Metalloprotease is not essential for Vibrio vulnificus virulence in mice

Vibrio vulnificus PlpA facilitates necrotic host cell death induced by the pore forming MARTX toxin

Opportunistic pathogen Vibrio vulnificus causes severe systemic infection in humans with high mortality. Although multiple exotoxins have been characterized in V. vulnificus, their interactions and potential synergistic roles in pathogen-induced host cell death have not been investigated previously. By employing a series of multiple exotoxin deletion mutants, we investigated whether specific exotoxins of the pathogen functioned together to achieve severe and rapid necrotic cell death. Human epithelial cells treated with V. vulnificus with a plpA deletion background exhibited an unusually prolonged cell blebbing, suggesting the importance of PlpA, a phospholipase A₂, in rapid necrotic cell death by this pathogen. Additional deletion of the rtxA gene encoding the multifunctional autoprocessing repeats-in-toxin (MARTX) toxin did not result in necrotic cell blebs. However, if the rtxA gene was engineered to produce an effector-free MARTX toxin, the cell blebbing was observed, indicating that the pore forming activity of the MARTX toxin is sufficient, but the MARTX toxin effector domains are not necessary, for the blebbing. When a recombinant PlpA was treated on the blebbed cells, the blebs were completely disrupted. Consistent with this, MARTX toxin-pendent rapid release of cytosolic lactate dehydrogenase was significantly delayed in the plpA deletion background. Mutations in other exotoxins such as elastase, cytolysin/hemolysin, and/or extracellular metalloprotease did not affect the bleb formation or disruption. Together, these findings indicate that the pore forming MARTX toxin and the phospholipase A₂, PlpA, cooperate sequentially to achieve rapid necrotic cell death by inducing cell blebbing and disrupting the blebs, respectively.

Lrp, a global regulator, regulates the virulence of Vibrio vulnificus

Background

An attenuated mutant (designated NY303) of Vibrio vulnificus, which causes serious wound infection and septicemia in humans, was isolated fortuitously from a clinical strain YJ016. This mutant was defective in cytotoxicity, migration on soft agar and virulence in the mouse. The purpose of this study was to map the mutation in this attenuated mutant and further explore how the gene thus identified is involved in virulence.

Methods

The whole genome sequence of mutant NY303 determined by next-generation sequencing was compared with that of strain YJ016 to map the mutations. By isolating and characterizing the specific gene-knockout mutants, the gene associated with the phenotype of mutant NY303 was identified. This gene encodes a global regulator, Lrp. A mutant, YH01, deficient in Lrp was isolated and examined in vitro, in vivo and ex vivo to find the affected virulence mechanisms. The target genes of Lrp were further identified by comparing the transcriptomes, which were determined by RNA-seq, of strain YJ016 and mutant YH01. The promoters bound by Lrp were identified by genome footprinting-sequencing, and those related with virulence were further examined by electrophoretic mobility shift assay.

Results

A mutation in lrp was shown to be associated with the reduced cytotoxicity, chemotaxis and virulence of mutant NY303. Mutant YH01 exhibited a phenotype resembling that of mutant NY303, and was defective in colonization in the mouse and growth in mouse serum, but not the antiphagocytosis ability. 596 and 95 genes were down- and up-regulated, respectively, in mutant YH01. Many of the genes involved in secretion of the MARTX cytotoxin, chemotaxis and iron-acquisition were down-regulated in mutant YH01. The lrp gene, which was shown to be negatively autoregulated, and 7 down-regulated virulence-associated genes were bound by Lrp in their promoters. A 14-bp consensus sequence, mkCrTTkwAyTsTG, putatively recognized by Lrp was identified in the promoters of these genes.

Conclusions

Lrp is a global regulator involved in regulation of cytotoxicity, chemotaxis and iron-acquisition in V. vulnificus. Down-regulation of many of the genes associated with these properties may be responsible, at least partly, for loss of virulence in mutant NY303.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-017-0361-9) contains supplementary material, which is available to authorized users.

MARTX Toxin in the Zoonotic Serovar of Vibrio vulnificus Triggers an Early Cytokine Storm in Mice

Vibrio vulnificus biotype 2-serovar E is a zoonotic clonal complex that can cause death by sepsis in humans and fish. Unlike other biotypes, Bt2 produces a unique type of MARTX_Vv (Multifunctional-Autoprocessive-Repeats-in-Toxin; RtxA1₃), which is encoded by a gene duplicated in the pVvBt2 plasmid and chromosome II. In this work, we analyzed the activity of this toxin and its role in human sepsis by performing in vitro, ex vivo, and in vivo assays. First, we demonstrated that the ACD domain, present exclusively in this toxin variant, effectively has an actin-cross-linking activity. Second, we determined that the whole toxin caused death of human endotheliocytes and monocytes by lysis and apoptosis, respectively. Finally, we tested the hypothesis that RtxA1₃ contributes to human death caused by this zoonotic serovar by triggering an early cytokine storm in blood. To this end, we used a Bt2-SerE strain (R99) together with its rtxA1₃ deficient mutant, and a Bt1 strain (YJ016) producing RtxA1₁ (the most studied MARTX_Vv) together with its rtxA1₁ deficient mutant, as controls. Our results showed that RtxA1₃ was essential for virulence, as R99ΔΔrtxA1₃ was completely avirulent in our murine model of infection, and that R99, but not strain YJ016, induced an early, strong and dysregulated immune response involving the up-regulation of a high number of genes. This dysregulated immune response was directly linked to RtxA1₃. Based on these results and those obtained ex vivo (human blood), we propose a model of infection for the zoonotic serovar of V. vulnificus, in which RtxA1₃ would act as a sepsis-inducing toxin.

