Requirement of the Shigella flexneri virulence plasmid in the ability to induce trafficking of neutrophils across polarized monolayers of the intestinal epithelium

Immune evasion and persistence in enteric bacterial pathogens

The major function of the mammalian immune system is to prevent and control infections caused by enteropathogens that collectively have altered human destiny. In fact, as the gastrointestinal tissues are the major interface of mammals with the environment, up to 70% of the human immune system is dedicated to patrolling them The defenses are multi-tiered and include the endogenous microflora that mediate colonization resistance as well as physical barriers intended to compartmentalize infections. The gastrointestinal tract and associated lymphoid tissue are also protected by sophisticated interleaved arrays of active innate and adaptive immune defenses. Remarkably, some bacterial enteropathogens have acquired an arsenal of virulence factors with which they neutralize all these formidable barriers to infection, causing disease ranging from mild self-limiting gastroenteritis to in some cases devastating human disease.

Epithelial and Neutrophil Interactions and Coordinated Response to Shigella in a Human Intestinal Enteroid-Neutrophil Coculture Model

Polymorphonuclear neutrophils (PMN) are recruited to the gastrointestinal mucosa in response to inflammation, injury, and infection. Here, we report the development and the characterization of an ex vivo tissue coculture model consisting of human primary intestinal enteroid monolayers and PMN, and a mechanistic interrogation of PMN-epithelial cell interaction and response to Shigella, a primary cause of childhood dysentery. Cellular adaptation and tissue integration, barrier function, PMN phenotypic and functional attributes, and innate immune responses were examined. PMN within the enteroid monolayers acquired a distinct activated/migratory phenotype that was influenced by direct epithelial cell contact as well as by molecular signals. Seeded on the basal side of the intestinal monolayer, PMN were intercalated within the epithelial cells and moved paracellularly toward the apical side. Cocultured PMN also increased basal secretion of interleukin 8 (IL-8). Shigella added to the apical surface of the monolayers evoked additional PMN phenotypic adaptations, including increased expression of cell surface markers associated with chemotaxis and cell degranulation (CD47, CD66b, and CD88). Apical Shigella infection triggered rapid transmigration of PMN to the luminal side, neutrophil extracellular trap (NET) formation, and bacterial phagocytosis and killing. Shigella infection modulated cytokine production in the coculture; apical monocyte chemoattractant protein (MCP-1), tumor necrosis factor alpha (TNF-α), and basolateral IL-8 production were downregulated, while basolateral IL-6 secretion was increased. We demonstrated, for the first time, PMN phenotypic adaptation and mobilization and coordinated epithelial cell-PMN innate response upon Shigella infection in the human intestinal environment. The enteroid monolayer-PMN coculture represents a technical innovation for mechanistic interrogation of gastrointestinal physiology, host-microbe interaction, innate immunity, and evaluation of preventive/therapeutic tools.

SepA Enhances Shigella Invasion of Epithelial Cells by Degrading Alpha-1 Antitrypsin and Producing a Neutrophil Chemoattractant

Shigella spp. are highly adapted pathogens that cause bacillary dysentery in human and nonhuman primates. An unusual feature of Shigella pathogenesis is that this organism invades the colonic epithelia from the basolateral pole. Therefore, it has evolved the ability to disrupt the intestinal epithelial barrier to reach the basolateral surface. We have shown previously that the secreted serine protease A (SepA), which belongs to the family of serine protease autotransporters of Enterobacteriaceae, is responsible for the initial destabilization of the intestinal epithelial barrier that facilitates Shigella invasion. However, the mechanisms used by SepA to regulate this process remain unknown. To investigate the protein targets cleaved by SepA in the intestinal epithelium, we incubated a sample of homogenized human colon with purified SepA or with a catalytically inactive mutant of this protease. We discovered that SepA targets an array of 18 different proteins, including alpha-1 antitrypsin (AAT), a major circulating serine proteinase inhibitor in humans. In contrast to other serine proteases, SepA cleaved AAT without forming an inhibiting complex, which resulted in the generation of a neutrophil chemoattractant. We demonstrated that the products of the AAT-SepA reaction induce a mild but significant increase in neutrophil transepithelial migration in vitro. Moreover, the presence of AAT during Shigella infection stimulated neutrophil migration and dramatically enhanced the number of bacteria invading the intestinal epithelium in a SepA-dependent manner. We conclude that by cleaving AAT, SepA releases a chemoattractant that promotes neutrophil migration, which in turn disrupts the intestinal epithelial barrier to enable Shigella invasion. IMPORTANCE Shigella is the second leading cause of diarrheal death globally. In this study, we identified the host protein targets of SepA, Shigella's major protein secreted in culture. We demonstrated that by cleaving AAT, a serine protease inhibitor important to protect surrounding tissue at inflammatory sites, SepA releases a neutrophil chemoattractant that enhances Shigella invasion. Moreover, SepA degraded AAT without becoming inhibited by the cleaved product, and SepA catalytic activity was enhanced at higher concentrations of AAT. Activation of SepA by an excess of AAT may be physiologically relevant at the early stages of Shigella infection, when the amount of synthesized SepA is very low compared to the concentration of AAT in the intestinal lumen. This observation may also help to explain the adeptness of Shigella infectivity at low dose, despite the requirement of reaching the basolateral side to invade and colonize the colonic epithelium.

A Versatile Human Intestinal Organoid-Derived Epithelial Monolayer Model for the Study of Enteric Pathogens

Gastrointestinal infections cause significant morbidity and mortality worldwide. The complexity of human biology and limited insights into host-specific infection mechanisms are key barriers to current therapeutic development. Here, we demonstrate that two-dimensional epithelial monolayers derived from human intestinal organoids, combined with in vivo-like bacterial culturing conditions, provide significant advancements for the study of enteropathogens. Monolayers from the terminal ileum, cecum, and ascending colon recapitulated the composition of the gastrointestinal epithelium, in which several techniques were used to detect the presence of enterocytes, mucus-producing goblet cells, and other cell types following differentiation. Importantly, the addition of receptor activator of nuclear factor kappa-B ligand (RANKL) increased the presence of M cells, critical antigen-sampling cells often exploited by enteric pathogens. For infections, bacteria were grown under in vivo-like conditions known to induce virulence. Overall, interesting patterns of tissue tropism and clinical manifestations were observed. Shigella flexneri adhered efficiently to the cecum and colon; however, invasion in the colon was best following RANKL treatment. Both Salmonella enterica serovars Typhi and Typhimurium displayed different infection patterns, with S. Typhimurium causing more destruction of the terminal ileum and S. Typhi infecting the cecum more efficiently than the ileum, particularly with regard to adherence. Finally, various pathovars of Escherichia coli validated the model by confirming only adherence was observed with these strains. This work demonstrates that the combination of human-derived tissue with targeted bacterial growth conditions enables powerful analyses of human-specific infections that could lead to important insights into pathogenesis and accelerate future vaccine development. IMPORTANCE While traditional laboratory techniques and animal models have provided valuable knowledge in discerning virulence mechanisms of enteric pathogens, the complexity of the human gastrointestinal tract has hindered our understanding of physiologically relevant, human-specific interactions; and thus, has significantly delayed successful vaccine development. The human intestinal organoid-derived epithelial monolayer (HIODEM) model closely recapitulates the diverse cell populations of the intestine, allowing for the study of human-specific infections. Differentiation conditions permit the expansion of various cell populations, including M cells that are vital to immune recognition and the establishment of infection by some bacteria. We provide details of reproducible culture methods and infection conditions for the analyses of Shigella, Salmonella, and pathogenic Escherichia coli in which tissue tropism and pathogen-specific infection patterns were detected. This system will be vital for future studies that explore infection conditions, health status, or epigenetic differences and will serve as a novel screening platform for therapeutic development.

