Vibrio cholerae hemagglutinin/protease (HA/protease) causes morphological changes in cultured epithelial cells and perturbs their paracellular barrier function

Upregulation of occludin by cytolethal distending toxin facilitates Glaesserella parasuis adhesion to respiratory tract cells

Virulent Glaesserella parasuis may engender systemic infection characterized by fibrinous polyserositis and pneumonia. G. parasuis causes systemic disease through upper respiratory tract infection, but the mechanism has not been fully characterized. Tight junction (TJ) proteins maintain the integrity and impermeability of the epithelial barriers. In this work, we applied the recombinant cytolethal distending toxin (CDT) holotoxin and cdt-deficient mutants to assess whether CDT interacted with TJ proteins of airway tract cells. Our results indicated that CDT induced the TJ occludin (OCLN) expression in newborn pig tracheal epithelial cells within the first 3 hours of bacterial infection, followed by a significant decrease. Overexpression of OCLN in target cells made them more susceptible to G. parasuis adhesion, whereas ablation of OCLN expression by CRISPR/Cas 9 gene editing technology in target cells decreased their susceptibility to bacterial adhesion. In addition, CDT treatment could upregulate the OCLN levels in the lung tissue of C57/BL6 mice. In summary, highly virulent G. parasuis strain SC1401 stimulated the tight junction expression, resulting in higher bacterial adhesion to respiratory tract cells, and this process is closely related to CDT. Our results may provide novel insights into G. parasuis infection and CDT-mediated pathogenesis.

Vibrio cholerae, classification, pathogenesis, immune response, and trends in vaccine development

Vibrio cholerae is the causative agent of cholera, a highly contagious diarrheal disease affecting millions worldwide each year. Cholera is a major public health problem, primarily in countries with poor sanitary conditions and regions affected by natural disasters, where access to safe drinking water is limited. In this narrative review, we aim to summarize the current understanding of the evolution of virulence and pathogenesis of V. cholerae as well as provide an overview of the immune response against this pathogen. We highlight that V. cholerae has a remarkable ability to adapt and evolve, which is a global concern because it increases the risk of cholera outbreaks and the spread of the disease to new regions, making its control even more challenging. Furthermore, we show that this pathogen expresses several virulence factors enabling it to efficiently colonize the human intestine and cause cholera. A cumulative body of work also shows that V. cholerae infection triggers an inflammatory response that influences the development of immune memory against cholera. Lastly, we reviewed the status of licensed cholera vaccines, those undergoing clinical evaluation, and recent progress in developing next-generation vaccines. This review offers a comprehensive view of V. cholerae and identifies knowledge gaps that must be addressed to develop more effective cholera vaccines.

Unraveling enterohemorrhagic Escherichia coli infection: the promising role of dietary compounds and probiotics in bacterial elimination and host innate immunity boosting

The innate immune system has developed sophisticated strategies to defense against infections. Host cells utilize the recognition machineries such as toll-like receptors and nucleotide binding and oligomerization domain-like receptors to identify the pathogens and alert immune system. However, some pathogens have developed tactics to evade host defenses, including manipulation of host inflammatory response, interference with cell death pathway, and highjack of phagocytosis signaling for a better survival and colonization in host. Enterohemorrhagic Escherichia coli (EHEC) is a notorious foodborne pathogen that causes severe tissue damages and gastrointestinal diseases, which has been reported to disturb host immune responses. Diverse bioactive compounds such as flavonoids, phenolic acids, alkaloids, saccharides, and terpenoids derived from food varieties and probiotics have been discovered and investigated for their capability of combating bacterial infections. Some of them serve as novel antimicrobial agents and act as immune boosters that harness host immune system. In this review, we will discuss how EHEC, specifically E. coli O157:H7, hijacks the host immune system and interferes with host signaling pathway; and highlight the promising role of food-derived bioactive compounds and probiotics in harnessing host innate immunity and eliminating E. coli O157:H7 infection with multiple strategies.

Cross-kingdom inhibition of bacterial virulence and communication by probiotic yeast metabolites

BACKGROUND

Probiotic milk-fermented microorganism mixtures (e.g., yogurt, kefir) are perceived as contributing to human health, and possibly capable of protecting against bacterial infections. Co-existence of probiotic microorganisms are likely maintained via complex biomolecular mechanisms, secreted metabolites mediating cell-cell communication, and other yet-unknown biochemical pathways. In particular, deciphering molecular mechanisms by which probiotic microorganisms inhibit proliferation of pathogenic bacteria would be highly important for understanding both the potential benefits of probiotic foods as well as maintenance of healthy gut microbiome.

RESULTS

The microbiome of a unique milk-fermented microorganism mixture was determined, revealing a predominance of the fungus Kluyveromyces marxianus. We further identified a new fungus-secreted metabolite-tryptophol acetate-which inhibits bacterial communication and virulence. We discovered that tryptophol acetate blocks quorum sensing (QS) of several Gram-negative bacteria, particularly Vibrio cholerae, a prominent gut pathogen. Notably, this is the first report of tryptophol acetate production by a yeast and role of the molecule as a signaling agent. Furthermore, mechanisms underscoring the anti-QS and anti-virulence activities of tryptophol acetate were elucidated, specifically down- or upregulation of distinct genes associated with V. cholerae QS and virulence pathways.

CONCLUSIONS

This study illuminates a yet-unrecognized mechanism for cross-kingdom inhibition of pathogenic bacteria cell-cell communication in a probiotic microorganism mixture. A newly identified fungus-secreted molecule-tryptophol acetate-was shown to disrupt quorum sensing pathways of the human gut pathogen V. cholerae. Cross-kingdom interference in quorum sensing may play important roles in enabling microorganism co-existence in multi-population environments, such as probiotic foods and the gut microbiome. This discovery may account for anti-virulence properties of the human microbiome and could aid elucidating health benefits of probiotic products against bacterially associated diseases. Video Abstract.

DksA coordinates bile-mediated regulation of virulence-associated phenotypes in type three secretion system-positive Vibrio cholerae

In order to cause disease, pathogenic strains of Vibrio cholerae rely on intricate regulatory networks to orchestrate the transition between their native aquatic environment and the human host. For example, bacteria in a nutrient-starved environment undergo a metabolic shift called the stringent response, which is mediated by the alarmone ppGpp and an RNA-polymerase binding transcriptional factor, DksA. In O1 serogroup strains of V. cholerae, which use the toxin co-regulated pilus (TCP) and cholera toxin (CT) as primary virulence factors, DksA was reported to have additional functions as a mediator of virulence gene expression. However, little is known about the regulatory networks coordinating virulence phenotypes in pathogenic strains that use TCP/CT-independent virulence mechanisms. We therefore investigated whether functions of DksA outside of the stringent response are conserved in type three secretion system (T3SS)-positive V. cholerae. In using the T3SS-positive clinically isolated O39 serogroup strain AM-19226, we observed an increase in dksA expression in the presence of bile at 37 °C. However, DksA was not required for wild-type levels of T3SS structural gene expression, or for colonization in vivo. Rather, data indicate that DksA positively regulates the expression of master regulators in the motility hierarchy. Interestingly, the ΔdksA strain forms a less robust biofilm than the WT parent strain at both 30 and 37 °C. We also found that DksA regulates the expression of hapR, encoding a major regulator of biofilm formation and protease expression. Athough DksA does not appear to modulate T3SS virulence factor expression, its activity is integrated into existing regulatory networks governing virulence-related phenotypes. Strain variations therefore may take advantage of conserved ancestral proteins to expand regulons responding to in vivo signals and thus coordinate multiple phenotypes important for infection.