Genotypic Diversity and Population Structure of Vibrio vulnificus Strains Isolated in Taiwan and Korea as Determined by Multilocus Sequence Typing

The genetic diversity and population structure of Vibrio vulnificus isolates from Korea and Taiwan were investigated using PCR-based assays targeting putative virulence-related genes and multilocus sequence typing (MLST). BOX-PCR genomic fingerprinting identified 52 unique genotypes in 84 environmental and clinical V. vulnificus isolates. The majority (> 50%) of strains had pathogenic genotypes for all loci tested; moreover, many environmental strains had pathogenic genotypes. Although significant (p < 0.05) inter-relationships among the genotypes were observed, the association between genotype and strain source (environmental or clinical) was not significant, indicating that genotypic characteristics alone are not sufficient to predict the isolation source or the virulence of a given V. vulnificus strain and vice versa. MLST revealed 23–35 allelic types per locus analyzed, resulting in a total of 44 unique sequence types (STs). Two major monophyletic groups (lineages A and B) corresponding to the two known lineages of V. vulnificus were observed; lineage A had six STs that were exclusively environmental, whereas lineage B had STs from both environmental and clinical sources. Pathogenic and nonpathogenic genotypes predominated in MLST lineages B and A, respectively. In addition, V. vulnificus was shown to be in linkage disequilibrium (p < 0.05), although two different recombination tests (PHI and Sawyer’s tests) detected significant evidence of recombination. Tajima’s D test also indicated that V. vulnificus might be comprised of recently sub-divided lineages. These results suggested that the two lineages revealed by MLST correspond to two distinct ecotypes of V. vulnificus.

The opportunistic marine pathogen Vibrio parahaemolyticus becomes virulent by acquiring a plasmid that expresses a deadly toxin

Acute hepatopancreatic necrosis disease (AHPND) is a severe, newly emergent penaeid shrimp disease caused by Vibrio parahaemolyticus that has already led to tremendous losses in the cultured shrimp industry. Until now, its disease-causing mechanism has remained unclear. Here we show that an AHPND-causing strain of V. parahaemolyticus contains a 70-kbp plasmid (pVA1) with a postsegregational killing system, and that the ability to cause disease is abolished by the natural absence or experimental deletion of the plasmid-encoded homologs of the Photorhabdus insect-related (Pir) toxins PirA and PirB. We determined the crystal structure of the V. parahaemolyticus PirA and PirB (PirA(vp) and PirB(vp)) proteins and found that the overall structural topology of PirA(vp)/PirB(vp) is very similar to that of the Bacillus Cry insecticidal toxin-like proteins, despite the low sequence identity (<10%). This structural similarity suggests that the putative PirAB(vp) heterodimer might emulate the functional domains of the Cry protein, and in particular its pore-forming activity. The gene organization of pVA1 further suggested that pirAB(vp) may be lost or acquired by horizontal gene transfer via transposition or homologous recombination.

Bacterial virulence analysis using brine shrimp as an infection model in relation to the importance of quorum sensing and proteases

Brine shrimp are aquatic crustaceans belonging to a genus of Artemia. This organism is widely used for testing the toxicity of chemicals. In this study, brine shrimp were evaluated as an infection model organism to study bacterial virulence. Artemia nauplii were infected with various pathogenic bacteria, such as Vibrio vulnificus, Pseudomonas aeruginosa, Burkholderia vietnamiensis, Staphylococcus aureus, and Escherichia coli, and the susceptibility to these bacteria was investigated by counting the survival of the infected nauplii. While all of the tested bacteria have significant virulence to brine shrimp, killing the nauplii in a few days, V. vulnificus showed the strongest virulence. P. aeruginosa also showed a dose-dependent virulence to brine shrimp, but the virulence was weaker than that of V. vulnificus. The virulence tests using the virulence-attenuated mutants of V. vulnificus and P. aeruginosa, such as quorum sensing (QS) mutants or protease-deficient mutants showed a significant attenuation of virulence, demonstrating that the QS mechanism is important in the virulence of these bacteria to brine shrimp. B. vietnamiensis, S. aureus, and E. coli were also virulent to brine shrimp and the virulence was correlated with dosage within 24 h under our conditions. Salmonella enterica Typhimurium and Bacillus subtilis were also virulent to brine shrimp, but the virulence was weak and slowly exerted compared with that of other bacteria. Taken together, we suggest that brine shrimp are a good infection model to assay bacterial virulence, especially for V. vulnificus and P. aeruginosa, and QS is important in the bacterial virulence to brine shrimp.

The Fish Pathogen Vibrio vulnificus Biotype 2: Epidemiology, Phylogeny, and Virulence Factors Involved in Warm-Water Vibriosis

Vibrio vulnificus biotype 2 is the etiological agent of warm-water vibriosis, a disease that affects eels and other teleosts, especially in fish farms. Biotype 2 is polyphyletic and probably emerged from aquatic bacteria by acquisition of a transferable virulence plasmid that encodes resistance to innate immunity of eels and other teleosts. Interestingly, biotype 2 comprises a zoonotic clonal complex designated as serovar E that has extended worldwide. One of the most interesting virulence factors produced by serovar E is RtxA1 3 , a multifunctional protein that acts as a lethal factor for fish, an invasion factor for mice, and a survival factor outside the host. Two practically identical copies of rtxA1 3 are present in all biotype 2 strains regardless of the serovar, one in the virulence plasmid and the other in chromosome II. The plasmid also contains other genes involved in survival and growth in eel blood: vep07 , a gene for an outer membrane (OM) lipoprotein involved in resistance to eel serum and vep20 , a gene for an OM receptor specific for eel-transferrin and, probably, other related fish transferrins. All the three genes are highly conserved within biotype 2, which suggests that they are under a strong selective pressure. Interestingly, the three genes are related with transferable plasmids, which emphasizes the role of horizontal gene transfer in the evolution of V. vulnificus in nutrient-enriched aquatic environments, such as fish farms.