Escherichia albertii Pathogenesis

Escherichia albertii is an emerging enteropathogen of humans and many avian species. This bacterium is a close relative of Escherichia coli and has been frequently misidentified as enteropathogenic or enterohemorrhagic E. coli due to their similarity in phenotypic and genetic features, such as various biochemical properties and the possession of a type III secretion system encoded by the locus of enterocyte effacement. This pathogen causes outbreaks of gastroenteritis, and some strains produce Shiga toxin. Although many genetic and phenotypic studies have been published and the genome sequences of more than 200 E. albertii strains are now available, the clinical significance of this species is not yet fully understood. The apparent zoonotic nature of the disease requires a deeper understanding of the transmission routes and mechanisms of E. albertii to develop effective measures to control its transmission and infection. Here, we review the current knowledge of the phylogenic relationship of E. albertii with other Escherichia species and the biochemical and genetic properties of E. albertii, with particular emphasis on the repertoire of virulence factors and the mechanisms of pathogenicity, and we hope this provides a basis for future studies of this important emerging enteropathogen.

Bacteriophage Therapy Testing Against Shigella flexneri in a Novel Human Intestinal Organoid-Derived Infection Model

OBJECTIVE

Enteric bacterial pathogens cause diarrheal disease and mortality at significant rates throughout the world, particularly in children younger than 5 years. Our ability to combat bacterial pathogens has been hindered by antibiotic resistance, a lack of effective vaccines, and accurate models of infection. With the renewed interest in bacteriophage therapy, we sought to use a novel human intestinal model to investigate the efficacy of a newly isolated bacteriophage against Shigella flexneri.

METHODS

An S. flexneri 2457T-specific bacteriophage was isolated and assessed through kill curve experiments and infection assays with colorectal adenocarcinoma HT-29 cells and a novel human intestinal organoid-derived epithelial monolayer model. In our treatment protocol, organoids were generated from intestinal crypt stem cells, expanded in culture, and seeded onto transwells to establish 2-dimensional monolayers that differentiate into intestinal cells.

RESULTS

The isolated bacteriophage efficiently killed S. flexneri 2457T, other S. flexneri strains, and a strain of 2457T harboring an antibiotic resistance cassette. Analyses with laboratory and commensal Escherichia coli strains demonstrated that the bacteriophage was specific to S. flexneri, as observed under co-culture conditions. Importantly, the bacteriophage prevented both S. flexneri 2457T epithelial cell adherence and invasion in both infection models.

CONCLUSIONS

Bacteriophages offer feasible alternatives to antibiotics for eliminating enteric pathogens, confirmed here by the bacteriophage-targeted killing of S. flexneri. Furthermore, application of the organoid model has provided important insight into Shigella pathogenesis and bacteriophage-dependent intervention strategies. The screening platform described herein provides proof-of-concept analysis for the development of novel bacteriophage therapies to target antibiotic-resistant pathogens.

Evaluating Shigella flexneri Pathogenesis in the Human Enteroid Model

The enteric pathogen Shigella is one of the leading causes of moderate-to-severe diarrhea and death in young children in developing countries. Transformed cell lines and animal models have been widely used to study Shigella pathogenesis.

The enteric pathogen Shigella is one of the leading causes of moderate-to-severe diarrhea and death in young children in developing countries. Transformed cell lines and animal models have been widely used to study Shigella pathogenesis. In addition to altered physiology, transformed cell lines are composed of a single cell type that does not sufficiently represent the complex multicellular environment of the human colon. Most available animal models do not accurately mimic human disease. The human intestinal enteroid model, derived from LGR5⁺ stem cell-containing intestinal crypts from healthy subjects, represents a technological leap in human gastrointestinal system modeling and provides a more physiologically relevant system that includes multiple cell types and features of the human intestine. We established the utility of this model for studying basic aspects of Shigella pathogenesis and host responses. In this study, we show that Shigellaflexneri is capable of infecting and replicating intracellularly in human enteroids derived from different segments of the intestine. Apical invasion by S. flexneri is very limited but increases ∼10-fold when enteroids are differentiated to include M cells. Invasion via the basolateral surface was at least 2-log₁₀ units more efficient than apical infection. Increased secretion of interleukin-8 and higher expression levels of the mucin glycoprotein Muc2 were observed in the enteroids following S. flexneri infection. The human enteroid model promises to bridge some of the gaps between traditional cell culture, animal models, and human infection.

Shigella depends on SepA to destabilize the intestinal epithelial integrity via cofilin activation

Shigella is unique among enteric pathogens, as it invades colonic epithelia through the basolateral pole. Therefore, it has evolved the ability to breach the intestinal epithelial barrier to deploy an arsenal of effector proteins, which permits bacterial invasion and leads to a severe inflammatory response. However, the mechanisms used by Shigella to regulate epithelial barrier permeability remain unknown. To address this question, we used both an intestinal polarized model and a human ex-vivo model to further characterize the early events of host-bacteria interactions. Our results showed that secreted Serine Protease A (SepA), which belongs to the serine protease autotransporter of Enterobacteriaceae family, is responsible for critically disrupting the intestinal epithelial barrier. Such disruption facilitates bacterial transit to the basolateral pole of the epithelium, ultimately fostering the hallmarks of the disease pathology. SepA was found to cause a decrease in active LIM Kinase 1 (LIMK1) levels, a negative inhibitor of actin-remodeling proteins, namely cofilin. Correspondingly, we observed increased activation of cofilin, a major actin-polymerization factor known to control opening of tight junctions at the epithelial barrier. Furthermore, we resolved the crystal structure of SepA to elucidate its role on actin-dynamics and barrier disruption. The serine protease activity of SepA was found to be required for the regulatory effects on LIMK1 and cofilin, resulting in the disruption of the epithelial barrier during infection. Altogether, we demonstrate that SepA is indispensable for barrier disruption, ultimately facilitating Shigella transit to the basolateral pole where it effectively invades the epithelium.

Escherichia albertii, a novel human enteropathogen, colonizes rat enterocytes and translocates to extra-intestinal sites

Diarrhea is the second leading cause of death of children up to five years old in the developing countries. Among the etiological diarrheal agents are atypical enteropathogenic Escherichia coli (aEPEC), one of the diarrheagenic E. coli pathotypes that affects children and adults, even in developed countries. Currently, genotypic and biochemical approaches have helped to demonstrate that some strains classified as aEPEC are actually E. albertii, a recently recognized human enteropathogen. Studies on particular strains are necessary to explore their virulence potential in order to further understand the underlying mechanisms of E. albertii infections. Here we demonstrated for the first time that infection of fragments of rat intestinal mucosa is a useful tool to study the initial steps of E. albertii colonization. We also observed that an E. albertii strain can translocate from the intestinal lumen to Mesenteric Lymph Nodes and liver in a rat model. Based on our finding of bacterial translocation, we investigated how E. albertii might cross the intestinal epithelium by performing infections of M-like cells in vitro to identify the potential in vivo translocation route. Altogether, our approaches allowed us to draft a general E. albertii infection route from the colonization till the bacterial spreading in vivo.