Measurement of novel intestinal secretory and barrier pathways and effects of proteases

The epithelial lining of the gastrointestinal (GI) tract in conjunction with the enteric nervous system (ENS) plays an important role in mediating solute absorption and secretion. A dysregulated ionic movement across the epithelium can result in GI diseases that manifest as either watery diarrhea or constipation. Hirschsprung disease is an example of an ENS disorder characterized by absence of enteric ganglia in distal gut resulting in obstructive phenotype. Receptor rearranged during transfection (RET) gene variants are the most commonly recognized genetic associations with Hirschsprung disease. In this issue of Neurogastroenterology and Motility, Russell et al demonstrate that RET mediates colonic ion transport through modulation of cholinergic nerves. They go on to show inhibition of RET can attenuate accelerated transit in a rat model. Normalizing secretory and absorptive defects has been an attractive therapeutic strategy. In addition to the intrinsic regulation of secretory processes, luminal mediators like bile acids, short-chain fatty acids, and proteases can affect both secretion and barrier function of the intestinal epithelium. Elevated levels of proteases have been identified in a wide range of GI diseases including irritable bowel syndrome. Proteases are known to cause visceral hypersensitivity and barrier disruption in vitro and in animal models. The goals of this review are to describe fundamental concepts related to intestinal epithelial secretion, the utility of Ussing chambers to measure ionic mechanisms and to discuss examples of novel signaling pathways; namely the RET signaling cascade in secretomotor neurons and effects of luminal proteases on barrier and ionic secretion.

Molecular insights into Vibrio cholerae's intra-amoebal host-pathogen interactions

Vibrio cholerae, which causes the diarrheal disease cholera, is a species of bacteria commonly found in aquatic habitats. Within such environments, the bacterium must defend itself against predatory protozoan grazers. Amoebae are prominent grazers, with Acanthamoeba castellanii being one of the best-studied aquatic amoebae. We previously showed that V. cholerae resists digestion by A. castellanii and establishes a replication niche within the host's osmoregulatory organelle. In this study, we decipher the molecular mechanisms involved in the maintenance of V. cholerae's intra-amoebal replication niche and its ultimate escape from the succumbed host. We demonstrate that minor virulence features important for disease in mammals, such as extracellular enzymes and flagellum-based motility, have a key role in the replication and transmission of V. cholerae in its aqueous environment. This work, therefore, describes new mechanisms that provide the pathogen with a fitness advantage in its primary habitat, which may have contributed to the emergence of these minor virulence factors in the species V. cholerae.

A study of the damage of the intestinal mucosa barrier structure and function of Ctenopharyngodon idella with Aeromonas hydrophila

The aim of this study is to explore the effect of Aeromonas hydrophila on the intestinal mucosal barrier structure and intestinal permeability in grass carp (Ctenopharyngodon idella). Histopathological examinations showed that A. hydrophila induced severe intestinal lesions, including inflammatory cell infiltration and intestinal villus fusion and swelling. Messenger RNA (mRNA) expression of the inflammatory cytokines TNF-α, IL-1β, IL-8, IL-10 and MyD88 was significantly increased after infection with A. hydrophila. The permeability of intestinal mucosa was determined using Evans blue (EB) and D-lactic acid. The results indicated that the levels of EB and serum D-lactic acid were significantly increased after infection with A. hydrophila (p < 0.05). Our results also indicated that the intestinal mucosal barrier injury induced by A. hydrophila infection was closely associated with the expression of the tight junction (TJ) protein zonula occludens-1 (ZO-1), occludin, claudin b and claudin c as well as the activity of Na⁺, K⁺-ATPase and Ca²⁺, Mg²⁺-ATPase. Lower mRNA levels of occludin and lower Na⁺, K⁺-ATPase and Ca²⁺, Mg²⁺-ATPase activity in the intestines were observed after challenge. ZO-1 and claudin c were significantly increased 24 h after infection with A. hydrophila. The most interesting finding was that claudin b also significantly increased 24 h after challenge and then decreased to lower levels at 72, 120 and 168 h post-infection compared to the PBS-treated control group. The results demonstrated that grass carp infection with A. hydrophila induced intestinal inflammation and impaired the structure and function of the intestinal mucosal barrier.

Treatment with Entinostat Heals Experimental Cholera by Affecting Physical and Chemical Barrier Functions of Intestinal Epithelia

We have shown previously that oral treatment with sodium butyrate or phenylbutyrate in an experimental model of shigellosis improves clinical outcomes and induces the expression of the antimicrobial peptide CAP-18 in the large intestinal epithelia. In a subsequent study, we found that entinostat, an aroylated phenylenediamine compound, has similar therapeutic potential against shigellosis. In this study, we aimed to evaluate entinostat as a potential candidate for host-directed therapy against cholera in an experimental model. Vibrio cholerae-infected rabbits were treated with two different dose regimens of entinostat: either 0.5 mg twice daily for 2 days or 1 mg once daily for 2 days. The effects of treatment on clinical outcomes and V. cholerae shedding (CFU count in stool) were observed. Immunohistochemical analysis was carried out to assess CAP-18 expression in ileal and jejunal mucosae. The serum zonulin level was measured by an enzyme-linked immunosorbent assay (ELISA) to evaluate gut permeability. Infection of rabbits with V. cholerae downregulated CAP-18 expression in the ileal epithelium; the expression was replenished by oral treatment with entinostat at either dose regimen. The level of zonulin, a marker of gut permeability, in serum was upregulated after infection, and this upregulation was counteracted after treatment with entinostat. Entinostat treatment also led to recovery from cholera and a decline in the V. cholerae count in stool. In conclusion, the improved clinical outcome of cholera for rabbits treated with entinostat is associated with the induction of CAP-18 and the reduction of gut epithelial permeability.

Effect of Bacillus subtilis on Aeromonas hydrophila-induced intestinal mucosal barrier function damage and inflammation in grass carp (Ctenopharyngodon idella)

Our study explored the effect of oral intubation of Bacillus subtilis on Aeromonas hydrophila-induced intestinal mucosal barrier function damage and inflammation in grass carp. The mid-intestine mucosal tissue was collected for ATPase activity measurement. Intestinal mucosa was also ultrastructurally examined with transmission electron microscope (TEM), and its permeability was determined using Evans blue (EB) and D-lactic acid. The mid-intestine pro-inflammation cytokine, MyD88 and tight junction (TJ) protein mRNA expression levels were measured using real-time quantitative PCR. The results revealed that B. subtilis was found to prevent the decrease in the activity of Na+, K+-ATPase and Ca2+, Mg2+-ATPase, as well as the increase in EB and D-lactic acid concentration and inflammation induced by A. hydrophila in grass carp. Compared with A. hydrophila groups, B. subtilis safeguarded the integrity of intestinal villi and tight junction structure and restrained A. hydrophila-induced down-regulation of TJ proteins zonula occludens-1 (ZO-1) and occludin. B. subtilis also restrained up-regulation of TJ protein claudin b, pro-inflammation cytokine tumour necrosis factor α (TNF-α), cytokine interleukin 8 (IL-8), IL-1β, and adaptor protein myeloid differentiation factor 88 (MyD88) mRNA levels. Thus, oral intubation of B. subtilis could reduce A. hydrophila-induced intestinal mucosal barrier function damage and inflammation.

Role of stress and intestinal barrier dysfunction in inflammatory bowel disease

Stress, provoked by stressors, can cause a series of physical and psychological reactions and initiate both central and peripheral responses to maintain homeostasis. The causes of inflammatory bowel disease are poorly understood, which may involve complex interactions among environmental, psychological, dietary, immunological and pharmacological factors. This paper reviews the epidemiological and clinical data on how stress may precipitate in the attack, relapse and recurrence of inflammatory bowel disease. A possible biological mechanism is the impaired intestinal barrier including physical barrier, biologic barrier, immunological barrier and chemical barrier. Clinical epidemiological data and many animal studies show that stress may reduce mucous secretion and increase colonic permeability, which are the characteristic changes of inflammatory bowel disease. Understanding the relationship between stress and inflammatory bowel disease can improve the health related quality of life and lower the medical cost.

