Metalloprotease is not essential for Vibrio vulnificus virulence in mice

Vibrio vulnificus RTX toxin kills host cells only after contact of the bacteria with host cells

Vibrio vulnificus causes acute cell death and a fatal septicaemia. In this study, we show that contact with host cells is a prerequisite to the acute cytotoxicity. We screened transposon mutants defective in the contact-dependent cytotoxicity. Two mutants had insertions within two open reading frames in a putative RTX toxin operon, the rtxA1 or rtxD encoding an RTX toxin (4701 amino acids) or an ABC type transporter (467 amino acids). An rtxA1 mutation resulted in a cytotoxicity defect, which was fully restored by in trans complementation. The expression of RtxA1 toxin increased after host cell contact in a time-dependent manner. The RtxA1 toxin induced cytoskeletal rearrangements and plasma membrane blebs, which culminated in a necrotic cell death. RtxA1 colocalized with actin and caused actin aggregation coinciding with a significant decrease in the F/G actin ratio. The RtxA1 toxin caused haemolysis through pore formation (radius 1.63 nm). The rtxA1 deletion mutant was defective in invading the blood stream from ligated ileal loops of CD1 mice. The rtxA1 null mutation resulted in over 100-fold increase in both intragastric and intraperitoneal LD(50)s against mice. Overall, these results show that the RtxA1 toxin is a multifunctional cytotoxin and plays an essential role in the pathogenesis of V. vulnificus infections.

Production of Vibrio vulnificus metalloprotease VvpE begins during the early growth phase: Usefulness of gelatin-zymography

Recent studies have demonstrated that expression of the vvpE gene begins during the early growth phase albeit at low levels. However, we found that the traditional protease assay method that is used to measure caseinolytic activity in culture supernatants is not suitable for the measurement of extracellular VvpE that is produced at low levels during the early growth phase. By using gelatin-zymography in place of the protease assay, we could specifically detect only VvpE of several proteases produced by Vibrio vulnificus. Moreover, we could sensitively measure VvpE produced at low levels during the early growth phase, which was consistent with transcription of the vvpE gene. The extracellular production of VvpE was reduced or delayed by mutation of the pilD gene which encodes for the type IV leader peptidase/N-methyltransferase associated with the type II general secretion system; the delayed production of VvpE was recovered by in trans complementation of the wild-type pilD gene. These results indicate that VvpE begins to be produced during the early growth phase via the PilD-mediated type II general secretion system, and that the use of gelatin-zymography is recommended as a simple method for the sensitive and specific detection of VvpE production.

Identification of OmpU of Vibrio vulnificus as a fibronectin-binding protein and its role in bacterial pathogenesis

Vibrio vulnificus is a pathogenic bacterium that causes gastroenteritis and primary septicemia. To identify factors involved in microbial adherence to the host cells, we investigated bacterial proteins capable of binding to fibronectin, one of the main components comprised of the extracellular matrix of mammalian cells. A protein of approximately 35 kDa was purified from the extracts of V. vulnificus by its property to bind to immobilized fibronectin. This protein was identified as OmpU, one of the major outer membrane proteins of V. vulnificus. In binding assays using immobilized fibronectin, the number of ompU mutant cells bound to fibronectin was only 4% of that of wild-type cells bound to fibronectin. In addition, the exogenous addition of antibodies against OmpU resulted in a decreased ability of wild-type V. vulnificus to adhere to fibronectin. The ompU mutant was also defective in its adherence to RGD tripeptide (5% of the adherence of the wild type to RGD), cytoadherence to HEp-2 cells (7% of the adherence of the wild type to HEp-2), cytotoxicity to cell cultures (39% of the cytotoxicity of the wild type), and mortality in mice (10-fold increase in the 50% lethal dose). The ompU mutant complemented with the intact ompU gene restored its abilities for adherence to fibronectin, RGD tripeptide, and HEp-2 cells; cytotoxicity to HEp-2 cells; and mouse lethality. This study indicates that OmpU is an important virulence factor involved in the adherence of V. vulnificus to the host cells.