The synthesis of OspD3 (ShET2) in Shigella flexneri is independent of OspC1

Shigella flexneri is a Gram-negative pathogen that invades the colonic epithelium and causes millions of cases of watery diarrhea or bacillary dysentery predominately in children under the age of 5 years in developing countries. The effector Shigella enterotoxin 2 (ShET2), or OspD3, is encoded by the sen or ospD3 gene on the virulence plasmid. Previous literature has suggested that ospD3 is in an operon downstream of the ospC1 gene, and expression of both genes is controlled by a promoter upstream of ospC1. Since the intergenic region is 328 bases in length and contains several putative promoter regions, we hypothesized the genes are independently expressed. Here we provide data that ospD3 and ospC1 are not co-transcribed and that OspC1 is not required for OspD3/ShET2 function. Most importantly, we identified strong promoter activity in the intergenic region and demonstrate that OspD3/ShET2 can be expressed and secreted independently of OspC1. This work increases our understanding of the synthesis of a unique virulence factor and provides further insights into Shigella pathogenesis.

Cytotoxic and Inflammatory Responses Induced by Outer Membrane Vesicle-Associated Biologically Active Proteases from Vibrio cholerae

Proteases in Vibrio cholerae have been shown to play a role in its pathogenesis. V. cholerae secretes Zn-dependent hemagglutinin protease (HAP) and calcium-dependent trypsin-like serine protease (VesC) by using the type II secretion system (TIISS). Our present studies demonstrated that these proteases are also secreted in association with outer membrane vesicles (OMVs) and transported to human intestinal epithelial cells in an active form. OMV-associated HAP induces dose-dependent apoptosis in Int407 cells and an enterotoxic response in the mouse ileal loop (MIL) assay, whereas OMV-associated VesC showed a hemorrhagic fluid response in the MIL assay, necrosis in Int407 cells, and an increased interleukin-8 (IL-8) response in T84 cells, which were significantly reduced in OMVs from VesC mutant strain. Our results also showed that serine protease VesC plays a role in intestinal colonization of V. cholerae strains in adult mice. In conclusion, our study shows that V. cholerae OMVs secrete biologically active proteases which may play a role in cytotoxic and inflammatory responses.

Serratia marcescens is injurious to intestinal epithelial cells

Diarrhea causes substantial morbidity and mortality in children in low-income countries. Although numerous pathogens cause diarrhea, the etiology of many episodes remains unknown. Serratia marcescens is incriminated in hospital-associated infections, and HIV/AIDS associated diarrhea. We have recently found that Serratia spp. may be found more commonly in the stools of patients with diarrhea than in asymptomatic control children. We therefore investigated the possible enteric pathogenicity of S. marcescens in vitro employing a polarized human colonic epithelial cell (T84) monolayer. Infected monolayers were assayed for bacterial invasion, transepithelial electrical resistance (TEER), cytotoxicity, interleukin-8 (IL-8) release and morphological changes by scanning electron microscopy. We observed significantly greater epithelial cell invasion by S. marcescens compared to Escherichia coli strain HS (p = 0.0038 respectively). Cell invasion was accompanied by reduction in TEER and secretion of IL-8. Lactate dehydrogenase (LDH) extracellular concentration rapidly increased within a few hours of exposure of the monolayer to S. marcescens. Scanning electron microscopy of S. marcescens-infected monolayers demonstrated destruction of microvilli and vacuolization. Our results suggest that S. marcescens interacts with intestinal epithelial cells in culture and induces dramatic alterations similar to those produced by known enteric pathogens.

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

In vitro coculture assay to assess pathogen induced neutrophil trans-epithelial migration

Mucosal surfaces serve as protective barriers against pathogenic organisms. Innate immune responses are activated upon sensing pathogen leading to the infiltration of tissues with migrating inflammatory cells, primarily neutrophils. This process has the potential to be destructive to tissues if excessive or held in an unresolved state. Cocultured in vitro models can be utilized to study the unique molecular mechanisms involved in pathogen induced neutrophil trans-epithelial migration. This type of model provides versatility in experimental design with opportunity for controlled manipulation of the pathogen, epithelial barrier, or neutrophil. Pathogenic infection of the apical surface of polarized epithelial monolayers grown on permeable transwell filters instigates physiologically relevant basolateral to apical trans-epithelial migration of neutrophils applied to the basolateral surface. The in vitro model described herein demonstrates the multiple steps necessary for demonstrating neutrophil migration across a polarized lung epithelial monolayer that has been infected with pathogenic P. aeruginosa (PAO1). Seeding and culturing of permeable transwells with human derived lung epithelial cells is described, along with isolation of neutrophils from whole human blood and culturing of PAO1 and nonpathogenic K12 E. coli (MC1000). The emigrational process and quantitative analysis of successfully migrated neutrophils that have been mobilized in response to pathogenic infection is shown with representative data, including positive and negative controls. This in vitro model system can be manipulated and applied to other mucosal surfaces. Inflammatory responses that involve excessive neutrophil infiltration can be destructive to host tissues and can occur in the absence of pathogenic infections. A better understanding of the molecular mechanisms that promote neutrophil trans-epithelial migration through experimental manipulation of the in vitro coculture assay system described herein has significant potential to identify novel therapeutic targets for a range of mucosal infectious as well as inflammatory diseases.

A new understanding of enteroaggregative Escherichia coli as an inflammatory pathogen

Enteroaggregative Escherichia coli (EAEC) is an important cause of endemic and epidemic diarrheal disease worldwide. Although not classically considered an inflammatory pathogen in the style of Shigella and Salmonella species, clinical data from patients suggests that inflammatory responses may play an important role during EAEC disease. However, the specific role of inflammation during EAEC pathogenesis has not been investigated in detail. To better understand how EAEC may induce inflammation, we have focused our attention on the intimate interactions between EAEC and the host epithelium and the subsequent induction of host cell signaling events leading to innate immune responses. Here, we discuss our recent findings on the signaling pathway by which EAEC promotes transepithelial migration of polymorphonuclear leukocytes (PMNs), the role of aggregative adherence fimbriae in triggering this event and the implementation of human intestinal xenografts in immunodeficient mice for studying EAEC pathogenesis in vivo. Our findings suggest that EAEC shares conserved mechanisms of inducing PMN recruitment with other intestinal pathogens, providing new insight into the potential pathological consequences of EAEC-induced inflammation.

Shigella flexneri effectors OspE1 and OspE2 mediate induced adherence to the colonic epithelium following bile salts exposure

Shigella flexneri is a Gram-negative pathogen that invades the colonic epithelium. While invasion has been thoroughly investigated, it is unknown how Shigella first attaches to the epithelium. Previous literature suggests that Shigella utilizes adhesins that are induced by environmental signals, including bile salts, encountered in the small intestine prior to invasion. We hypothesized that bile would induce adherence factors to facilitate attachment to colonic epithelial cells. To test our hypothesis, S. flexneri strain 2457T was subcultured in media containing bile salts, and the ability of the bacteria to adhere to the apical surface of polarized T84 epithelial cells was measured. We observed a significant increase in adherence, which was absent in a virulence plasmid-cured strain and a type-III secretion system mutant. Microarray expression analysis indicated that the ospE1/ospE2 genes were induced in the presence of bile, and bile-induced adherence was lost in a ΔospE1/ΔospE2 mutant. Further studies demonstrated that the OspE1/OspE2 proteins were localized to the bacterial outer membrane following exposure to bile salts. The data presented are the first demonstration that the OspE1/OspE2 proteins promote initial adherence to the intestinal epithelium. The adhesins required for Shigella attachment to the colonic epithelium may serve as ideal targets for vaccine development.