Transcriptome sequencing reveals the virulence and environmental genetic programs of Vibrio vulnificus exposed to host and estuarine conditions

Vibrio vulnificus is a natural inhabitant of estuarine waters worldwide and is of medical relevance due to its ability to cause grievous wound infections and/or fatal septicemia. Genetic polymorphisms within the virulence-correlated gene (vcg) serve as a primary feature to distinguish clinical (C-) genotypes from environmental (E-) genotypes. C-genotypes demonstrate superior survival in human serum relative to E-genotypes, and genome comparisons have allowed for the identification of several putative virulence factors that could potentially aid C-genotypes in disease progression. We used RNA sequencing to analyze the transcriptome of C-genotypes exposed to human serum relative to seawater, which revealed two divergent genetic programs under these two conditions. In human serum, cells displayed a distinct "virulence profile" in which a number of putative virulence factors were upregulated, including genes involved in intracellular signaling, substrate binding and transport, toxin and exoenzyme production, and the heat shock response. Conversely, the "environmental profile" exhibited by cells in seawater revealed upregulation of transcription factors such as rpoS, rpoN, and iscR, as well as genes involved in intracellular signaling, chemotaxis, adherence, and biofilm formation. This dichotomous genetic switch appears to be largely governed by cyclic-di-GMP signaling, and remarkably resembles the dual life-style of V. cholerae as it transitions from host to environment. Furthermore, we found a "general stress response" module, known as the stressosome, to be upregulated in seawater. This signaling system has been well characterized in Gram-positive bacteria, however its role in V. vulnificus is not clear. We examined temporal gene expression patterns of the stressosome and found it to be upregulated in natural estuarine waters indicating that this system plays a role in sensing and responding to the environment. This study advances our understanding of gene regulation in V. vulnificus, and brings to the forefront a number of previously overlooked genetic networks.

Host-nonspecific iron acquisition systems and virulence in the zoonotic serovar of Vibrio vulnificus

The zoonotic serovar of Vibrio vulnificus (known as biotype 2 serovar E) is the etiological agent of human and fish vibriosis. The aim of the present work was to discover the role of the vulnibactin- and hemin-dependent iron acquisition systems in the pathogenicity of this zoonotic serovar under the hypothesis that both are host-nonspecific virulence factors. To this end, we selected three genes for three outer membrane receptors (vuuA, a receptor for ferric vulnibactin, and hupA and hutR, two hemin receptors), obtained single and multiple mutants as well as complemented strains, and tested them in a series of in vitro and in vivo assays, using eels and mice as animal models. The overall results confirm that hupA and vuuA, but not hutR, are host-nonspecific virulence genes and suggest that a third undescribed host-specific plasmid-encoded system could also be used by the zoonotic serovar in fish. hupA and vuuA were expressed in the internal organs of the animals in the first 24 h of infection, suggesting that they may be needed to achieve the population size required to trigger fatal septicemia. vuuA and hupA were sequenced in strains representative of the genetic diversity of this species, and their phylogenies were reconstructed by multilocus sequence analysis of selected housekeeping and virulence genes as a reference. Given the overall results, we suggest that both genes might form part of the core genes essential not only for disease development but also for the survival of this species in its natural reservoir, the aquatic environment.

Extracellular proteolytic enzymes produced by human pathogenic vibrio species

Bacteria in the genus Vibrio produce extracellular proteolytic enzymes to obtain nutrients via digestion of various protein substrates. However, the enzymes secreted by human pathogenic species have been documented to modulate the bacterial virulence. Several species including Vibrio cholerae and V. vulnificus are known to produce thermolysin-like metalloproteases termed vibriolysin. The vibriolysin from V. vulnificus, a causative agent of serious systemic infection, is a major toxic factor eliciting the secondary skin damage characterized by formation of the hemorrhagic brae. The vibriolysin from intestinal pathogens may play indirect roles in pathogenicity because it can activate protein toxins and hemagglutinin by the limited proteolysis and can affect the bacterial attachment to or detachment from the intestinal surface by degradation of the mucus layer. Two species causing wound infections, V. alginolyticus and V. parahaemolyticus, produce another metalloproteases so-called collagenases. Although the detailed pathological roles have not been studied, the collagenase is potent to accelerate the bacterial dissemination through digestion of the protein components of the extracellular matrix. Some species produce cymotrypsin-like serine proteases, which may also affect the bacterial virulence potential. The intestinal pathogens produce sufficient amounts of the metalloprotease at the small intestinal temperature; however, the metalloprotease production by extra-intestinal pathogens is much higher around the body surface temperature. On the other hand, the serine protease is expressed only in the absence of the metalloprotease.