Construction of a Vibrio splendidus mutant lacking the metalloprotease gene vsm by use of a novel counterselectable suicide vector

Vibrio splendidus is a dominant culturable Vibrio in seawater, and strains related to this species are also associated with mortality in a variety of marine animals. The determinants encoding the pathogenic properties of these strains are still poorly understood; however, the recent sequencing of the genome of V. splendidus LGP32, an oyster pathogen, provides an opportunity to decipher the basis of the virulence properties by disruption of candidate genes. We developed a novel suicide vector based on the pir-dependent R6K replicative origin, which potentially can be transferred by RP4-based conjugation to any Vibrio strain and which also carries the plasmid F toxin ccdB gene under control of the PBAD promoter. We demonstrated that this genetic system allows efficient counterselection of integrated plasmids in the presence of arabinose in both V. splendidus and Vibrio cholerae and thus permits efficient markerless allelic replacement in these species. We used this technique to construct several mutants of V. splendidus LGP32, including a derivative with a secreted metalloprotease gene, vsm, deleted. We found that this gene is essential for LGP32 extracellular product toxicity when the extracellular products are injected into oysters but is not necessary for virulence of bacteria in the oyster infection model when bacteria are injected.

Vibrio vulnificus vulnibactin, but not metalloprotease VvpE, is essentially required for iron-uptake from human holotransferrin

The roles of metalloprotease (VvpE) and catechol-siderophore (vulnibactin) in the uptake of iron from human transferrins by Vibrio vulnificus have been determined using different experimental conditions and methods. Therefore, in this study, we attempted to elucidate the roles of VvpE and vulnibactin using the same methods and experimental conditions, in an in vitro and a human ex vivo system, and in accordance with the molecular version of Koch's postulates. Neither vvpE mutation nor in trans vvpE complementation affected vulnibactin production, iron-assimilation from human holotransferrin (HT), and bacterial growth in a HT-containing deferrated Heart-Infusion medium (HT-DF-HI) or a HT-containing cirrhotic ascites (HT-CA). In contrast, the mutation of fur gene encoding Fur, a repressor regulating expression of the vulnibactin-mediated iron-uptake system, derepressed vulnibactin production, and facilitated iron-assimilation from HT and bacterial growth in HT-DF-HI or HT-CA. The mutation of vis gene encoding isochorismate synthase required for vulnibactin synthesis abolished vulnibactin production, iron-assimilation from HT and bacterial growth in HT-DF-HI or HT-CA. These results demonstrate that vulnibactin is essentially required for iron-assimilation from transferrin, and that VvpE has no direct effect on facilitating vulnibactin-mediated iron-assimilation from transferrin in vitro or in a human ex vivo system.

Specificity of a Vibrio vulnificus aminopeptidase toward kinins and other peptidyl substrates

Recently, phosphoglucose isomerase with a lysyl aminopeptidase (PGI-LysAP) activity was identified in Vibrio vulnificus. In this paper, we demonstrate the proteolytic cleavage of human-derived peptides by PGI-LysAP of V. vulnificus using three approaches: (i) a quantitative fluorescent ninhydrin assay for free lysine, (ii) matrix-assisted laser desorption ionization-two-stage time of flight mass spectrometry (MALDI-TOF-TOF), and (iii) Tricine gel electrophoresis. PGI-LysAP hydrolyzed bradykinin, Lys-bradykinin, Lys-(des-Arg9)-bradykinin, neurokinin A, Met-Lys-bradykinin, histatin 8, and a myosin light chain fragment. We detected the proteolytic release of free L-lysine from peptide digests using a rapid, simple, sensitive, and quantitative fluorescent ninhydrin assay, and results were confirmed by MALDI-TOF-TOF. The use of the fluorescent ninhydrin assay to quantitatively detect free lysine hydrolyzed from peptides is the first application of its kind and serves as a paradigm for future studies. The visualization of peptide hydrolysis was accomplished by Tricine gel electrophoresis. Proteolytic processing of kinins alters their affinities toward specific cellular receptors and initiates signal transduction mechanisms responsible for inflammation, vasodilation, and enhanced vascular permeability. By applying novel approaches to determine the proteolytic potential of bacterial enzymes, we demonstrate that PGI-LysAP has broad exopeptidase activity which may enhance V. vulnificus invasiveness by altering peptides involved in signal transduction pathways.