Evolutionary, ecological and biotechnological perspectives on plasmids resident in the human gut mobile metagenome

Numerous mobile genetic elements (MGE) are associated with the human gut microbiota and collectively referred to as the gut mobile metagenome. The role of this flexible gene pool in development and functioning of the gut microbial community remains largely unexplored, yet recent evidence suggests that at least some MGE comprising this fraction of the gut microbiome reflect the co-evolution of host and microbe in the gastro-intestinal tract. In conjunction, the high level of novel gene content typical of MGE coupled with their predicted high diversity, suggests that the mobile metagenome constitutes an immense and largely unexplored gene-space likely to encode many novel activities with potential biotechnological or pharmaceutical value, as well as being important to the development and functioning of the gut microbiota. Of the various types of MGE that comprise the gut mobile metagenome, plasmids are of particular importance since these elements are often capable of autonomous transfer between disparate bacterial species, and are known to encode accessory functions that increase bacterial fitness in a given environment facilitating bacterial adaptation. In this article current knowledge regarding plasmids resident in the human gut mobile metagenome is reviewed, and available strategies to access and characterize this portion of the gut microbiome are described. The relative merits of these methods and their present as well as prospective impact on our understanding of the human gut microbiota is discussed.

Enteroaggregative Escherichia coli promotes transepithelial migration of neutrophils through a conserved 12-lipoxygenase pathway

Enteroaggregative Escherichia coli (EAEC) induces release of pro-inflammatory markers and disruption of intestinal epithelial barriers in vitro, suggesting an inflammatory aspect to EAEC infection. However, the mechanisms underlying EAEC-induced mucosal inflammatory responses and the extent to which these events contribute to pathogenesis is not well characterized. Employing an established in vitro model we demonstrated that EAEC prototype strain 042 induces migration of polymorphonuclear neutrophils (PMNs) across polarized T84 cell monolayers. This event was mediated through a conserved host cell signalling cascade involving the 12/15-LOX pathway and led to apical secretion of an arachidonic acid-derived lipid PMN chemoattractant, guiding PMNs across the epithelia to the site of infection. Moreover, supporting the hypothesis that inflammatory responses may contribute to EAEC pathogenesis, we found that PMN transepithelial migration promoted enhanced attachment of EAEC 042 to T84 cells. These findings suggest that EAEC-induced PMN infiltration may favour colonization and thus pathogenesis of EAEC.

Analysis of Shigella flexneri-mediated infections in model organism Caenorhabditis elegans

BACKGROUND

Shigella flexneri is the causative agent of bacillary dysentery and generates a significant global disease burden. The aim of this study was to analyze the pathogenesis and host immune response, at both the physiological and molecular level, using the model organism Caenorhabditis elegans, in response to S. flexneri. C. elegans is a nematode that responds to infection with a simple innate immune system, key aspects of which have been shown to be conserved.

METHODS

S. flexneri-mediated infection of C. elegans was performed in both solid and liquid assays. The expression and subsequent regulation of host candidate antimicrobial genes such as lysozymes, C-type lectins and pathogen virulence genes were kinetically analyzed in the S. flexneri-exposed nematode.

RESULTS

In solid assays, worms fed with S. flexneri showed complete killing at 153 ± 9 h. The kinetic studies showed that S. flexneri killed the worms upon continuous exposure at 41 ± 1.7 h. However, short-time exposure of the host to S. flexneri indicated that 14 h of exposure resulted in a loss of progeny, and death occurred after 46 h. Semi-quantitative reverse transcriptase polymerase chain reaction (RT-PCR) analysis revealed that mRNA levels of host candidate antimicrobial genes and pathogen virulence genes varied significantly at the time of early infection.

CONCLUSIONS

The killing of C. elegans requires live bacteria, and a minimal exposure time is sufficient for S. flexneri to have a lethal effect. The candidate antimicrobial genes and virulence genes are kinetically regulated within C. elegans during S. flexneri-mediated infections, thereby exhibiting their role and contribution in the host innate immune system.

Direction of neutrophil movements by Campylobacter-infected intestinal epithelium

Campylobacter jejuni and Campylobacter coli together represent the leading bacterial cause of human enteritis. However, the pathogenesis of this disease is poorly understood. Infection results in the formation of crypt abscesses resulting from the migration of neutrophils across the intestinal epithelium and into the intestinal crypts. In this study, we model this process in vitro and show that Campylobacter infection of epithelium results in a quantifiable increase in the directed movement of neutrophils from the basolateral to apical surface of the epithelium. This process is dependent on both bacterially derived n-formyl peptides and on the host cell enzyme 12-lipoxygenase (12-LOX).

The human gut mobile metagenome: a metazoan perspective

Using the culture independent TRACA system in conjunction with a comparative metagenomic approach, we have recently explored the pool of plasmids associated with the human gut mobile metagenome. This revealed that some plasmids or plasmid families are present in the gut microbiomes of geographically isolated human hosts with a broad global distribution (America, Japan and Europe), and are potentially unique to the human gut microbiome. Functions encoded by the most widely distributed plasmid (pTRACA22) were found to be enriched in the human gut microbiome when compared to microbial communities from other environments, and of particular interest was the increased prevalence of a putative RelBE toxin-antitoxin (TA) addiction module. Subsequent analysis revealed that this was most closely related to putative TA modules from gut associated bacteria belonging to the Firmicutes, but homologues of the RelE toxin were associated with all major bacterial divisions comprising the human gut microbiota. In this addendum, functions of the gut mobile metagenome are considered from the perspective of the human host, and within the context of the hologenome theory of human evolution. In doing so, our original analysis is also extended to include the gut metagenomes of a further 124 individuals comprising the METAHIT dataset. Differences in the incidence and relative abundance of pTRACA22 and associated TA modules between healthy individuals and those with inflammatory bowel diseases are explored, and potential functions of pTRACA22 type RelBE modules in the human gut microbiome are discussed.

The Salmonella enterica serotype Typhi Vi capsular antigen is expressed after the bacterium enters the ileal mucosa

Salmonella enterica serotype Typhi, the etiological agent of typhoid fever, produces the Vi capsular antigen, a virulence factor absent in Salmonella enterica serotype Typhimurium. Previous studies suggest that the capsule-encoding viaB locus reduces inflammatory responses in intestinal tissue; however, there are currently no data regarding the in vivo expression of this locus. Here we implemented direct and indirect methods to localize and detect Vi antigen expression within polarized intestinal epithelial cells and in the bovine ileal mucosa. We report that tviB, a gene necessary for Vi production in S. Typhi, was significantly upregulated during invasion of intestinal epithelial cells in vitro. During infection of bovine ligated loops, tviB was expressed at levels significantly higher in calf tissue than those in the inoculum. The presence of the Vi capsular antigen was detected in calf ileal tissue via fluorescence microscopy. Together, these results support the concept that expression of the Vi capsular antigen is induced when S. Typhi transits from the intestinal lumen into the ileal mucosa.