Protection against Vibrio vulnificus infection by active and passive immunization with the C-terminal region of the RtxA1/MARTXVv protein

Vibrio vulnificus is a foodborne pathogen that is prevalent in coastal waters worldwide. Infection with V. vulnificus causes septicemia with fatality rates exceeding 50% even with aggressive antibiotic therapy. Several vaccine studies to prevent V. vulnificus infection have been performed but have had limited success. In this study, we identified the C-terminal region (amino acids 3491 to 4701) of the V. vulnificus multifunctional autoprocessing RTX (MARTXVv or RtxA1) protein, RtxA1-C, as a promising antigen that induces protective immune responses against V. vulnificus. Vaccination of mice with recombinant RtxA1-C protein with adjuvant elicited a robust antibody response and a dramatic reduction in blood bacterial load in mice infected intraperitoneally. Vaccination resulted in significant protection against lethal challenge with V. vulnificus. Furthermore, intraperitoneal passive immunization with serum raised against the recombinant RtxA1-C protein demonstrated marked efficacy in both prophylaxis and therapy. These results suggest that active and passive immunization against the C-terminal region of the RtxA1 protein may be an effective approach in the prevention and therapy of V. vulnificus infections.

Transcriptome profiling analysis of Vibrio vulnificus during human infection

Vibrio vulnificus is a waterborne pathogen that was responsible for an outbreak of severe soft-tissue infections among fish farmers and fish consumers in Israel. Several factors have been shown to be associated with virulence. However, the transcriptome profile of the pathogen during human infection has not been determined yet. We compared the transcriptome profile, using RNA sequencing, of a human-pathogenic strain harvested directly from tissue of a patient suffering from severe soft-tissue infection with necrotizing fasciitis, with the same strain and three other environmental strains grown in vitro. The five sequenced libraries were aligned to the reference genomes of V. vulnificus strains CMCP6 and YJ016. Approximately 47.8 to 62.3 million paired-end raw reads were generated from the five runs. Nearly 84 % of the genome was covered by reads from at least one of the five runs, suggesting that nearly 16 % of the genome is not transcribed or is transcribed at low levels. We identified 123 genes that were differentially expressed during the acute phase of infection. Sixty-three genes were mapped to the large chromosome, 47 genes mapped to the small chromosome and 13 genes mapped to the YJ016 plasmid. The 123 genes fell into a variety of functional categories including transcription, signal transduction, cell motility, carbohydrate metabolism, intracellular trafficking and cell envelope biogenesis. Among the genes differentially expressed during human infection we identified genes encoding bacterial toxin (RtxA1) and genes involved in flagellar components, Flp-coding region, GGDEF family protein, iron acquisition system and sialic acid metabolism.

Regulation of haemolysin (VvhA) production by ferric uptake regulator (Fur) in Vibrio vulnificus: repression of vvhA transcription by Fur and proteolysis of VvhA by Fur-repressive exoproteases

VvhA produced by Vibrio vulnificus exhibits cytolytic activity to human cells including erythrocytes. Since haemolysis by VvhA may provide iron for bacterial growth and pathogenicity, we investigated the expression of VvhA to elucidate the regulatory roles of Fur, a major transcription factor controlling iron-homeostasis. Fur repressed the transcription of vvhBA operon via binding to the promoter region. However, haemolysin content and haemolytic activity were lowered in cell-free supernatant of fur mutant. This discrepancy between the levels of vvhA transcript and VvhA protein in fur mutant was caused by exoproteolytic activities of the elastase VvpE and another metalloprotease VvpM, which were also regulated by Fur. vvpE gene expression was repressed by Fur via binding to the Fur-box homologous region. Regulation of VvpM expression by Fur did not occur at the level of vvpM transcription. In vitro proteolysis assays showed that both proteases efficiently degraded VvhA. In addition, the extracellular levels of VvhA were higher in culture supernatants of vvpE or vvpM mutants than in the wild type. Thus this study demonstrates that Fur regulates haemolysin production at the transcription level of the vvhBA operon and at the post-translation level by regulating the expressions of two VvhA-degrading exoproteases, VvpE and VvpM.

Presence of virulence markers in environmental Vibrio vulnificus strains

AIMS

This work aims to demonstrate the presence of several genes and factors associated with virulence in strains isolated from the environment at Pueblo Viejo Lagoon, State of Veracruz, Mexico.

METHODS AND RESULTS

In this study, we investigated the production of V. vulnificus virulence factors, as cytolysin (haemolysin), RTX toxin, metalloprotease, siderophores, capsular polysaccharide, adhesion structures (like type IV pili), and polar and lateral flagella, involved in swimming and swarming (or, at least, the presence of genes encoding some of them) in 40 strains of V. vulnificus isolated from water and food. The results indicate that strains of environmental origin possess potential virulence characteristics.

CONCLUSIONS

Caution should be exercised when consuming raw shellfish (especially by those more susceptible risk groups).

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first work focused on the evaluation of V. vulnificus virulence factors in Mexico.

The regulator HlyU, the repeat-in-toxin gene rtxA1, and their roles in the pathogenesis of Vibrio vulnificus infections

HlyU is a master regulator that plays an essential role in the virulence of the human pathogen Vibrio vulnificus. One of the most noteworthy characteristics of HlyU regulation in this organism is its positive control of the expression of the repeat-in-toxin (RtxA1) gene, one of the most important virulence factors accounting for the fulminating and damaging nature of V. vulnificus infections. In this work, we reviewed the latest studies of RtxA1 in this bacterium and highlight the mechanism of gene regulation of rtxA1 expression by HlyU under a broader gene regulatory network.