Human serum albumin enhances the hemolytic activity of Vibrio vulnificus

Vibrio vulnificus hemolysin (VvhA) is inactivated in the late growth phase by its oligomerization. Albumin is known to affect the activities of many bacterial toxins. In this study, we investigated the effects of human or bovine serum albumin (HSA or BSA) on the production and activity of VvhA. HSA did not affect V. vulnificus growth and vvhA transcription. However, VvhA hemolytic activity in culture supernatants was significantly higher in the presence of HSA than in the absence of HSA. By Western blot analysis, the oligomerization of VvhA was inhibited and the remaining active VvhA monomer was increased in culture supernatants containing HSA. BSA produced similar results. These findings indicate that both HSA and BSA stabilize VvhA and delay VvhA inactivation by oligomerization, and thus enhance VvhA activity.

Virulence and stress susceptibility of clinical and environmental strains of Vibrio vulnificus isolated from samples from Taiwan and the United States

Vibrio vulnificus is an estuarine bacterium that causes severe wound infection and septicemia with high mortality. It also can be transmitted through the consumption of raw contaminated seafood and is an important foodborne pathogen. A total of 40 environmental and clinical V. vulnificus strains isolated from the United States and Taiwan were analyzed for virulence in animals, the presence of virulence-associated factors, and susceptibility to environmental stresses. Virulence in mice was exhibited by 85% of the environmental strains and 95% of the clinical strains. Strains from environmental or clinical sources were similar in virulence-associated phenotypes (protease activity, utilization of transferrin-bound iron, hemolysis, and inactivation in serum) and susceptibility to various stresses (4 and 52 degrees C, 0.1 and 10% NaCl, and pH 3.2), except freeze-thaw treatment. The clinical strains killed experimental animals after a shorter incubation time than did the environmental strains. Most of the 15 virulence-associated genes examined were present in most of the strains, regardless of their sources or virulence, with the exception of vvh, flgF, and purH. vvh was significantly more common in clinical strains than in environmental strains, and vvh, flgF, and purH were more common in virulent strains than in nonvirulent strains. These data may be helpful in devising strategies to manage or reduce the presence of V. vulnificus in foods.

Vibrio vulnificus secretes a broad-specificity metalloprotease capable of interfering with blood homeostasis through prothrombin activation and fibrinolysis

Vibrio vulnificus is a causative agent of serious food-borne diseases in humans related to the consumption of raw seafood. It secretes a metalloprotease that is associated with skin lesions and serious hemorrhagic complications. In this study, we purified and characterized an extracellular metalloprotease (designated as vEP) having prothrombin activation and fibrinolytic activities from V. vulnificus ATCC 29307. vEP could cleave various blood clotting-associated proteins such as prothrombin, plasminogen, fibrinogen, and factor Xa, and the cleavage could be stimulated by addition of 1 mM Mn2+ in the reaction. The cleavage of prothrombin produced active thrombin capable of converting fibrinogen to fibrin. The formation of active thrombin appeared to be transient, with further cleavage resulting in a loss of activity. The cleavage of plasminogen, however, did not produce an active plasmin. vEP could cleave all three major chains of fibrinogen without forming a clot. It could cleave fibrin polymer formed by thrombin as well as the cross-linked fibrin formed by factor XIIIa. In addition, vEP could also cleave plasma proteins such as bovine serum albumin and gamma globulin, and its broad specificity is reflected in the cleavage sites, which include Asp207-Phe208 and Thr272-Ala273 bonds in prothrombin and a Tyr80-Leu81 bond in plasminogen. Taken together, the data suggest that vEP is a broad-specificity protease that could function as a prothrombin activator and a fibrinolytic enzyme to interfere with blood homeostasis as part of the mechanism associated with the pathogenicity of V. vulnificus in humans and thereby facilitate the development of systemic infection.

Inactivation of Vibrio vulnificus hemolysin by oligomerization but not proteolysis

Vibrio vulnificus extracellular protease (VvpE) is believed to destroy its hemolysin (VvhA) in the late growth phase, without obvious experimental evidence. So, we attempted to elucidate the mechanism. The hemolytic activity steeply increased with the expression of the VvhA in the early growth phase, and then abruptly declined with the expression of VvpE in the late growth phase. However, the VvhA activity also abruptly declined in a VvpE-deficient mutant. In Western blot, the degradation of VvhA was not observed; instead, the oligomerization of VvhA increased with the concomitant loss of hemolytic activity. These results evidently indicate that the inactivation of VvhA is due to the novel oligomerization of VvhA by unknown mechanism, but not to the destruction of VvhA by VvpE, so that the routine functional assay measuring hemolytic activity cannot reflect the actual production of VvhA.