Vibrio cholerae hemagglutinin(HA)/protease: An extracellular metalloprotease with multiple pathogenic activities

Vibrio cholerae of serogroup O1 and O139, the etiological agent of the diarrheal disease cholera, expresses the extracellular Zn-dependent metalloprotease hemagglutinin (HA)/protease also reported as vibriolysin. This enzyme is also produced by non-O1/O139 (non-cholera) strains that cause mild, sporadic illness (i.e. gastroenteritis, wound or ear infections). Orthologs of HA/protease are present in other members of the Vibrionaceae family pathogenic to humans and fish. HA/protease belongs to the M4 neutral peptidase family and displays significant amino acid sequence homology to Pseudomonas aeruginosa elastase (LasB) and Bacillus thermoproteolyticus thermolysin. It exhibits a broad range of potentially pathogenic activities in cell culture and animal models. These activities range from the covalent modification of other toxins, the degradation of the protective mucus barrier and disruption of intestinal tight junctions. Here we review (i) the structure and regulation of HA/protease expression, (ii) its interaction with other toxins and the intestinal mucosa and (iii) discuss the possible role(s) of HA/protease in the pathogenesis of cholera.

Interplay among cyclic diguanylate, HapR, and the general stress response regulator (RpoS) in the regulation of Vibrio cholerae hemagglutinin/protease

Vibrio cholerae secretes the Zn-dependent metalloprotease hemagglutinin (HA)/protease (mucinase), which is encoded by hapA and displays a broad range of potential pathogenic activities. Expression of HA/protease has a stringent requirement for the quorum-sensing regulator HapR and the general stress response regulator RpoS. Here we report that the second messenger cyclic diguanylic acid (c-di-GMP) regulates the production of HA/protease in a negative manner. Overexpression of a diguanylate cyclase to increase the cellular c-di-GMP pool resulted in diminished expression of HA/protease and its positive regulator, HapR. The effect of c-di-GMP on HapR was independent of LuxO but was abolished by deletion of the c-di-GMP binding protein VpsT, the LuxR-type regulator VqmA, or a single-base mutation in the hapR promoter that prevents autorepression. Though expression of HapR had a positive effect on RpoS biosynthesis, direct manipulation of the c-di-GMP pool at a high cell density did not significantly impact RpoS expression in the wild-type genetic background. In contrast, increasing the c-di-GMP pool severely inhibited RpoS expression in a ΔhapR mutant that is locked in a regulatory state mimicking low cell density. Based on the above findings, we propose a model for the interplay between HapR, RpoS, and c-di-GMP in the regulation of HA/protease expression.

Tight junctions and viral entry

A wide variety of different viruses use tight junction (TJ) proteins in the course of infection and different mechanisms of pathogen–TJ interactions have been described; pathogens may induce the reorganization or degradation of distinct TJ proteins, reorganization of the cell cytoskeleton, activation of host-cell signaling pathways and/or use TJ proteins as receptors to enter host cells. Most recently, the TJ proteins claudin-1 and occludin have been identified as essential host factors for HCV entry. Furthermore, TJ protein occludin has been shown to play an important role in the species specificity of HCV infection. Recent data suggest that claudin-1 is a promising target for antiviral strategies. The aim of this article is to elucidate the impact of the interplay between pathogens and TJ proteins for pathogen–host interactions, summarize recent findings regarding the role of TJ proteins in HCV entry and highlight the relevance of TJ proteins for the development of novel antiviral strategies.

Regulation of hemagglutinin/protease expression by the VarS/VarA-CsrA/B/C/D system in Vibrio cholerae

In this study, through the analysis of Vibrio cholerae 2740-80 mutant strains produced by the cholera toxin subunit B gene containing Mariner-based transposon, we found that disruption of the varS gene, a member of the recently reported sensory system VarS/VarA-CsrA/B/C/D, resulted in altered expression of hemagglutinin/protease A. To further investigate the connection between VarS and HapA, we generated an additional varS mutant, V. cholerae 2740-80-VS, and examined the effect of this mutation on expression of HapA and of genes in the VarS/VarA-CsrA/B/C/D system. 2740-80-VS showed decreased expression of varS, csrB/C, hapR, and hapA along with increased biofilm production. Interestingly, expression of the alternative sigma factor sigma(s), which is important for adaptation to environmental stress, was also decreased in this mutant. These results indicate that the VarS/VarA-CsrA/B/C/D system is involved in the control of HapA expression and biofilm production in V. cholerae 2740-80 through HapR regulation, and also that VarS/VarA controls expression of sigma(s) for HapA regulation.

Effect of a matrix metalloproteinase sequestering biomaterial on Caco-2 epithelial cell barrier integrity in vitro

A novel matrix metalloproteinase sequestering biomaterial (MI Theramer beads) restored the epithelial barrier in a double chamber in vitro test system after disruption by Cytochalasin D and the secretion of the metalloproteinase MMP-2. MI beads are chemically modified (hydroxamated) poly(methacrylic acid-co-methyl methacrylate). We are exploring the utility of this material in inflammatory bowel disease (IBD), in which one manifestation is a compromised intestinal epithelial barrier. In a first step towards this goal we incubated MI beads (or polymethyl methacrylate control beads) with Caco-2 epithelial cells and mesenchymal 3T3 fibroblasts on two sides of a Matrigel((R))-coated filter and used Cytochalasin D (Cyto D) to activate MMP-2 (secreted by the 3T3 cells), disrupt actin filaments of Caco-2 cells and render the epithelial barrier leaky, as measured by dextran fluorescein equilibration. Addition of MI beads to Cyto D-treated cells inhibited active MMP-2 and prevented equilibration of dextran fluorescein. This study is the first step in showing a potential benefit to local (as opposed to systemic) inhibition of metalloproteinases in IBD or other intestinal inflammatory diseases.

Intestinal barrier function: molecular regulation and disease pathogenesis

The intestinal epithelium is a single-cell layer that constitutes the largest and most important barrier against the external environment. It acts as a selectively permeable barrier, permitting the absorption of nutrients, electrolytes, and water while maintaining an effective defense against intraluminal toxins, antigens, and enteric flora. The epithelium maintains its selective barrier function through the formation of complex protein-protein networks that mechanically link adjacent cells and seal the intercellular space. The protein networks connecting epithelial cells form 3 adhesive complexes: desmosomes, adherens junctions, and tight junctions. These complexes consist of transmembrane proteins that interact extracellularly with adjacent cells and intracellularly with adaptor proteins that link to the cytoskeleton. Over the past decade, there has been increasing recognition of an association between disrupted intestinal barrier function and the development of autoimmune and inflammatory diseases. In this review we summarize the evolving understanding of the molecular composition and regulation of intestinal barrier function. We discuss the interactions between innate and adaptive immunity and intestinal epithelial barrier function, as well as the effect of exogenous factors on intestinal barrier function. Finally, we summarize clinical and experimental evidence demonstrating intestinal epithelial barrier dysfunction as a major factor contributing to the predisposition to inflammatory diseases, including food allergy, inflammatory bowel diseases, and celiac disease.