MARTX of Vibrio vulnificus biotype 2 is a virulence and survival factor

Vibrio vulnificus biotype 2 is a polyphyletic group whose virulence for fish relies on a plasmid. This plasmid contains an rtxA gene duplicated in the small chromosome that encodes a MARTX (Multifunctional, Autoprocessing Repeats-in-Toxin) unique within the species in domain structure (MARTX type III). To discover the role of this toxin in the fitness of this biotype in the fish-farming environment, single- and double-knockout mutants were isolated from a zoonotic strain and analysed in a series of in vivo and in vitro experiments with eel, fish cell lines and amoebae isolated from gills. Mice, murine and human cell lines were also assayed for comparative purposes. The results suggest that MARTX type III is involved in the lysis of a wide range of eukaryotic cells, including the amoebae, erythrocytes, epithelial cells and phagocytes after bacterium-cell contact. In fish, MARTX type III may act as a toxin involved in the onset of septic shock, while in mice it may promote bacterial colonization by preventing phagocytosis of bacterial cells. Moreover, this toxin could protect bacteria from predation by amoebae, which would increase bacterial survival outside the host and would explain the fitness of this biotype in the fish-farming environment.

Functional characterization of EpsC, a component of the type II secretion system, in the pathogenicity of Vibrio vulnificus

EpsC, one of the components comprising the type II secretion system (T2SS), was isolated from a human-pathogenic bacterium, Vibrio vulnificus, to evaluate its role in eliciting virulence. An espC-deleted mutant of V. vulnificus displayed a reduced cytotoxicity to the human cell line HEp-2 and an attenuated virulence in a mouse model. This mutant exhibited dramatic defects in the secretion of diverse extracellular proteins, such as outer membrane proteins, transporters, and the known secreted factors, notably, a hemolysin (VvhA) and an elastase (VvpE). A defect in its secretion of proteins was restored by in trans complementation of the intact epsC gene. Analyses of cellular fractions revealed that VvhA and VvpE of the ΔepsC mutant were not excreted outside the cell but were present mainly in the periplasmic space. Examination of a V. vulnificus mutant deficient in TolC, a component of the T1SS, showed that it is not involved in the secretion of VvhA and VvpE but that it is necessary for the secretion of another major toxin of V. vulnificus, RtxA. Therefore, the T2SS is required for V. vulnificus pathogenicity, which is mediated by at least two secreted factors, VvhA and VvpE, via facilitating the secretion and exposure of these factors to host cells.

Proteases produced by vibrios

Bacteria of the genus Vibrio are normal habitants of the aquatic environment but the some species are believed to be human pathogens. Pathogenic vibrios produce various pathogenic factors, and the proteases are also recognized to play pathogenic roles in the infection: the direct roles by digesting many kinds of host proteins or indirect roles by processing other pathogenic protein factors. Especially VVP from Vibrio vulnificus is thought to be a major pathogenic factor of the vibrio. Although HA/P, the V. cholerae hemagglutinin/protease, is not a direct toxic factor of cholera vibrio, its significance is an undeniable fact. Production of HA/P is regulated together with major pathogenic factors such as CT (cholera toxin) or TCP (toxin co-regulated pilus) by a quorum-sensing system. HA/P is necessary for full expression of pathogenicity of the vibrio by supporting growth and translocation in the digestive tract. Processing of protein toxins such as CT or El Tor hemolysin is also an important pathogenic role.

Regulation of cytotoxicity by quorum-sensing signaling in Vibrio vulnificus is mediated by SmcR, a repressor of hlyU

Cytotoxicity is an important virulence determinant in the pathogenesis of Vibrio vulnificus, and two cytotoxins, RTX (encoded by rtxA1) and cytolysin/hemolysin (encoded by vvhA), have been identified in this organism. We showed that the quorum-sensing regulator LuxO controlled the cytotoxicity of this organism: a ΔluxO mutant exhibited low cytotoxicity, whereas a constitutively activated luxO mutant, luxO(D47E), remained highly cytotoxic. The cytotoxicity of the ΔluxO mutant was restored when smcR, a Vibrio harveyi luxR homologue repressed by luxO, was further deleted. SmcR then was shown to repress the expression of both rtxA1 and vvhA. A DNA library of V. vulnificus was screened in Escherichia coli for clones that upregulated vvhA in the presence of SmcR, and hlyU, which has been shown to positively regulate rtxA1 and vvhA, was identified. We demonstrated that SmcR repressed the expression of hlyU and bound to a region upstream of hlyU in V. vulnificus. The deletion of hlyU resulted in the loss of cytotoxicity and reduced cytolysin/hemolysin production in the ΔsmcR mutant. The ΔsmcR ΔhlyU mutant regained cytotoxicity and cytolysin/hemolysin activity when hns, which has been shown to repress the transcription of rtxA1 and interfere with hlyU, was further removed. Collectively, our data suggest that SmcR mediates the regulation of cytotoxicity by quorum-sensing signaling in V. vulnificus by repressing hlyU, an activator of rtxA1 and vvhA.

Putative virulence factors identified in Vibrio vulnificus strains isolated from oysters and seawater in Mexico

The presence of Vibrio vulnificus was analyzed in oyster and estuarine water samples from Mexico by PCR amplification of the vvhA gene and some putative virulence factors were tested. Samples were collected from 12 different sampling points over a one-year period; 31% samples were positive for V. vulnificus and all isolates were identified as biotype 1. All strains were cytotoxic and proteolytic, 98% showed adherence to epithelial cells, 91.4% were DNase-positive, 77.6% were mucinase-positive, 97.8% were lecithinase-positive and 79.8% were lipase positive. Regarding colony morphology, 51% strains were opaque, 20% were translucid, 28% were both opaque and translucid, and 80.8% showed a capsule. This is the first report on the isolation of V. vulnificus strains from environmental samples in Mexico, which may pose a health risk for local fisherman and seafood consumers.