Pathogenic Factors of Vibrios with Special Emphasis on Vibrio vulnificus

Bacteria of the genus Vibrio are normal habitants of the aquatic environment and play roles for biocontrole of aquatic ecosystem, but some species are believed to be human pathogens. These species can be classified into two groups according to the types of diseases they cause: the gastrointestinal infections and the extraintestinal infections. The pathogenic species produce various pathogenic factors including enterotoxin, hemolysin, cytotoxin, protease, siderophore, adhesive factor, and hemagglutinin. We studied various pathogenic factors of vibrios with special emphasis on protease and hemolysin of V. vulnificus. V. vulnificus is now recognized as being among the most rapidly fatal of human pathogens, although the infection is appeared in patients having underlying disease(s) such as liver dysfunction, alcoholic cirrhosis or haemochromatosis. V. vulnificus protease (VVP) is thought to be a major toxic factor causing skin damage in the patients having septicemia. VVP is a metalloprotease and degrades a number of biologically important proteins including elastin, fibrinogen, and plasma proteinase inhibitors of complement components. VVP causes skin damages through activation of the Factor XII-plasma kallikrein-kinin cascade and/or exocytotic histamine release from mast cells, and a haemorrhagic lesion through digestion of the vascular basement membrane. Thus, the protease is the most probable candidate for tissue damage and bacterial invasion during an infection. Pathogenic roles and functional mechanism of other factors including hemolysins of V. vulnificus and V. mimicus are also shown in this review article.

Essential role of an adenylate cyclase in regulating Vibrio vulnificus virulence

Vibrio vulnificus, a halophilic estuarine bacterium, causes a fatal septicemia and necrotizing wound infection. To investigate the role of cAMP in V. vulnificus virulence regulation, an in-frame deletion mutant of the cya gene encoding adenylate cyclase was constructed. The cya null mutation resulted in a pleiotropic change of virulence phenotypes. The production of hemolysin and protease, the motility, and the cytotoxicity were decreased by the cya mutation. The defects in the cya mutant were functionally complemented in trans by a plasmid carrying the wild type cya allele. The V. vulnificus cya mutant exhibited a 100-fold increase in LD50 to mice. The result indicates that cAMP plays an essential role in the global regulation of V. vulnificus virulence.

Vibrio vulnificusmetalloprotease VvpE has no direct effect on the iron-assimilation from human holotransferrin

In order to elucidate the role of Vibrio vulnificus metalloprotease VvpE in the uptake of iron from human transferrin, we constructed a VvpE-deficient mutant and a merozygotic vvpE-transcriptional reporter from the wild type strain MO6-24/O. All three strains were able to grow only in deferrated Heart Infusion broth (DF-HI) with human holotransferrin (HT), but not in DF-HI containing partially iron-saturated transferrin or apotransferrin, without noticeable differences among the strains. All strains consumed most iron in the early growth phase. Both the transcription and extracellular production of VvpE proceeded at undetectable levels when bacterial growth was severely retarded in the DF-HI. When HT or FeCl(3) was added to the DF-HI, the retarded bacterial growth was restored and vvpE transcription dramatically increased in the late growth phase, but the extracellular VvpE production was negligible as compared to its transcription. All strains were unable to degrade HT even in normal HI broth containing HT, in which extracellular VvpE activity was remarkably high. The uptake of iron from HT in all strains was consistent with the production of catechol-siderophore rather than hydroxamate-siderophore. Similar results were also observed when clinical isolates from septicemic patients were used. In conclusion, we determined that VvpE was not directly involved in the siderophore-mediated iron-uptake from human transferrin. In addition, the discrepancy between the transcription and extracellular production of VvpE suggests that additional posttranscriptional events are involved in the extracellular production of VvpE.