Spa15 of Shigella flexneri is secreted through the type III secretion system and prevents staurosporine-induced apoptosis

Shigella flexneri is a gram-negative, facultative intracellular pathogen that invades the colonic epithelium and causes bacillary dysentery. We previously demonstrated that S. flexneri inhibits staurosporine-induced apoptosis in infected epithelial cells and that a DeltamxiE mutant is unable to inhibit apoptosis. Therefore, we hypothesized that an MxiE-regulated gene was responsible for protection of epithelial cells from apoptosis. Analysis of all MxiE-regulated genes yielded no mutants that lacked the ability to prevent apoptosis. Spa15, which is defined as a type III secretion system chaperone, was analyzed since it associates with MxiE. A Deltaspa15 mutant was unable to prevent staurosporine-induced apoptosis. C-terminal hemagglutinin-tagged spa15 was secreted by S. flexneri within 2 h in the Congo red secretion assay, and secretion was dependent on the type III secretion system. Spa15 was also secreted by Shigella in infected epithelial cells, as verified by immunofluorescence analysis. Spa15 secretion was decreased in the DeltamxiE mutant, which demonstrates why this mutant is unable to prevent staurosporine-induced apoptosis. Our data are the first to show that Spa15 is secreted in a type III secretion system-dependent fashion, and the absence of Spa15 in the Deltaspa15 mutant results in the loss of protection from staurosporine-induced apoptosis in epithelial cells. Thus, Spa15 contributes to the intracellular survival of Shigella by blocking apoptosis in the infected host cell.

Shigella flexneri type III secretion system effectors OspB and OspF target the nucleus to downregulate the host inflammatory response via interactions with retinoblastoma protein

OspF, OspG and IpaH(9.8) are type III secretion system (T3SS) effectors of Shigella flexneri that downregulate the host innate immune response. OspF modifies mitogen-activated protein kinase pathways and polymorphonuclear leucocyte transepithelial migration associated with Shigella invasion. OspF also localizes in the nucleus to mediate chromatin remodelling, resulting in reduced transcription of inflammatory cytokines. We now report that OspB can be added to the set of S. flexneri T3SS effectors required to modulate the innate immune response. T84 cells infected with a Delta ospB mutant resulted in reduced polymorphonuclear leucocyte transepithelial migration and mitogen-activated protein kinase signalling. Tagged versions of OspB localized with endosomes and the nucleus. Further, T84 cells infected with the Delta ospB mutant showed increased levels of secreted IL-8 compared with wild-type infected cells. Both GST-OspB and GST-OspF coprecipitated retinoblastoma protein from host cell lysates. Because Delta ospB and Delta ospF mutants share similar phenotypes, and OspB and OspF share a host binding partner, we propose that OspB and OspF facilitate the remodelling of chromatin via interactions with retinoblastoma protein, resulting in diminished inflammatory cytokine production. The requirement of multiple T3SS effectors to modulate the innate immune response correlates to the complexity of the human immune system.

Distinct isoforms of phospholipase A2 mediate the ability of Salmonella enterica serotype typhimurium and Shigella flexneri to induce the transepithelial migration of neutrophils

Salmonella spp. and Shigella spp. are responsible for millions of cases of enteric disease each year worldwide. While these pathogens have evolved distinct strategies for interacting with the human intestinal epithelium, they both induce significant proinflammatory responses that result in massive transepithelial migration of neutrophils across the intestinal mucosa. It has previously been shown with Salmonella enterica serotype Typhimurium that the process of neutrophil transmigration is mediated in part by the secretion of hepoxilin A(3) (HXA(3); 8-hydroxy-11,12-epoxy-eicosatetraenoic acid), a potent neutrophil chemoattractant, from the apical surface of infected model intestinal epithelium. This study confirms that HXA(3) is also secreted in response to infection by Shigella flexneri, that it is produced by a pathway involving 12/15-lipoxygenase (12/15-LOX), and that S. enterica serovar Typhimurium and S. flexneri share certain elements in the mechanism(s) that underlies the otherwise separate signal transduction pathways that are engaged to induce polymorphonuclear leukocyte (PMN) transepithelial migration (protein kinase C and extracellular signal-regulated kinases 1 and 2, respectively). PMN transepithelial migration in response to infection with S. flexneri was dependent on 12/15-LOX activity, the enzyme responsible for the initial metabolism of arachidonic acid to HXA(3). Probing further into this pathway, we also found that S. enterica serovar Typhimurium and S. flexneri activate different subtypes of phospholipase A(2), a critical enzyme involved in the liberation of arachidonic acid from cellular membranes. Thus, although S. enterica serovar Typhimurium and S. flexneri utilize different mechanisms for triggering the induction of PMN transepithelial migration, we found that their reliance on 12/15-LOX is conserved, suggesting that enteric pathogens may ultimately stimulate similar pathways for the synthesis and release of HXA(3).

Saccharomyces boulardii interferes with Shigella pathogenesis by postinvasion signaling events

Saccharomyces boulardii is gaining in popularity as a treatment for a variety of diarrheal diseases as well as inflammatory bowel disease. This study was designed to examine the effect of this yeast on infection by Shigella flexneri, a highly infectious and human host-adapted enteric pathogen. We investigated key interactions between the bacteria and host cells in the presence of the yeast in addition to a number of host responses including proinflammatory events and markers. Although the presence of the yeast during infection did not alter the number of bacteria that was able to attach or invade human colon cancer-derived T-84 cells, it did positively impact the tight junction protein zonula occluden-2 and significantly increase the barrier integrity of model epithelia. The yeast also decreased ERK, JNK, and NF-kappaB activation in response to S. flexneri, events likely responsible for the observed reductions in IL-8 secretion and the transepithelial migration of polymorphonuclear leukocytes across T-84 monolayers. These results, suggesting that the yeast allowed for a dampened inflammatory response, were confirmed in vivo utilizing a highly relevant model of human fetal colonic tissue transplanted into scid mice. Furthermore, a cell-free S. boulardii culture supernatant was also capable of reducing IL-8 secretion by infected T-84 cells. These data suggest that although the use of S. boulardii during infection with S. flexneri may alleviate symptoms associated with the inflammatory response of the host, it would not prevent infection.

Molecular pathogenesis of Shigella spp.: controlling host cell signaling, invasion, and death by type III secretion

Shigella spp. are gram-negative pathogenic bacteria that evolved from harmless enterobacterial relatives and may cause devastating diarrhea upon ingestion. Research performed over the last 25 years revealed that a type III secretion system (T3SS) encoded on a large plasmid is a key virulence factor of Shigella flexneri. The T3SS determines the interactions of S. flexneri with intestinal cells by consecutively translocating two sets of effector proteins into the target cells. Thus, S. flexneri controls invasion into EC, intra- and intercellular spread, macrophage cell death, as well as host inflammatory responses. Some of the translocated effector proteins show novel biochemical activities by which they intercept host cell signal transduction pathways. An understanding of the molecular mechanisms underlying Shigella pathogenesis will foster the development of a safe and efficient vaccine, which, in parallel with improved hygiene, should curb infections by this widespread pathogen.

nadA and nadB of Shigella flexneri 5a are antivirulence loci responsible for the synthesis of quinolinate, a small molecule inhibitor of Shigella pathogenicity

