect protein members are worthy of further analysis for their potential role as surface adhesins. Analysis of the molecular basis of adherence properties of isolates of L. monocytogenes from food-associated facilities may help identify sanitation regimens to prevent cell attachment and biofilm formation on abiotic surfaces that could play a role in reducing foodborne illness resulting from Listeria biofilms.

Epithelial cell extrusion: Pathways and pathologies

To remove dying or unwanted cells from an epithelium while preserving the barrier function of the layer, epithelia use a unique process called cell extrusion. To extrude, the cell fated to die emits the lipid Sphingosine 1 Phosphate (S1P), which binds the G-protein-coupled receptor Sphingosine 1 Phosphate receptor 2 (S1P₂) in the neighboring cells that activates Rho-mediated contraction of an actomyosin ring circumferentially and basally. This contraction acts to squeeze the cell out apically while drawing together neighboring cells and preventing any gaps to the epithelial barrier. Epithelia can extrude out cells targeted to die by apoptotic stimuli to repair the barrier in the face of death or extrude live cells to promote cell death when epithelial cells become too crowded. Indeed, because epithelial cells naturally turn over by cell death and division at some of the highest rates in the body, epithelia depend on crowding-induced live cell extrusion to preserve constant cell numbers. If extrusion is defective, epithelial cells rapidly lose contact inhibition and form masses. Additionally, because epithelia act as the first line of defense in innate immunity, preservation of this barrier is critical for preventing pathogens from invading the body. Given its role in controlling constant cell numbers and maintaining barrier function, a number of different pathologies can result when extrusion is disrupted. Here, we review mechanisms and signaling pathways that control epithelial extrusion and discuss how defects in these mechanisms can lead to multiple diseases. We also discuss tactics pathogens have devised to hijack the extrusion process to infect and colonize epithelia.

Multiplex detection of nine food-borne pathogens by mPCR and capillary electrophoresis after using a universal pre-enrichment medium

Routine microbiological quality analyses in food samples require, in some cases, an initial incubation in pre-enrichment medium. This is necessary in order to ensure that small amounts of pathogenic strains are going to be detected. In this work, a universal pre-enrichment medium has been developed for the simultaneous growth of Bacillus cereus, Campylobacter jejuni, Clostridium perfringens, Cronobacter sakazakii, Escherichia coli, Enterobacteriaceae family (38 species, 27 genera), Listeria monocytogenes, Staphylococcus aureus, Salmonella spp. (two species, 13 strains). Growth confirmation for all these species was achieved in all cases, with excellent enrichments. This was confirmed by plating on the corresponding selective agar media for each bacterium. This GVUM universal pre-enrichment medium could be useful in food microbiological analyses, where different pathogenic bacteria must be detected after a pre-enrichment step. Following, a mPCR reaction for detection of all these pathogens was developed, after designing a set of nine oligonucleotide pairs from specific genetic targets on gDNA from each of these bacteria, covering all available strains already sequenced in GenBank for each pathogen type. The detection limits have been 1 Genome Equivalent (GE), with the exception of the Fam. Enterobacteriaceae (5 GEs). We obtained amplification for all targets (from 70 to 251 bp, depending on the bacteria type), showing the capability of this method to detect the most important industrial and sanitary food-borne pathogens from a universal pre-enrichment medium. This method includes an initial pre-enrichment step (18 h), followed by a mPCR (2 h) and a capillary electrophoresis (30 min); avoiding the tedious and long lasting growing on solid media required in traditional analysis (1–4 days, depending on the specific pathogen and verification procedure). An external testing of this method was conducted in order to compare classical and mPCR methods. This evaluation was carried out on five types of food matrices (meat, dairy products, prepared foods, canned fish, and pastry products), which were artificially contaminated with each one of the microorganisms, demonstrating the equivalence between both methods (coincidence percentages between both methods ranged from 78 to 92%).