Role of GacA in virulence of Vibrio vulnificus

The GacS/GacA two-component signal transduction system regulates virulence, biofilm formation and symbiosis in Vibrio species. The present study investigated this regulatory pathway in Vibrio vulnificus, a human pathogen that causes life-threatening disease associated with the consumption of raw oysters and wound infections. Small non-coding RNAs (csrB1, csrB2, csrB3 and csrC) commonly regulated by the GacS/GacA pathway were decreased (P<0.0003) in a V. vulnificus CMCP6 ΔgacA : : aph mutant compared with the wild-type parent, and expression was restored by complementation of the gacA deletion mutation in trans. Of the 20 genes examined by RT-PCR, significant reductions in the transcript levels of the mutant in comparison with the wild-type strain were observed only for genes related to motility (flaA), stationary phase (rpoS) and protease (vvpE) (P=0.04, 0.01 and 0.002, respectively). Swimming motility, flagellation and opaque colony morphology indicative of capsular polysaccharide (CPS) were unchanged in the mutant, while cytotoxicity, protease activity, CPS phase variation and the ability to acquire iron were decreased compared with the wild-type (P<0.01). The role of gacA in virulence of V. vulnificus was also demonstrated by significant impairment in the ability of the mutant strain to cause either skin (P<0.0005) or systemic infections (P<0.02) in subcutaneously inoculated, non-iron-treated mice. However, the virulence of the mutant was equivalent to that of the wild-type in iron-treated mice, demonstrating that the GacA pathway in V. vulnificus regulates the virulence of this organism in an iron-dependent manner.

Virulence characteristics of sucrose-fermenting Vibrio vulnificus strains

We identified 6 sucrose-fermenting Vibrio vulnificus strains and examined their virulence characteristics. They were all encapsulated, motile, capable of producing toxins and utilizing transferrin-bound iron, cytotoxic to cultured cells, and virulent enough to kill mice. They could be definitely identified only by genetic identification methods such as PCR, and not by conventional culture-based identification methods such as API 20E (bioMérieux, France). These results indicate that it is essential to adopt genetic approaches as early as possible in order to avoid misdiagnosis of such strains, especially in clinical situations.

Diversity and impact of prokaryotic toxins on aquatic environments: a review

Microorganisms are ubiquitous in all habitats and are recognized by their metabolic versatility and ability to produce many bioactive compounds, including toxins. Some of the most common toxins present in water are produced by several cyanobacterial species. As a result, their blooms create major threats to animal and human health, tourism, recreation and aquaculture. Quite a few cyanobacterial toxins have been described, including hepatotoxins, neurotoxins, cytotoxins and dermatotoxins. These toxins are secondary metabolites, presenting a vast diversity of structures and variants. Most of cyanobacterial secondary metabolites are peptides or have peptidic substructures and are assumed to be synthesized by non-ribosomal peptide synthesis (NRPS), involving peptide synthetases, or NRPS/PKS, involving peptide synthetases and polyketide synthases hybrid pathways. Besides cyanobacteria, other bacteria associated with aquatic environments are recognized as significant toxin producers, representing important issues in food safety, public health, and human and animal well being. Vibrio species are one of the most representative groups of aquatic toxin producers, commonly associated with seafood-born infections. Some enterotoxins and hemolysins have been identified as fundamental for V. cholerae and V. vulnificus pathogenesis, but there is evidence for the existence of other potential toxins. Campylobacter spp. and Escherichia coli are also water contaminants and are able to produce important toxins after infecting their hosts. Other bacteria associated with aquatic environments are emerging as toxin producers, namely Legionella pneumophila and Aeromonas hydrophila, described as responsible for the synthesis of several exotoxins, enterotoxins and cytotoxins. Furthermore, several Clostridium species can produce potent neurotoxins. Although not considered aquatic microorganisms, they are ubiquitous in the environment and can easily contaminate drinking and irrigation water. Clostridium members are also spore-forming bacteria and can persist in hostile environmental conditions for long periods of time, contributing to their hazard grade. Similarly, Pseudomonas species are widespread in the environment. Since P. aeruginosa is an emergent opportunistic pathogen, its toxins may represent new hazards for humans and animals. This review presents an overview of the diversity of toxins produced by prokaryotic microorganisms associated with aquatic habitats and their impact on environment, life and health of humans and other animals. Moreover, important issues like the availability of these toxins in the environment, contamination sources and pathways, genes involved in their biosynthesis and molecular mechanisms of some representative toxins are also discussed.

Regulation of the Vibrio vulnificus vvpE expression by cyclic AMP-receptor protein and quorum-sensing regulator SmcR

In Vibrio vulnificus, cAMP-receptor protein (CRP) and the quorum-sensing regulator SmcR are simultaneously and cooperatively required for the metalloprotease vvpE gene expression, rather than sequentially in a regulatory cascade. However, this study shows a new temporal and functional sequence between the two factors in regulating vvpE expression. A crp mutation inhibited vvpE expression with growth impairment from early stage. In contrast, a smcR mutation inhibited vvpE expression only at the late stage with no effect on growth. A crp-smcR double mutation severely inhibited vvpE expression with growth impairment from early stage. The inhibited vvpE expression was restored only at the early stage by a crp single complementation, but not at all by a smcR complementation. These results indicate that CRP functions as an essential activator, whereas SmcR functions in the presence of CRP for full vvpE expression.