The evolution of bacterial pathogens from commensal organisms involves virulence gene acquisition followed by pathoadaptation to the new host, including inactivation of antivirulence loci (AVL). AVL are core ancestral genes whose expression is incompatible with the pathogenic lifestyle. Previous studies identified cadA (encoding lysine decarboxylase) as an AVL of Shigella spp. In this study, AVL of Shigella were identified by examining a phenotypic difference from its non-pathogenic ancestor, Escherichia coli. Unlike most E. coli strains, Shigella spp. are nicotinic acid auxotrophs, the pathway for the de novo synthesis of NAD being uniformly defective. In Shigella flexneri, this defect is due to alterations in the nadA and/or nadB genes encoding the enzyme complex that converts L-aspartate to quinolinate, a precursor to NAD synthesis. Quinolinate was found to inhibit invasion and cell-to-cell spread of Sh. flexneri 5a and its ability to induce polymorphonuclear neutrophil transepithelial migration. Virulence of other Shigella species was also inhibited by quinolinate. Introduction of functional nadA and nadB genes from E. coli K-12 into Sh. flexneri 5a restored its ability to synthesize quinolinate but also resulted in strong attenuation of virulence in this strain. The results define nadA and nadB as AVL in Shigella and validate the concept of pathoadaptive evolution of bacteria from commensal ancestors by inactivation of AVL. They also suggest that studies focusing on this form of bacterial evolution can identify novel inhibitors of virulence in other bacterial pathogens.

The NleE/OspZ family of effector proteins is required for polymorphonuclear transepithelial migration, a characteristic shared by enteropathogenic Escherichia coli and Shigella flexneri infections

Enteropathogenic Escherichia coli (EPEC) and Shigella flexneri are human host-specific pathogens that infect intestinal epithelial cells. However, each bacterial species employs a different infection strategy within this environmental niche. EPEC attaches to the apical surface of small intestine enterocytes, causing microvillus effacement and rearrangement of the host cell cytoskeleton beneath adherent bacteria. In contrast, S. flexneri invades the large intestine epithelium at the basolateral membrane, replicates, and spreads cell to cell. Both EPEC and S. flexneri rely on type three secretion systems (T3SS) to secrete effectors into host cells, and both pathogens recruit polymorphonuclear leukocytes (PMNs) from the submucosa to the lumen of the intestine. In this report, we compared the virulence functions of the EPEC T3SS effector NleE and the homologous Shigella protein Orf212. We discovered that Orf212 was secreted by the S. flexneri T3SS and renamed this protein OspZ. Infection of polarized T84 intestinal epithelial cells with an ospZ deletion mutant of S. flexneri resulted in reduced PMN transepithelial migration compared to infection by the wild type. An nleE deletion mutant of EPEC showed a similar reduction of PMN migration. The ability to induce PMN migration was restored in both mutants when either ospZ or nleE was expressed from a plasmid. An infection of T84 cells with the delta ospZ mutant resulted in reduced extracellular signal-related kinase phosphorylation and NF-kappaB activation compared to infection with the wild type. Therefore, we conclude that OspZ and NleE have similar roles in the upstream induction of host signaling pathways required for PMN transepithelial migration in Shigella and EPEC infections.

OspF and OspC1 are Shigella flexneri type III secretion system effectors that are required for postinvasion aspects of virulence

Shigella flexneri is the causative agent of dysentery, and its pathogenesis is mediated by a type III secretion system (T3SS). S. flexneri secretes effector proteins into the eukaryotic cell via the T3SS, and these proteins usurp host cellular functions to the benefit of the bacteria. OspF and OspC1 are known to be secreted by S. flexneri, but their functions are unknown. We transformed S. flexneri with a plasmid that expresses a two-hemagglutinin tag (2HA) in frame with OspF or OspC1 and verified that these proteins are secreted in a T3SS-dependent manner. Immunofluorescence of HeLa cells infected with S. flexneri expressing OspF-2HA or OspC1-2HA revealed that both proteins localize in the nucleus and cytoplasm of host cells. To elucidate the function of these T3SS effectors, we constructed DeltaospF and DeltaospC1 deletion mutants by allelic exchange. We found that DeltaospF and DeltaospC1 mutants invade host cells and form plaques in confluent monolayers similar to wild-type S. flexneri. However, in the polymorphonuclear (PMN) cell migration assay, a decrease in neutrophil migration was observed for both mutants in comparison to the migration of wild-type bacteria. Moreover, infection of polarized T84 intestinal cells infected with DeltaospF and DeltaospC1 mutants resulted in decreased phosphorylation of extracellular signal-regulated kinase 1/2 in comparison to that of T84 cells infected with wild-type S. flexneri. To date, these are the first examples of T3SS effectors implicated in mitogen-activated protein kinase kinase/extracellular signal-regulated kinase pathway activation. Ultimately, OspF and OspC1 are essential for PMN transepithelial migration, a phenotype associated with increased inflammation and bacterial access to the submucosa, which are fundamental aspects of S. flexneri pathogenesis.

Bacterial gut infections

Infections of the bowel as a result of bacterial enteropathogens are one of the most common medical problems. The use of novel molecular biology techniques and the recent development of new antimicrobial drugs and vaccines are helping us to identify, understand, treat and prevent these infections.

Interferon gamma induces translocation of commensal Escherichia coli across gut epithelial cells via a lipid raft-mediated process

BACKGROUND & AIMS

The "leaky gut" hypothesis proposes that leakage of enteric bacteria into the body resulting from disruption of the epithelial barrier is a critical step in the pathophysiology of various disorders such as inflammatory bowel disease and sepsis. However, the pathways and underlying mechanisms by which commensal bacteria cross the epithelial barrier in inflammatory conditions remain unclear. This study investigated the mechanisms of interferon gamma-mediated bacterial translocation across human colonic epithelial monolayers.

METHODS

Caco-2 and T84 monolayers were exposed to interferon gamma. Barrier function was assessed by transepithelial electrical resistance and lucifer yellow permeability. Internalization and translocation of Escherichia coli strain C25 were measured by quantitative bacterial culture. Expression and distribution of junctional proteins were assessed by immunoblotting and confocal imaging.

RESULTS

Minimal apical to basolateral translocation of C25 was observed in untreated T84 and Caco-2 monolayers. Interferon gamma caused a dramatic, dose-dependent increase in C25 translocation, which was uncoupled from cytokine-induced increases in paracellular permeability and disruption of tight junction proteins at low interferon gamma concentrations. These effects were associated with increased internalization of viable bacteria into, but not adherence to, Caco-2 cells. Interferon gamma-mediated bacterial translocation was abolished by pretreatment with the cholesterol-disrupting drugs filipin and methyl-beta-cyclodextrin, whereas these agents had no effect on infection of Caco-2 by the enteric pathogen Shigella sonnei.

CONCLUSIONS

Normally poorly invasive enteric bacteria may, in situations of inflammatory stress, exploit lipid raft-mediated transcytotic pathways to cross the intestinal epithelium, and these effects may precede cytokine-induced disruption of tight junctions.