Translocation of a Vibrio cholerae type VI secretion effector requires bacterial endocytosis by host cells

The type VI secretion system (T6SS) is a virulence mechanism common to several Gram-negative pathogens. In Vibrio cholerae, VgrG-1 is required for T6SS-dependent secretion. VgrG-1 is also secreted by T6SS and displays a C-terminal actin crosslinking domain (ACD). Using a heterologous reporter enzyme in place of the ACD, we show that the effector and secretion functions of VgrG-1 are genetically dissociable with the ACD being dispensable for secretion but required for T6SS-dependent phenotypes. Furthermore, internalization of bacteria is required for ACD translocation into phagocytic target cells. Inhibiting bacterial uptake abolishes actin crosslinking, while improving intracellular survival enhances it. Otherwise resistant nonphagocytic cells become susceptible to T6SS-mediated actin crosslinking when engineered to take up bacteria. Our results support a model for translocation of VgrG C-terminal effector domains into target cell cytosol by a process that requires trafficking of bacterial cells into an endocytic compartment where translocation is triggered by an unknown signal.

Ultrastructural changes in the small intestine of suckling mice, caused by vibrio cholerae hemagglutinin/protease

Suckling mice aged 4-5 days were injected with Vibrio cholerae hemagglutinin/protease and ultrastructural changes in their small intestine were studied after 5 h. The preparation caused a statistically significant accumulation of fluid in the intestine, appearance of large gaps along cell-cell spaces in the villi and crypts, intense production and secretion of the mucus by goblet cells. The formation of interepithelial cavities was paralleled by vascular changes, supplemented by extravasal disorders caused by mast cell reaction. The role of enterochromaffin cells and lipofibroblasts, modulating the secretion in the intestine, is confirmed.

Vibrio zinc-metalloprotease causes photoinactivation of coral endosymbionts and coral tissue lesions

BACKGROUND

Coral diseases are emerging as a serious threat to coral reefs worldwide. Of nine coral infectious diseases, whose pathogens have been characterized, six are caused by agents from the family Vibrionacae, raising questions as to their origin and role in coral disease aetiology.

METHODOLOGY/PRINCIPAL FINDINGS

Here we report on a Vibrio zinc-metalloprotease causing rapid photoinactivation of susceptible Symbiodinium endosymbionts followed by lesions in coral tissue. Symbiodinium photosystem II inactivation was diagnosed by an imaging pulse amplitude modulation fluorometer in two bioassays, performed by exposing Symbiodinium cells and coral juveniles to non-inhibited and EDTA-inhibited supernatants derived from coral white syndrome pathogens.

CONCLUSION/SIGNIFICANCE

These findings demonstrate a common virulence factor from four phylogenetically related coral pathogens, suggesting that zinc-metalloproteases may play an important role in Vibrio pathogenicity in scleractinian corals.

Role of the histone-like nucleoid structuring protein in the regulation of rpoS and RpoS-dependent genes in Vibrio cholerae

Production of the Zn-metalloprotease hemagglutinin (HA)/protease by Vibrio cholerae has been reported to enhance enterotoxicity in rabbit ileal loops and the reactogenicity of live cholera vaccine candidates. Expression of HA/protease requires the alternate sigma factor sigma(S) (RpoS), encoded by rpoS. The histone-like nucleoid structuring protein (H-NS) has been shown to repress rpoS expression in Escherichia coli. In V. cholerae strains of the classical biotype, H-NS has been reported to silence virulence gene expression. In this study we examined the role of H-NS in the expression of HA/protease and motility in an El Tor biotype strain by constructing a Deltahns mutant. The Deltahns mutant exhibited multiple phenotypes, such as production of cholera toxin in nonpermissive LB medium, reduced resistance to high osmolarity, enhanced resistance to low pH and hydrogen peroxide, and reduced motility. Depletion of H-NS by overexpression of a dominant-negative allele or by deletion of hns resulted in diminished expression of HA/protease. Epistasis analysis of HA/protease expression in Deltahns, DeltarpoS, and Deltahns DeltarpoS mutants, analysis of RpoS reporter fusions, quantitative reverse transcription-PCR measurements, and ectopic expression of RpoS in DeltarpoS and DeltarpoS Deltahns mutants showed that H-NS posttranscriptionally enhances RpoS expression. The Deltahns mutant exhibited a lower degree of motility and lower levels of expression of flaA, flaC, cheR-2, and motX mRNAs than the wild type. Comparison of the mRNA abundances of these genes in wild-type, Deltahns, DeltarpoS, and Deltahns DeltarpoS strains revealed that deletion of rpoS had a more severe negative effect on their expression. Interestingly, deletion of hns in the rpoS background resulted in higher expression levels of flaA, flaC, and motX, suggesting that H-NS represses the expression of these genes in the absence of sigma(S). Finally, we show that the cyclic AMP receptor protein and H-NS act along the same pathway to positively affect RpoS expression.

Homology modeling of hemagglutinin/protease [HA/P (vibriolysin)] from Vibrio cholerae: sequence comparision, residue interactions and molecular mechanism

Vibrio cholerae produces a zinc-containing and calcium-stabilized soluble hemagglutinin/protease, which has been earlier shown to have the ability to cleave several physiologically important substrates including mucin, fibronectin and lactoferin. This study presents homology modeling of hemagglutinin/protease (vibriolysin) from Vibrio cholerae in the presence of inhibitor HPI [N-(1-carboxy-3-phenylpropyl)-phenylalanyl-alpha-aspargine]. The 3D structure was predicted based on its sequence homology with Pseudomonas aeruginosa elastase (PAE). Comparison of the 3D structures of PAE and HA/P reveals a remarkable similarity having a conserved alpha + beta domain. The inhibitor shows similar binding features as seen in other metalloproteases of M4 peptidase family. The study also highlights the key catalytic residues as well as the residues at the S1 and S1' binding sub-sites. The similarities between the two proteins provide support for the hypothesis that the two enzymes have similar three-dimensional structures and a common mechanism of action. The fact that both enzymes are secreted as zinc-containing proteases, led us to further hypothesize that they may play similar role in pathogenesis.

Coral pathogens identified for White Syndrome (WS) epizootics in the Indo-Pacific

BACKGROUND

White Syndrome (WS), a general term for scleractinian coral diseases with acute signs of advancing tissue lesions often resulting in total colony mortality, has been reported from numerous locations throughout the Indo-Pacific, constituting a growing threat to coral reef ecosystems.

METHODOLOGY/PRINCIPAL FINDINGS

Bacterial isolates were obtained from corals displaying disease signs at three ws outbreak sites: Nikko Bay in the Republic of Palau, Nelly Bay in the central Great Barrier Reef (GBR) and Majuro Atoll in the Republic of the Marshall Islands, and used in laboratory-based infection trials to satisfy Henle-Koch's postulates, Evan's rules and Hill's criteria for establishing causality. Infected colonies produced similar signs to those observed in the field following exposure to bacterial concentrations of 1x10(6) cells ml(-1). Phylogenetic 16S rRNA gene analysis demonstrated that all six pathogens identified in this study were members of the gamma-Proteobacteria family Vibrionacae, each with greater than 98% sequence identity with the previously characterized coral bleaching pathogen Vibrio coralliilyticus. Screening for proteolytic activity of more than 150 coral derived bacterial isolates by a biochemical assay and specific primers for a Vibrio family zinc-metalloprotease demonstrated a significant association between the presence of isolates capable of proteolytic activity and observed disease signs.

CONCLUSION/SIGNIFICANCE

This is the first study to provide evidence for the involvement of a unique taxonomic group of bacterial pathogens in the aetiology of Indo-Pacific coral diseases affecting multiple coral species at multiple locations. Results from this study strongly suggest the need for further investigation of bacterial proteolytic enzymes as possible virulence factors involved in Vibrio associated acute coral infections.

Probiotic Escherichia coli Nissle 1917 inhibits leaky gut by enhancing mucosal integrity

BACKGROUND

Probiotics are proposed to positively modulate the intestinal epithelial barrier formed by intestinal epithelial cells (IECs) and intercellular junctions. Disruption of this border alters paracellular permeability and is a key mechanism for the development of enteric infections and inflammatory bowel diseases (IBDs).