Functional characterization of the IlpA protein of Vibrio vulnificus as an adhesin and its role in bacterial pathogenesis

Vibrio vulnificus is a Gram-negative bacterium that causes a fatal septicemia. One of its virulence factors is a membrane-bound lipoprotein, IlpA, which can induce cytokine production in human immune cells. In the present study, the role of IlpA as an adhesion molecule was investigated. An ilpA-deleted V. vulnificus mutant showed significantly decreased adherence to INT-407 human intestinal epithelial cells, which in turn resulted in reduced cytotoxicity. The DeltailpA mutant recovered the adherence ability of the wild type by complementation in trans with the intact ilpA gene. In addition, pretreatment of V. vulnificus with anti-IlpA polyclonal antibodies resulted in a significant reduction of bacterial adherence. To localize the domain of IlpA required for cytoadherence, three truncated recombinant IlpA polypeptides were constructed and tested for the ability to adhere to human cells by a ligand-binding immunoblot assay and fluorescence microscopy. The polypeptide containing the carboxy (C)-terminal hydrophilic domain exhibited direct binding to INT-407 cells. Therefore, the C-terminal domain of IlpA allows this protein to be an adhesion molecule of V. vulnificus.

A recombinant metalloprotease antigen of Vibrio vulnificus elicits protective antibodies in a murine model

AIMS

To investigate whether Vibrio vulnificus metalloprotease (VvpE) can induce the production of specific anti-VvpE antibody to confer effective protection against Vibrio vulnificus infection and to evaluate the possibility of VvpE as a potential vaccine candidate against disease caused by V. vulnificus.

METHODS AND RESULTS

The gene encoding the 65-kDa VvpE of V. vulnificus was amplified by PCR and cloned into the expression vector pET21(b). The recombinant VvpE of V. vulnificus was expressed in Escherichia coli BL21(DE3). This His(6)-tagged VvpE was purified and injected intramuscularly into mice to evaluate its ability to stimulate immune response. Specific antibody levels were measured by ELISA. The 75% protective efficacy of recombinant VvpE was evaluated by active immunization and intraperitoneal challenge with V. vulnificus in mice.

CONCLUSIONS

The recombinant His(6)-tagged VvpE of V. vulnificus is capable of inducing high antibody response in mice to confer effective protection against lethal challenge with V. vulnificus. VvpE might be a potential vaccine candidate to against V. vulnificus infection.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study uses His(6)-tagged VvpE to act as vaccine that successfully induces effective and specific anti-VvpE antibody and offers an option for the potential vaccine candidate against V. vulnificus infection.

Quorum sensing negatively regulates hemolysin transcriptionally and posttranslationally in Vibrio cholerae

Recent work has shown that in addition to cholera toxin (CT) and the toxin-coregulated pilus (TCP), other cytotoxic proteins in Vibrio cholerae also cause disease symptoms, and this is particularly evident in strains lacking CT. One such protein is the hemolysin encoded by hlyA. Here we show that, like CT and TCP, HlyA is repressed by the quorum-sensing-regulated transcription factor HapR. This repression occurs on two levels: one at the transcriptional level that is independent of the metalloprotease HapA and one at the posttranslational level that is mediated by HapA. The transcriptional regulation is significantly more apparent on solid media than in liquid cultures. This is the first time that hemolysis has been shown to be directly regulated by quorum sensing in V. cholerae, and it is interesting that, like other virulence factors, HlyA is also repressed by HapR, which is expressed late in infection.

The extracellular metalloprotease of Vibrio tubiashii directly inhibits its extracellular haemolysin

Vibrio tubiashii is a re-emerging pathogen of molluscs that secretes a variety of extracellular products (ECPs), including a metalloprotease and a cytolysin/haemolysin. Previously, we reported that the V. tubiashii haemolysin locus consists of two ORFs (vthB and vthA), similar to that of the homologous haemolysin genes (vvhB and vvhA) found in Vibrio vulnificus. Here, we demonstrate that the concomitant expression of both V. tubiashii genes resulted in significantly higher haemolytic activity than the vthA gene alone. In addition, we created a VthAB- mutant strain of V. tubiashii that was virtually devoid of haemolytic activity in liquid media. Interestingly, significant production of an additional haemolysin(s) was observed on blood plates. Moreover, we have previously reported that in V. tubiashii, proteolytic and haemolytic activities are inversely produced during bacterial growth. Here, we study this correlation in more detail and present evidence that the VtpA metalloprotease inhibits haemolytic activity in culture supernatants, based on the following evidence: (i) loss of metalloprotease activity by either mutation or EDTA inhibition resulted in increased haemolytic activity; (ii) overexpression of the vtpA gene resulted in decreased haemolytic activity; (iii) purified VtpA metalloprotease directly diminished haemolytic activity by purified VthA haemolysin. Importantly, we found not only that vthAB gene expression remained high throughout growth but also that there were no dramatic differences in vthAB gene expression between the parent and VtpA- mutant strains. Thus, our results strongly suggest that the V. tubiashii metalloprotease directly targets its haemolysin.

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.

Two forms of Vibrio vulnificus metalloprotease VvpE are secreted via the type II general secretion system

Vibrio vulnificus has been known to secrete one form of metalloprotease VvpE (45 kDa) that is cleaved to 34 kDa-VvpE and 11 kDa-C-terminal propeptide via extracellular autoproteolysis. However, we found that extracellular secretion of both the 34 and 45 kDa forms of VvpE began in the early growth phase; moreover, 34 kDa-VvpE existed as the major form in V. vulnificus cell lysates and culture supernatants. In addition, extracellular secretion of both 34 and 45 kDa-VvpE was blocked by mutation of the pilD gene, which encodes for the type IV leader peptidase/N-methyltransferase of the type II general secretion system, and the blocked VvpE secretion was recovered by in trans-complementation of the wild-type pilD gene. These results indicate that 34 kDa-VvpE is the major form secreted along with 45 kDa-VvpE from the early growth phase via the PilD-mediated type II general secretion system.