Intestinal epithelial defense systems protect against bacterial threats

Numerous bacterial species inhabit the lumen of the human intestine. The epithelial cells that line the intestinal barrier are in direct contact with many of these species and have developed sophisticated strategies to prevent bacterial invasion of host tissue beyond simply providing a physical blockade. Intestinal epithelial cells (IECs) possess receptors that are capable of recognizing bacterial products, and engagement of these receptors results in the production and secretion of immunoregulatory proteins, such as cytokines and chemokines, which mobilize immune effector cells, including macrophages, dendritic cells, and neutrophils. In addition, IECs can produce various lipid-based eicosanoids that can contribute to the orchestration of the immune response. A better understanding of how the intestinal epithelium interacts with its microbial constituents may provide insight into strategies for treatment of enteric infections and inflammatory bowel disease.

Shigella interaction with intestinal epithelial cells determines the innate immune response in shigellosis

Shigellae are Gram-negative bacilli that cause bacillary dysentery in humans. This review summarizes current knowledge of Shigella pathogenesis and pathogenicity factors, invasion of epithelial cells, intracellular motility and cell-to-cell spreading, as well as components of the host cell involved in innate immune responses.

Afa/Dr diffusely adhering Escherichia coli infection in T84 cell monolayers induces increased neutrophil transepithelial migration, which in turn promotes cytokine-dependent upregulation of decay-accelerating factor (CD55), the receptor for Afa/Dr adhesins

Ulcerative colitis and Crohn's disease are inflammatory bowel diseases thought to involve strains of Escherichia coli. We report here that two wild-type Afa/Dr diffusely adhering E. coli (DAEC) strains, C1845 and IH11128, which harbor the fimbrial F1845 adhesin and the Dr hemagglutinin, respectively, and the E. coli laboratory strain HB101, transformed with the pSSS1 plasmid to produce Afa/Dr F1845 adhesin, all induced interleukin-8 (IL-8) production and transepithelial migration of polymorphonuclear leukocytes (PMNL) in polarized monolayers of the human intestinal cell line T84 grown on semipermeable filters. We observed that after PMNL migration, expression of decay-accelerating factor (DAF, or CD55), the brush border-associated receptor for Afa/Dr adhesins, was strongly enhanced, increasing the adhesion of Afa/Dr DAEC bacteria. When examining the mechanism by which DAF expression was enhanced, we observed that the PMNL transepithelial migration induced epithelial synthesis of tumor necrosis factor alpha and IL-1beta, which in turn promoted the upregulation of DAF.

Shigella flexneri regulates tight junction-associated proteins in human intestinal epithelial cells

Shigella spp. are a group of Gram-negative enteric bacilli that cause acute dysentery in humans. We demonstrate that Shigella flexneri has evolved the ability to regulate functional components of tight junctions after interaction at the apical and basolateral pole of model intestinal epithelia. In the regulation of tight junctional protein assemblies, S. flexneri can engage serotype-specific mechanisms, which targets not only expression, but also cellular distribution and membrane association of components of tight junctions. Distinct mechanisms resulting in the regulation of tight junction-associated proteins are initiated after either apical or basolateral interactions. S. flexneri serotype 2a has the ability to remove claudin-1 from Triton X-insoluble protein fractions upon apical exposure to T-84 cell monolayers. S. flexneri serotype 2a and 5, but not the non-invasive Escherichia coli strain F-18, share the ability to regulate expression of ZO-1, ZO-2, E-cadherin and to dephosphorylate occludin. The disruption of tight junctions is dependent on direct interaction of living Shigella with intestinal epithelial cells and is supported by heat-stable secreted bacterial products. Intestinal epithelial cells have the ability to compensate in part for S. flexneri induced regulation of tight junction-associated proteins.

Shigella flexneri Interactions with the Basolateral Membrane Domain of Polarized Model Intestinal Epithelium: Role of Lipopolysaccharide in Cell Invasion and in Activation of the Mitogen-Activated Protein Kinase ERK

An early step governing Shigella flexneri pathogenesis is the invasion of the colonic epithelium from the basolateral surface followed by disruption of the colonic epithelial barrier. Despite recent insight into S. flexneri-host interactions, much remains to be determined regarding the nature of the initial contact between S. flexneri and the host epithelial basolateral membrane domain. Since the lipopolysaccharide (LPS) is located at the outermost part of the bacterial membrane, we considered that this component might be used by S. flexneri to attach to the basolateral surface of the intestinal epithelium and promote a proinflammatory response. Therefore, polarized human T84 intestinal epithelial cells were infected from the basolateral surface with either wild-type S. flexneri or one of its isogenic LPS-defective strains with mutations in either rfc, rfaL, or galU. We found that both adherence to and internalization into the basolateral surface of a polarized intestinal epithelium with S. flexneri were highly dependent on the length of the LPS (i.e., rfc > rfaL > galU). Furthermore, the addition of the anti-inflammatory LPS (RsDPLA) considerably decreased the invasion profile of wild-type S. flexneri by nearly 50%. Since LPS is associated with host inflammation, we further examined whether this molecule was involved in Shigella-induced inflammatory events. We found that S. flexneri LPS plays an important role in mediating epithelial-derived signaling, which leads to directed migration of polymorphonuclear leukocytes across model intestinal epithelium. This signaling most likely involves the activation of the mitogen-activated protein kinase extracellular regulated kinase. Thus, our findings have important implications on the understanding of the mechanisms by which S. flexneri can elicit mucosal inflammation.

Shiga toxin translocation across intestinal epithelial cells is enhanced by neutrophil transmigration

Shiga toxin-producing E. coli (STEC) is a food-borne pathogen that causes serious illness, including hemolytic-uremic syndrome (HUS). STEC colonizes the lower intestine and produces Shiga toxins (Stxs). Stxs appear to translocate across intestinal epithelia and affect sensitive endothelial cell beds at various sites. We have previously shown that Stxs cross polarized intestinal epithelial cells (IECs) via a transcellular route and remain biologically active. Since acute inflammatory infiltration of the gut and fecal leukocytes is seen in many STEC-infected patients and since polymorphonuclear leukocyte (PMN) transmigration across polarized IECs diminishes the IEC barrier function in vitro, we hypothesized that PMN transmigration may enhance Stx movement across IECs. We found that basolateral-to-apical transmigration of neutrophils significantly increased the movement of Stx1 and Stx2 across polarized T84 IECs in the opposite direction. The amount of Stx crossing the T84 barrier was proportional to the degree of neutrophil transmigration, and the increase in Stx translocation appears to be due to increases in paracellular permeability caused by migrating PMNs. STEC clinical isolates applied apically induced PMN transmigration across and interleukin-8 (IL-8) secretion from T84 cells. Of the 10 STEC strains tested, three STEC strains lacking eae and espB (eae- and espB-negative STEC strains) induced significantly more neutrophil transmigration and significantly greater IL-8 secretion than eae- and espB-positive STEC or enteropathogenic E. coli. This study suggests that STEC interaction with intestinal epithelia induces neutrophil recruitment to the intestinal lumen, resulting in neutrophil extravasation across IECs, and that during this process Stxs may pass in greater amounts into underlying tissues, thereby increasing the risk of HUS.

Molecular physiology and pathophysiology of tight junctions V. assault of the tight junction by enteric pathogens

Studies of the impact of enteric pathogens and their virulence factors on the proteins comprising the tight junction and zonula adherens offer a novel approach to dissection of tight junctional complex regulation. Most studies to date provide only tantalizing clues that select pathogens may indeed assault the tight junctional complex. Information on critical human pathogens such as Campylobacter jejuni and Shigella and Salmonella subspecies is lacking. Mechanistic studies are currently sparse, but available results on pathogenic Escherichia coli and specific virulence factors such as the Rho-modifying and protease bacterial toxins indicate four major mechanisms by which these pathogens may act: 1) direct cleavage of tight junctional structural proteins; 2) modification of the actin cytoskeleton; 3) activation of cellular signal transduction; and 4) triggering transmigration of polymorphonuclear cells across the epithelial cell barrier. New therapeutics may evolve from detailed studies of these pathogens and the cellular processes and proteins they disrupt.