METHODOLOGY AND PRINCIPAL FINDINGS

To study the in vivo effect of probiotic Escherichia coli Nissle 1917 (EcN) on the stabilization of the intestinal barrier under healthy conditions, germfree mice were colonized with EcN or K12 E. coli strain MG1655. IECs were isolated and analyzed for gene and protein expression of the tight junction molecules ZO-1 and ZO-2. Then, in order to analyze beneficial effects of EcN under inflammatory conditions, the probiotic was orally administered to BALB/c mice with acute dextran sodium sulfate (DSS) induced colitis. Colonization of gnotobiotic mice with EcN resulted in an up-regulation of ZO-1 in IECs at both mRNA and protein levels. EcN administration to DSS-treated mice reduced the loss of body weight and colon shortening. In addition, infiltration of the colon with leukocytes was ameliorated in EcN inoculated mice. Acute DSS colitis did not result in an anion secretory defect, but abrogated the sodium absorptive function of the mucosa. Additionally, intestinal barrier function was severely affected as evidenced by a strong increase in the mucosal uptake of Evans blue in vivo. Concomitant administration of EcN to DSS treated animals resulted in a significant protection against intestinal barrier dysfunction and IECs isolated from these mice exhibited a more pronounced expression of ZO-1.

CONCLUSION AND SIGNIFICANCE

This study convincingly demonstrates that probiotic EcN is able to mediate up-regulation of ZO-1 expression in murine IECs and confer protection from the DSS colitis-associated increase in mucosal permeability to luminal substances.

Role of nitric oxide during rotavirus infection

The pathophysiological mechanisms behind rotavirus-induced diarrhoea still remain incomplete. Current views suggest that the non-structural protein 4 (NSP4) of rotavirus and the enteric nervous system (ENS) participate in water secretion and diarrhoea. In the present work the role of nitric oxide (NO) in rotavirus infection and disease has been studied in vitro, mice and humans. Incubation of human intestinal epithelial cells (HT-29) with purified NSP4 but not with infectious virus produced NO2/NO3 accumulation in the incubation media. The NSP4-induced release of NO metabolites occurred within the first minutes after the addition of the toxin. Mice infected with murine rotavirus (strain EDIM) accumulated NO2/NO3 in the urine at the onset for diarrhoea. Following rotavirus infection, inducible nitric oxide synthetase (iNOS) mRNA was upregulated in ileum, but not in duodenum or jejunum of newborn pups within 5 days post-infection. A prospective clinical study including 46 children with acute rotavirus infection and age-matched controls concluded that rotavirus infection stimulates NO production during the course of the disease (P < 0.001). These observations identify NO as an important mediator of host responses during rotavirus infection.

Contribution of hemagglutinin/protease and motility to the pathogenesis of El Tor biotype cholera

Vibrio cholerae is a highly motile organism that secretes a Zn-dependent metalloprotease, hemagglutinin/protease (HapA). HapA has been shown to have mucinase activity and contribute to the reactogenicity of live vaccine candidates, but its role in cholera pathogenesis is not yet clear. The contribution of motility to pathogenesis is not fully understood, since conflicting results have been obtained with different strains, mutants, and animal models. The objective of this work was to determine the contribution of HapA and motility to the pathogenesis of El Tor biotype cholera. To this end we constructed isogenic motility (motY) and mucinase (hapA) single and double mutants of an El Tor biotype V. cholerae strain. Mutants were characterized for the expression of major virulence factors in vitro and in vivo. The motility mutant showed a remarkable increase in cholera toxin (CT), toxin coregulated pilus major subunit (TcpA), and HapA production in vitro. Increased TcpA and CT production could be explained by increased transcription of tcpA, ctxA, and toxT. No effect was detected on the transcription of hapA in the motility mutant. The sodium ionophore monensin diminished production of HapA in the parent but not in the motility mutant. Phenamil, a specific inhibitor of the flagellar motor, diminished CT production in the wild-type and motY strains. The hapA mutant showed increased binding to mucin. In contrast, the motY mutation diminished adherence to biotic and abiotic surfaces including mucin. Lack of HapA did not affect colonization in the suckling mouse model. The motility and mucinase defects did not prevent induction of ctxA and tcpA in the mouse intestine as measured by recombinase-based in vivo expression technology. Analysis of mutants in the rabbit ileal loop model showed that both V. cholerae motility and HapA were necessary for full expression of enterotoxicity.

Enterotoxigenicity of mature 45-kilodalton and processed 35-kilodalton forms of hemagglutinin protease purified from a cholera toxin gene-negative Vibrio cholerae non-O1, non-O139 strain

Cholera toxin gene-negative Vibrio cholerae non-O1, non-O139 strain PL-21 is the etiologic agent of cholera-like syndrome. Hemagglutinin protease (HAP) is one of the major secretory proteins of PL-21. The mature 45-kDa and processed 35-kDa forms of HAP were purified in the presence and absence of EDTA from culture supernatants of PL-21. Enterotoxigenicities of both forms of HAP were tested in rabbit ileal loop (RIL), Ussing chamber, and tissue culture assays. The 35-kDa HAP showed hemorrhagic fluid response in a dose-dependent manner in the RIL assay. Histopathological examination of 20 microg of purified protease-treated rabbit ileum showed the presence of erythrocytes and neutrophils in the upper part of the villous lamina propria. Treatment with 40 microg of protease resulted in gross damage of the villous epithelium with inflammation, hemorrhage, and necrosis. The 35-kDa form of HAP, when added to the lumenal surface of rat ileum loaded in an Ussing chamber, showed a decrease in the intestinal short-circuit current and a cell rounding effect on HeLa cells. The mature 45-kDa form of HAP showed an increase in intestinal short-circuit current in an Ussing chamber and a cell distending effect on HeLa cells. These results show that HAP may play a role in the pathogenesis of PL-21.

Vibrio parahaemolyticus disruption of epithelial cell tight junctions occurs independently of toxin production

Vibrio parahaemolyticus is a leading cause of seafood-borne gastroenteritis worldwide. Virulence is commonly associated with the production of two toxins, thermostable direct hemolysin (TDH) and TDH-related hemolysin (TRH). Although the majority of clinical isolates produce TDH and/or TRH, clinical samples lacking toxin genes have been identified. In the present study, we investigated the effects of V. parahaemolyticus on transepithelial resistance (TER) and paracellular permeability in Caco-2 cultured epithelial cells. We found that V. parahaemolyticus profoundly disrupts epithelial barrier function in Caco-2 cells and that this disruption occurs independently of toxin production. Clinical isolates with different toxin genotypes all led to a significant decrease in TER, which was accompanied by an increased flux of fluorescent dextran across the Caco-2 monolayer, and profound disruption of actin and the tight junction-associated proteins zonula occludin protein 1 and occludin. Purified TDH, even at concentrations eightfold higher than those produced by the bacteria, had no effect on either TER or paracellular permeability. We used lactate dehydrogenase release as a measure of cytotoxicity and found that this parameter did not correlate with the ability to disrupt tight junctions. As the effect on barrier function occurs independently of toxin production, we used PCR to determine the toxin genotypes of V. parahaemolyticus isolates obtained from both clinical and environmental sources, and we found that 5.6% of the clinical isolates were toxin negative. These data strongly indicate that the effect on tight junctions is not due to TDH and suggest that there are other virulence factors.