Role of the virulence plasmid pR99 and the metalloprotease Vvp in resistance of Vibrio vulnificus serovar E to eel innate immunity

Vibrio vulnificus biotype 2 serovar E (VSE) is a bacterial pathogen that produces a haemorrhagic septicaemia called vibriosis in eels. Its ability to grow in blood is conferred by a recently described virulence plasmid [Lee CT, Amaro C, Wu KM, Valiente E, Chang YF, Tsai SF, et al. A common virulence plasmid in biotype 2 Vibrio vulnificus and its dissemination aided by a conjugal plasmid. Journal of Bacteriology, submitted for publication.]. In this study, we analyzed the role of this plasmid together with the role played by the metalloprotease (Vvp) in the interaction between bacteria and eel innate immunity. To this end, we compared and statistically analyzed the differences in resistance to serum and mucus factors (complement, selected antimicrobial peptides, transferrin and lysozyme) and also to phagocytosis/opsonophagocytosis between one VSE strain and its derivatives: a plasmid-cured strain and a vvp-deficient mutant. The wild-type and the metalloprotease-deficient strains were resistant to both the bactericidal action of fresh serum and the phagocytosis and opsonophagocytosis by eel phagocytes, confirming that Vvp is not involved in resistance to eel innate immunity. In contrast, the cured strain was sensitive to both the bactericidal action of eel serum activated by the alternative pathway and phagocytosis/opsonophagocytosis. Since no plasmid-encoded ORF, with homology to known genes, is related to the resistance to innate immunity [Lee CT, Amaro C, Wu KM, Valiente E, Chang YF, Tsai SF, et al. A common virulence plasmid in biotype 2 Vibrio vulnificus and its dissemination aided by a conjugal plasmid. Journal of Bacteriology, submitted for publication.], this function could be codified by one or more new genes. Further studies are underway to characterize the plasmid-encoded system responsible for V. vulnificus resistance to the innate immune system of eels.

A common virulence plasmid in biotype 2 Vibrio vulnificus and its dissemination aided by a conjugal plasmid

Strains of Vibrio vulnificus, a marine bacterial species pathogenic for humans and eels, are divided into three biotypes, and those virulent for eels are classified as biotype 2. All biotype 2 strains possess one or more plasmids, which have been shown to harbor the biotype 2-specific DNA sequences. In this study we determined the DNA sequences of three biotype 2 plasmids: pR99 (68.4 kbp) in strain CECT4999 and pC4602-1 (56.6 kb) and pC4602-2 (66.9 kb) in strain CECT4602. Plasmid pC4602-2 showed 92% sequence identity with pR99. Curing of pR99 from strain CECT4999 resulted in loss of resistance to eel serum and virulence for eels but had no effect on the virulence for mice, an animal model, and resistance to human serum. Plasmids pC4602-2 and pR99 could be transferred to the plasmid-cured strain by conjugation in the presence of pC4602-1, which was self-transmissible, and acquisition of pC4602-2 restored the virulence of the cured strain for eels. Therefore, both pR99 and pC4602-2 were virulence plasmids for eels but not mice. A gene in pR99, which encoded a novel protein and had an equivalent in pC4602-2, was further shown to be essential, but not sufficient, for the resistance to eel serum and virulence for eels. There was evidence showing that pC4602-2 may form a cointegrate with pC4602-1. An investigation of six other biotype 2 strains for the presence of various plasmid markers revealed that they all harbored the virulence plasmid and four of them possessed the conjugal plasmid in addition.

Role of the metalloprotease Vvp and the virulence plasmid pR99 of Vibrio vulnificus serovar E in surface colonization and fish virulence

The virulence for eels of Vibrio vulnificus biotype 2 serovar E (VSE) is conferred by a plasmid that codifies ability to survive in eel serum and cause septicaemia. To find out whether the plasmid and the selected chromosomal gene vvp plays a role in the initial steps of infection, the VSE strain CECT4999, the cured strain CT218 and the Vvp-deficient mutant CT201 (obtained in this work by allelic exchange) were used in colonization and virulence experiments. The eel avirulent biotype 1 (BT1) strain YJ016, whose genome has been sequenced, was used for comparative purposes. The global results demonstrate that the plasmid does not play a significant role in surface colonization because (i) CECT4999 and CT218 were equally chemoattracted towards and adherent to eel mucus and gills, and (ii) CT218 persisted in gills from bath-infected eels 2 weeks post infection. In contrast, mutation in vvp gene reduced significantly chemoattraction and attachment to eel mucus and gills, as well as virulence degree by immersion challenge. Co-infection experiments by bath with CECT4999 and CT201 confirmed that Vvp was involved in eel colonization and persistence in gills, because CECT4999 was recovered at higher numbers compared with CT201 from both internal organs of moribund fish (ratio 4:1) and gills from survivors (ratio 50:1). Interestingly, YJ016 also showed chemoattraction and attachment to mucus, and complementation of CT201 with BT1-vvp gene restored both activities together with virulence degree by immersion challenge. Additional experiments with algae mucus and purified mucin gave similar results. In conclusion, the protease Vvp of V. vulnificus seems to play an essential role in colonization of mucosal surfaces present in aquatic environments. Among the V. vulnificus strains colonizing fish mucus, only those harbouring the plasmid could survive in blood and cause septicaemia.