Non-pathogenic bacteria elicit a differential cytokine response by intestinal epithelial cell/leucocyte co-cultures

BACKGROUND AND AIM

Intestinal epithelial cells (IEC) are thought to participate in the mucosal defence against bacteria and in the regulation of mucosal tissue homeostasis. Reactivity of IEC to bacterial signals may depend on interactions with immunocompetent cells. To address the question of whether non-pathogenic bacteria modify the immune response of the intestinal epithelium, we co-cultivated enterocyte-like CaCO-2 cells with human blood leucocytes in separate compartments of transwell cultures.

METHODS

CaCO-2/PBMC co-cultures were stimulated with non-pathogenic bacteria and enteropathogenic Escherichia coli. Expression of tumour necrosis factor alpha (TNF-alpha), interleukin (IL)-1beta, IL-8, monocyte chemoattracting protein 1 (MCP-1), and IL-10 was studied by enzyme linked immunosorbent assays (cytokine secretion) and by semiquantitative reverse transcription-polymerase chain reaction.

RESULTS

Challenge of CaCO-2 cells with non-pathogenic E coli and Lactobacillus sakei induced expression of IL-8, MCP-1, IL-1beta, and TNF-alpha mRNA in the presence of underlying leucocytes. Leucocyte sensitised CaCO-2 cells produced TNF-alpha and IL-1beta whereas IL-10 was exclusively secreted by human peripheral blood mononuclear cells. CaCO-2 cells alone remained hyporesponsive to the bacterial challenge. Lactobacillus johnsonii, an intestinal isolate, showed reduced potential to induce proinflammatory cytokines but increased transforming growth factor beta mRNA in leucocyte sensitised CaCO-2 cells. TNF-alpha was identified as one of the early mediators involved in cellular cross talk. In the presence of leucocytes, discriminative activation of CaCO-2 cells was observed between enteropathogenic E coli and non-pathogenic bacteria.

CONCLUSION

The differential recognition of non-pathogenic bacteria by CaCO-2 cells required the presence of underlying leucocytes. These results strengthen the hypothesis that bacterial signalling at the mucosal surface is dependent on a network of cellular interactions.

The pathogenesis of Shigella flexneri infection: lessons from in vitro and in vivo studies

Shigella flexneri is a Gram-negative facultatively intracellular pathogen responsible for bacillary dysentery in humans. More than one million deaths occur yearly due to infections with Shigella spp. and the victims are mostly children of the developing world. The pathogenesis of Shigella centres on the ability of this organism to invade the colonic epithelium where it induces severe mucosal inflammation. Much information that we have gained concerning the pathogenesis of Shigella has been derived from the study of in vitro models of infection. Using these techniques, a number of the molecular mechanisms by which Shigella invades epithelial cells and macrophages have been identified. In vivo models of shigellosis have been hampered since humans are the only natural hosts of Shigella. However, experimental infection of macaques as well as the murine lung and rabbit ligated ileal loop models have been important in defining some of the immune and inflammatory components of the disease. In particular, the murine lung model has shed light on the development of systemic and local immune protection against Shigella infection. It would be naive to believe that any one model of Shigella infection could adequately represent the complexity of the disease in humans, and more sophisticated in vivo models are now necessary. These models require the use of human cells and tissue, but at present such models remain in the developmental stage. Ultimately, however, it is with such studies that novel treatments and vaccine candidates for the treatment and prevention of shigellosis will be designed.

Inhibition of Shigella flexneri-induced transepithelial migration of polymorphonuclear leucocytes by cadaverine

Dysentery caused by Shigella species is characterized by infiltration of polymorphonuclear leucocytes (PMNs) into the colonic mucosa. Shigella spp. evolved into pathogens by the acquisition of virulence genes and by the deletion of 'antivirulence' genes detrimental to its pathogenic lifestyle. An example is cadA (encoding lysine decarboxylase), which is uniformly absent in Shigella spp., whereas it is present in nearly all isolates of the closely related non-pathogen Escherichia coli. Here, using monolayers of T84 cells to model the human intestinal epithelium, we determined that the introduction of cadA into S. flexneri and the expression of lysine decarboxylase attenuated the bacteria's ability to induce PMN influx across model intestinal epithelium. Such inhibition was caused by cadaverine generated from the decarboxylation of lysine. Cadaverine treatment of model intestinal epithelia specifically inhibited S. flexneri induction of PMN transepithelial migration, while having no effect on the ability of Salmonella or enteropathogenic E. coli (EPEC) to induce PMN migration. These observations not only provide insight into mechanisms of S. flexneri pathogen evolution and pathogenesis, but also suggest a potential for the use of cadaverine in the treatment of dysentery.

Orchestration of neutrophil movement by intestinal epithelial cells in response to Salmonella typhimurium can be uncoupled from bacterial internalization

Intestinal epithelial cells respond to Salmonella typhimurium by internalizing this pathogen and secreting, in a polarized manner, an array of chemokines which direct polymorphonuclear leukocyte (PMN) movement. Notably, interleukin-8 (IL-8) is secreted basolaterally and directs PMN through the lamina propria, whereas pathogen-elicited epithelial chemoattractant (PEEC) is secreted apically and directs PMN migration across the epithelial monolayer to the intestinal lumen. While most studies of S. typhimurium pathogenicity have focused on the mechanism by which this bacterium invades its host, the enteritis characteristically associated with salmonellosis appears to be more directly attributable to the PMN movement that occurs in response to this pathogen. Therefore, we sought to better understand the relationship between S. typhimurium invasion and epithelial promotion of PMN movement. First, we investigated whether S. typhimurium becoming intracellular was necessary or sufficient to induce epithelial promotion of PMN movement. Blocking S. typhimurium invasion by preventing, with cytochalasin D, the epithelial cytoskeletal rearrangements which mediate internalization did not reduce the epithelial promotion of PMN movement. Conversely, bacterial attainment of an intracellular position was not sufficient to induce model epithelia to direct PMN transmigration, since neither basolateral invasion by S. typhimurium nor apical internalization of an invasion-deficient mutant (achieved by inducing membrane ruffling with epidermal growth factor) induced this epithelial cell response. These results indicate that specific interactions between the apical surface of epithelial cells and S. typhimurium, rather than simply bacterial invasion, mediate the epithelial direction of PMN transmigration. To further investigate the means by which S. typhimurium induces epithelia to direct PMN movement, we investigated whether the same signaling pathways regulate secretion of IL-8 and PEEC. IL-8 secretion, but not PEEC secretion, was activated by phorbol myristate acetate and blocked by an inhibitor (mg-132) of the proteosome which mediates NF-kappabeta activation. Further, secretion of IL-8, but not PEEC, was activated by an entry-deficient (HilDelta) S. typhimurium mutant or by basolateral invasion of a wild-type strain. Together, these results indicate that distinct signaling pathways mediate S. typhimurium invasion, induction of IL-8 secretion, and induction of PEEC secretion in model intestinal epithelia.