Chronic inflammatory pain leads to increased blood-brain barrier permeability and tight junction protein alterations

The blood-brain barrier (BBB) maintains brain homeostasis by limiting entry of substances to the central nervous system through interaction of transmembrane and intracellular proteins that make up endothelial cell tight junctions (TJs). Recently it was shown that the BBB can be modulated by disease pathologies including inflammatory pain. This study examined the effects of chronic inflammatory pain on the functional and molecular integrity of the BBB. Inflammatory pain was induced by injection of complete Freund's adjuvant (CFA) into the right plantar hindpaw in female Sprague-Dawley rats under halothane anesthesia; control animals were injected with saline. Edema and hyperalgesia were assessed by plethysmography and infrared paw-withdrawal latency. At 72 h postinjection, significant edema formation and hyperalgesia were noted in the CFA-treated rats. Examination of permeability of the BBB by in situ perfusion of [14C]sucrose while rats were under pentobarbital anesthesia demonstrated that CFA treatment significantly increased brain sucrose uptake. Western blot analysis of BBB TJ proteins showed no change in expression of zonula occludens-1 (an accessory protein) or actin (a cytoskeletal protein) with CFA treatment. Expression of the transmembrane TJ proteins occludin and claudin-3 and -5 significantly changed with CFA treatment with a 60% decrease in occludin, a 450% increase in claudin-3, and a 615% increase in claudin-5 expression. This study demonstrates that during chronic inflammatory pain, alterations in BBB function are associated with changes in specific transmembrane TJ proteins.

Transcriptional regulation of Vibrio cholerae hemagglutinin/protease by the cyclic AMP receptor protein and RpoS

Vibrio cholerae secretes a Zn-dependent metalloprotease, hemagglutinin/protease (HA/protease), which is encoded by hapA and displays a broad range of potentially pathogenic activities. Production of HA/protease requires transcriptional activation by the quorum-sensing regulator HapR. In this study we demonstrate that transcription of hapA is growth phase dependent and specifically activated in the deceleration and stationary growth phases. Addition of glucose in these phases repressed hapA transcription by inducing V. cholerae to resume exponential growth, which in turn diminished the expression of a rpoS-lacZ transcriptional fusion. Contrary to a previous observation, we demonstrate that transcription of hapA requires the rpoS-encoded sigma(s) factor. The cyclic AMP (cAMP) receptor protein (CRP) strongly enhanced hapA transcription in the deceleration phase. Analysis of rpoS and hapR mRNA in isogenic CRP+ and CRP- strains suggested that CRP enhances the transcription of rpoS and hapR. Analysis of strains containing hapR-lacZ and hapA-lacZ fusions confirmed that hapA is transcribed in response to concurrent quorum-sensing and nutrient limitation stimuli. Mutations inactivating the stringent response regulator RelA and the HapR-controlled AphA regulator did not affect HA/protease expression. Electrophoretic mobility shift experiments showed that pure cAMP-CRP and HapR alone do not bind the hapA promoter. This result suggests that HapR activation of hapA differs from its interaction with the aphA promoter and could involve additional factors.

Induction of interleukin-8 in T84 cells by Vibrio cholerae

The induction of interleukin-8 (IL-8) in vitro has been suggested to correlate with the reactogenicity of Vibrio cholerae vaccine candidates. V. cholerae vaccine candidate 638, a hemagglutinin protease/hap-defective strain, was recently reported to be well tolerated in human volunteers, suggesting a role for Hap in reactogenicity. We examined the role of hap in the induction of IL-8 in intestinal epithelial T84 cells. Wild-type V. cholerae strains 3038 and C7258 and a vaccine candidate strain, JBK70, induced levels of IL-8 similar to those of their isogenic hap mutants. Supernatant containing Hap did not stimulate IL-8 production at a variety of concentrations tested, suggesting that Hap itself does not induce IL-8 production. Furthermore, supernatant from CVD115, which had deletions of hap and rtxA (encoding repeats in toxin) and was derived from a reactogenic strain, CVD110, induced IL-8 production in T84 cells in a dose-dependent manner. The IL-8-stimulating activity of CVD115 culture supernatants was growth phase dependent and was strongest in stationary phase cultures. This IL-8 stimulator(s) was resistant to heat treatment but sensitive to proteinase. Protease activity in vitro did not correlate with the reactogenicity of V. cholerae vaccine candidates. Our data suggest that Hap is not an IL-8 inducer in T84 cells and that the IL-8 stimulator in the supernatant of V. cholerae culture may play a role in reactogenicity.

Haemagglutinin/protease expression and mucin gel penetration in El Tor biotype Vibrio cholerae

Vibrio cholerae of both biotypes produce a soluble Zn(2+)-dependent metalloprotease: haemagglutinin/protease (Hap), encoded by hapA. Hap has been shown to have mucinolytic and cytotoxic activity. These activities are likely to play an important role in the pathogenesis of cholera and the reactogenicity of attenuated vaccine strains. Production of Hap requires transcriptional activation by the HapR regulator and is repressed by glucose. The present study shows that mucin purified from two sources, bile salts, and growth at 37 degrees C enhanced Hap protease production. Analysis of hapA and hapR promoter fusions with the lacZ gene showed both promoters to be activated in a cell-density-dependent pattern. Glucose repressed and mucin induced the hapA promoter by a HapR-independent mechanism. Bile had no effect on either hapR or hapA promoter activity. Expression of hapA was required for vibrios to translocate through a mucin-containing gel. These results suggest Hap to play an important role in cholera pathogenesis by promoting mucin gel penetration, detachment and spreading of infection along the gastrointestinal tract.

Mast cells disrupt epithelial barrier function during enteric nematode infection

We have investigated the influence of mast cells on the barrier function of intestinal epithelium during nematode infection. Trichinella spiralis infection induces a strong type 2 cytokine-mediated inflammation, resulting in a critical mucosal mastocytosis that is known to mediate expulsion of the parasites from the intestine. The host response to infection is also characterized by an increase in mucosal leakiness. We show here that intestinal epithelial permeability is markedly elevated during infection, with kinetics that mirror the adaptive immune response to primary and secondary infection. Furthermore, we have identified degradation of the tight junction protein, occludin, thereby providing a mechanism for increased paracellular permeability during helminth infection. We further demonstrate by using anti-c-kit antibody and IL-9 transgenic mice that mast cells are directly responsible for increasing epithelial paracellular permeability and that mice deficient in a mast cell-specific protease fail to increase intestinal permeability and fail to expel their parasite burden. These results provide the mechanism whereby mucosal mast cells mediate parasite expulsion from the intestine.

A constitutively active variant of the quorum-sensing regulator LuxO affects protease production and biofilm formation in Vibrio cholerae

Vibrio cholerae normally inhabits aquatic habitats but can cause a severe diarrheal illness in humans. Its arsenal of virulence factors includes a secreted hemagglutinin (HA) protease. An HA protease-deficient mutant of V. cholerae was isolated and designated E7946 mpc. E7946 mpc was found to contain a point mutation in the luxO quorum-sensing regulator. In accordance with this finding, E7946 mpc exhibits a defect in quorum sensing. The mutant luxO allele [luxO(Con)] produces a protein with a leucine-to-glutamine substitution at amino acid 104. Transfer of the luxO(Con) allele to an otherwise wild-type background was sufficient to eliminate HA protease expression; conversely, deletion of luxO(Con) from E7946 mpc restored protease activity. We demonstrate that LuxO(Con) constitutively represses the transcription of hapR, an essential positive regulator of HA protease. Interestingly, strains harboring luxO(Con) form enhanced biofilms, and enhanced biofilm formation does not appear to be dependent on reduced HA protease expression. Taken together, the results confirm the role of LuxO as a central "switch" that coordinately regulates virulence-related phenotypes such as protease production and biofilm formation.

Environmental signals controlling production of hemagglutinin/protease in Vibrio cholerae

Vibrio cholerae hemagglutinin/protease (Hap) was induced upon nutrient limitation and strongly repressed by glucose. Hap was not produced in a mutant defective in the cyclic AMP (cAMP) receptor protein, suggesting that intracellular cAMP levels mediate Hap expression. No difference was found in Hap production between an rpoS deletion mutant and its isogenic wild-type precursor, indicating that the alternate sigma(s) factor is not essential for Hap expression. Based on these and previous results, we discuss the role of Hap in the pathogenesis of cholera.

Vibrio cholerae-induced cellular responses of polarized T84 intestinal epithelial cells are dependent on production of cholera toxin and the RTX toxin

To study the utility of in vitro-polarized intestinal cell monolayers for modeling Vibrio cholerae-host cell interactions, we added live V. cholerae bacteria to the apical surfaces of polarized T84 cell monolayers and monitored changes in electrical properties. We found that both classical and El Tor strains produce cholera toxin after addition to the monolayer, but induction is most likely due to medium components rather than bacterium-cell interactions. We also found that the RTX toxin is produced by El Tor strains. This toxin caused a loss of the barrier function of the paracellular tight junction that was measured as a decrease in transepithelial resistance. This decrease occurred when bacteria were added to either the apical or basolateral surfaces, indicating that the RTX toxin receptor is expressed on both surfaces. These results are discussed with regard to the applicability of the polarized T84 cell monolayers as an in vitro model of host-pathogen interactions.

The cell envelope-bound metalloprotease (camelysin) from Bacillus cereus is a possible pathogenic factor

A novel membrane proteinase of the nosocomial important bacteria species Bacillus cereus (synonyms: camelysin, CCMP) was purified up to homogeneity as was shown by mass spectrometry in its amphiphilic form. Camelysin is a neutral metalloprotease with a molecular mass of 19 kDa. Its unique N-terminus Phe-Phe-Ser-Asp-Lys-Glu-Val-Ser-Asn-Asn-Thr-Phe-Ala-Ala-Gly-Thr-Leu-Asp-Leu-Thr-Leu-Asn-Pro-Lys-Thr-Leu-Val-Asp-(Ile-Lys-Asp)- was not detected in the protein data bases during BLAST searches, but in the partially sequenced genome of Bacillus anthracis, coding for an unknown protein. Cleavage sites of the membrane proteinase for the insulin A- and B-chains were determined by mass spectrometry and N-terminal sequencing. Camelysin prefers cleavage sites in front of aliphatic and hydrophilic amino acid residues (-OH, -SO3H, amido group), avoiding bulky aromatic residues. The internally quenched fluorogenic substrates of the matrix metalloproteases 2 and 7 were cleaved with the highest efficiency at the Leu-decrease-Gly or Leu-decrease-Ala bond with the smaller residue in the P1' position. The protein specificity is broad--all various kinds of casein were cleaved as well as acid-soluble collagen, globin and ovalbumin; intact insulin was destroyed only to a low extent. Actin, collagen type I, fibrinogen, fibrin, alpha2-antiplasmin and alpha1-antitrypsin were cleaved. The protease formed SDS-stable complexes with Glu-plasminogen and antithrombin III, visible after SDS electrophoresis by gold staining and Western blot. The CCMP-plasminogen complex caused a partial activation of plasminogen to plasmin. Camelysin interacts with proteins of the blood coagulation cascade and could facilitate the penetration of fibrin clots and of the extracellular matrix during bacterial invasion.

NSP4 enterotoxin of rotavirus induces paracellular leakage in polarized epithelial cells

The nonstructural NSP4 protein of rotavirus has been described as the first viral enterotoxin. In this study we have examined the effect of NSP4 on polarized epithelial cells (MDCK-1) grown on permeable filters. Apical but not basolateral administration of NSP4 was found to cause a reduction in the transepithelial electrical resistance, redistribution of filamentous actin, and an increase in paracellular passage of fluorescein isothiocyanate-dextran. Significant effects on transepithelial electrical resistance were noted after a 20- to 30-h incubation with 1 nmol of NSP4. Most surprisingly, the epithelium recovered its original integrity and electrical resistance upon removal of NSP4. Preincubation of nonconfluent MDCK-1 cells with NSP4 prevented not only development of a permeability barrier but also lateral targeting of the tight-junction-associated Zonula Occludens-1 (ZO-1) protein. Taken together, these data indicate new and specific effects of NSP4 on tight-junction biogenesis and show a novel effect of NSP4 on polarized epithelia.

Association of protease activity in Vibrio cholerae vaccine strains with decreases in transcellular epithelial resistance of polarized T84 intestinal epithelial cells

Culture supernatants prepared from reactogenic strains of Vibrio cholerae cause a decrease in the transcellular epithelial resistance of T84 intestinal cells. This decrease correlates with the presence of hemagglutinin/protease but not with the presence of other potential accessory toxins or proteases. These data suggest a possible role for hemagglutinin/protease in reactogenicity, although other factors may also contribute.

In vivo covalent cross-linking of cellular actin by the Vibrio cholerae RTX toxin

Enteric pathogens often export toxins that elicit diarrhea as a part of the etiology of disease, including toxins that affect cytoskeletal structure. Recently, we discovered that the intestinal pathogen Vibrio cholerae elicits rounding of epithelial cells that is dependent upon a gene we designated rtxA. Here we investigate the association of rtxA with the cell-rounding effect. We find that V. cholerae exports a large toxin, RTX (repeats-in-toxin) toxin, to culture supernatant fluids and that this toxin is responsible for cell rounding. Furthermore, we find that cell rounding is not due to necrosis, suggesting that RTX toxin is not a typical member of the RTX family of pore-forming toxins. Rather, RTX toxin causes depolymerization of actin stress fibers and covalent cross-linking of cellular actin into dimers, trimers and higher multimers. This RTX toxin-specific cross-linking occurs in cells previously rounded with cytochalasin D, indicating that G-actin is the toxin target. Although several models explain our observations, our simultaneous detection of actin cross-linking and depolymerization points toward a novel mechanism of action for RTX toxin, distinguishing it from all other known toxins.

Cloning of genes of nontypeable Haemophilus influenzae involved in penetration between human lung epithelial cells

Haemophilus influenzae penetrates between epithelial cells via an unknown mechanism. A chromosomal library of nonencapsulated H. influenzae strain A960053 DNA was constructed in Escherichia coli DH5alpha to identify bacterial genes contributing to this paracytosis. Two E. coli clones that contained open reading frames (ORFs) homologous to HI0636 to HI0641 of H. influenzae strain Rd and that showed an increased penetration in epithelial cell layers of the human bronchial epithelial cell line NCI-H292 were identified. ORFs HI0636 and HI0638, encoding two small proteins of unknown functions, were further investigated. The clone containing ORFs HI0636 and HI0637 as well as the clone containing ORF HI0638 showed a significant increase in penetration. Disruption of HI0638 by kanamycin box insertion in H. influenzae strain A960053 resulted in loss of penetration into the epithelial cell layers. Disruption of HI0636 had no effect on penetration in this model system. Since a role for HI0637 in the paracytosis of H. influenzae is very unlikely because it encodes TrpS, we conclude that the protein encoded by ORF HI0638 may function as a paracytin, while that encoded by HI0636 may have an auxiliary function.

Distinct effects of Vibrio cholerae haemagglutinin/protease on the structure and localization of the tight junction-associated proteins occludin and ZO-1

Vibrio cholerae produces a little-studied cytotoxin, haemagglutinin/protease (HA/P), in addition to several better-characterized enterotoxins, i.e. cholera toxin (CT), zonula occludens toxin (ZOT) and accessory cholera enterotoxin (Ace). We have found recently that HA/P perturbs the barrier function of Mardin-Darby canine kidney epithelial cell line I (MDCK-I) by affecting the intercellular tight junctions (TJs) and the F-actin cytoskeleton. In the present study we have assessed more specifically how TJs are affected by HA/P by investigating the cellular localization and biochemical integrity of two well-characterized TJ-associated proteins, occludin and ZO-1. Western blot analysis showed that occludin bands of 66-85 kDa were digested by HA/P to two predominant bands of around 50 kDa and 35 kDa, and that this degradation was greatly attenuated when the specific bacterial metalloproteinase inhibitor Zincov was co-administered. Trypsin, on the other hand, did not degrade occludin when it was applied in the same way, suggesting that the degradation of occludin by HA/P is an early and specific event. The other TJ-associated protein ZO-1 was not degraded by HA/P in parallel experiments, suggesting the selectivity of HA/P-associated protein degradation. Moreover, immunofluorescence labelling and confocal microscopy showed that ZO-1, but not occludin, around cell-cell boundaries was rearranged by HA/P treatment. Since ZO-1 is located on the inside of the plasma membrane and is directly associated with occludin, the results indicate that breakdown of occludin may send signals to ZO-1 that affect its organization and the structure of the F-actin cytoskeleton. Our finding that the zinc-containing metalloprotease of V. cholerae specifically degraded occludin suggests that specific degradation of important host proteins by bacterial zinc-containing metalloproteases may be an important mechanism in microbial pathogenesis.

DNA sequence of both chromosomes of the cholera pathogen Vibrio cholerae

Here we determine the complete genomic sequence of the gram negative, gamma-Proteobacterium Vibrio cholerae El Tor N16961 to be 4,033,460 base pairs (bp). The genome consists of two circular chromosomes of 2,961,146 bp and 1,072,314 bp that together encode 3,885 open reading frames. The vast majority of recognizable genes for essential cell functions (such as DNA replication, transcription, translation and cell-wall biosynthesis) and pathogenicity (for example, toxins, surface antigens and adhesins) are located on the large chromosome. In contrast, the small chromosome contains a larger fraction (59%) of hypothetical genes compared with the large chromosome (42%), and also contains many more genes that appear to have origins other than the gamma-Proteobacteria. The small chromosome also carries a gene capture system (the integron island) and host 'addiction' genes that are typically found on plasmids; thus, the small chromosome may have originally been a megaplasmid that was captured by an ancestral Vibrio species. The V. cholerae genomic sequence provides a starting point for understanding how a free-living, environmental organism emerged to become a significant human bacterial pathogen.

Preliminary assessment of the safety and immunogenicity of a new CTXPhi-negative, hemagglutinin/protease-defective El Tor strain as a cholera vaccine candidate

Vibrio cholerae 638 (El Tor, Ogawa), a new CTXPhi-negative hemagglutinin/protease-defective strain that is a cholera vaccine candidate, was examined for safety and immunogenicity in healthy adult volunteers. In a double-blind placebo-controlled study, no significant adverse reactions were observed in volunteers ingesting strain 638. Four volunteers of 42 who ingested strain 638 and 1 of 14 who received placebo experienced loose stools. The strain strongly colonized the human small bowel, as evidenced by its isolation from the stools of 37 of 42 volunteers. V. cholerae 638, at doses ranging from 4 x 10(7) to 2 x 10(9) vibrios, elicited significant serum vibriocidal antibody and anti-Ogawa immunoglobulin A antibody secreting cell responses.

Amino-acid sequence and three-dimensional structure of the Staphylococcus aureus metalloproteinase at 1.72 A resolution

BACKGROUND

Aureolysin is an extracellular zinc-dependent metalloproteinase from the pathogenic bacterium Staphylococcus aureus. This enzyme exhibits in vitro activity against several molecules of biological significance for the host, indicating that it is involved in the pathology of staphylococcal diseases.

RESULTS

Here we report the amino-acid sequence and inhibitor-free X-ray crystal structure of aureolysin, a member of the thermolysin family of zinc-dependent metalloproteinases. This enzyme, which binds one zinc and three calcium ions, comprises a single chain of 301 amino acids that consists of a beta-strand-rich upper domain and an alpha-helix-rich lower domain.

CONCLUSIONS

The overall structure of aureolysin is very similar to that of the other three members of this family whose structures are known - thermolysin (TLN) from Bacillus thermoproteolyticus, neutral protease (NP) from Bacillus cereus and elastase (PAE) from Pseudomonas aeruginosa. But an important difference has been encountered: in contrast to what has been observed in the other three members of this family (TLN, NP and PAE), inhibitor-free aureolysin displays a 'closed' active site cleft conformation. This new structure therefore raises questions about the universality of the hinge-bending motion model for the neutral metalloproteinases.

Endogenous nitric oxide in MDCK-I cells modulates the Vibrio cholerae haemagglutinin/protease (HA/P)-mediated cytotoxicity

Previously, we have shown that the Vibrio cholerae haemagglutinin/protease (HA/P) accounts for significant remaining toxicity of CVD110, an attenuated V. cholerae 01 El Tor live oral vaccine-strain. The present report demonstrates that endogenous nitric oxide (NO) production modulates HA/P-mediated cytotoxicity in Madin-Darby canine kidney cell strain I (MDCK-I) epithelial cells. The basal levels of endogenous NO suppressed the cytotoxicity of HA/P, whereas inhibition of NO production with nitro-L-arginine methyl-ester (L-NAME) made the MDCK-I cells susceptible even to low concentrations of the cytotoxin. The inhibition of NO production caused a reinforcement of the HA/P- mediated distortion of a tight junction-associated protein ZO-1 and increment of filamentous actin at the apical and the lateral membrane domains. The mechanism by which NO exerts its modulatory action is not likely to be from its direct interaction with the zinc-containing catalytic domain of HA/P, since two NO donors, sodium nitroprusside (SNP) and S-nitroso-N-acetyl-D, L-penicillamine (SNAP), did not affect the proteolytic activity of HA/P. In conclusion, the endogenous NO in the MDCK-I cells has a modulating effect on the cytotoxicity of HA/P and thus protects the cells against the cytotoxin.

The rat mucosal mast cell chymase, RMCP-II, alters epithelial cell monolayer permeability in association with altered distribution of the tight junction proteins ZO-1 and occludin

Mucosal mast cells undergo hyperplasia in a variety of inflammatory bowel diseases including nematode infection in man and animals. The intra-epithelial localization of these cells make their soluble mediators prime candidates for modulators of epithelial function. In particular previous in vivo and ex vivo studies have established a link between the release of the highly soluble mast cell granule chymases and increased mucosal permeability. The hypothesis that the rat mast cell protease, RMCP-II, directly increases permeability to macromolecules via the paracellular route is tested in this study. Monolayers of epithelial cells (Madin-Darby canine kidney cell line) were exposed to varying concentrations of RMCP-II in vitro, in the absence of other cell types or mediators, and the effect on permeability and tight junction associated proteins was investigated. Basolateral, but not apical, exposure of polarized MDCK monolayers on porous supports to RMCP-II led to concentration- (> 100 microg/ml) and time-dependent increases in electrical conductance and permeability to mannitol (MW182) and inulin (MW5000), which was accompanied by decreases in the immunostaining of the tight junction-associated proteins occludin and ZO-1. Furthermore, prolonged exposure to RMCP-II (> 12 hours) resulted in the formation of identifiable gaps separating adjacent epithelial cells, in the absence of evidence of cytotoxicity. Inhibition of RMCP-II with Soya bean trypsin inhibitor completely abrogated the response, demonstrating that proteolysis was required. These data provide direct evidence that the rat mast cell chymase RMCP-II can, in the absence of other inflammatory mediators, increase epithelial permeability via an effect on the paracellular route.