A Vibrio vulnificus type IV pilin contributes to biofilm formation, adherence to epithelial cells, and virulence

Vibrio vulnificus expresses a multitude of cell-associated and secreted factors that potentially contribute to pathogenicity, although the specific roles of most of these factors have been difficult to define. Previously we have shown that a mutation in pilD (originally designated vvpD), which encodes a type IV prepilin peptidase/N-methyltransferase, abolishes expression of surface pili, suggesting that they belong to the type IV class. In addition, a pilD mutant exhibits reduced adherence to HEp-2 cells, a block in secretion of several exoenzymes that follow the type II secretion pathway, and decreased virulence. In this study, we have cloned and characterized a V. vulnificus type IV pilin (PilA) that shares extensive homology to group A type IV pilins expressed by many pathogens, including Vibrio cholerae (PilA), Pseudomonas aeruginosa (PilA), and Aeromonas hydrophila (TapA). The V. vulnificus pilA gene is part of an operon and is clustered with three other pilus biogenesis genes, pilBCD. Inactivation of pilA reduces the ability of V. vulnificus to form biofilms and significantly decreases adherence to HEp-2 cells and virulence in iron dextran-treated mice. Southern blot analysis demonstrates the widespread presence of both pilA and pilD in clinical as well as environmental strains of V. vulnificus.

Vibrio vulnificus in Taiwan

Residents in Taiwan are often exposed to marine microorganisms through seafood and occupational exposure. The number of reported cases of infection attributable to this organism has increased since the first case was reported in 1985. The increasing number of cases may be caused by greater disease activity or improved recognition by clinicians or laboratory workers. We analyze a clinical-case series of 84 patients with Vibrio vulnificus infection from 1995 to 2000 and describe the molecular epidemiologic features of pathogens isolated from these patients. The spectrum of clinical manifestations and outcomes, options of antimicrobial therapy, and virulence mechanisms were investigated. Results of molecular typing of isolates from humans and marine environment in this country had a high genetic divergence among these isolates. Education and measures are needed to prevent this emerging disease.

A calcium-calmodulin antagonist blocks experimental Vibrio vulnificus cytolysin-induced lethality in an experimental mouse model

We demonstrated that trifluoperazine, a calcium-calmodulin antagonist, blocked the hyperpermeability induced by Vibrio vulnificus cytolysin in in vitro-modeled endothelium and prevented the deaths of mice. Furthermore, compared to tetracycline alone, tetracycline combined with trifluoperazine enhanced the survival rate of V. vulnificus-infected mice, indicating the role of the cytolysin as an important factor in pathogenesis.

Cloning and characterization of the lipase and lipase activator protein from Vibrio vulnificus CKM-1

The gene (lipA) encoding the extracellular lipase and its downstream gene (lipB) from Vibrio vulnificus CKM-1 were cloned and sequenced. Nucleotide sequence analysis and alignments of amino acid sequences suggest that Lip Ais a member of bacterial lipase family I.1 and that LipB is a lipase activator of LipA. The active LipA was produced in recombinant Escherichia coli cells only in the presence of the lipB. In the hydrolysis of p-nitrophenyl esters and triacylglycerols, using the reactivated LipA, the optimum chain lengths for the acyl moiety on the substrate were C14 for ester hydrolysis and C10 to C12 for triacylglycerol hydrolysis.

High growing ability of Vibrio vulnificus biotype 1 is essential for production of a toxic metalloprotease causing systemic diseases in humans

Vibrio vulnificus biotype 1, a causative agent of fatal septicemia or wound infection in humans, is known to produce a toxic metalloprotease as an important virulence determinant. V. vulnificus biotype 2 (serovar E), a primary eel pathogen, was found to elaborate an extracellular metalloprotease that was indistinguishable from that of biotype 1. The potential of V. vulnificus biotype 1 for production of the metalloprotease was compared with biotype 2 and other human non-pathogenic Vibrio species (Vibrio anguillarum and Vibrio proteolyticus). When cultivated at 25 degrees C in tryptone-yeast extract broth supplemented with 0.9% NaCl, all bacteria multiplied sufficiently and secreted significant amounts of the metalloprotease. However, at 37 degrees C with 0.9% NaCl, V. anguillarum neither grew nor produced the metalloprotease. In human serum, only V. vulnificus biotype 1 revealed a steady multiplication accompanied with production of the extracellular metalloprotease. This prominent ability of biotype 1 in growth and protease production may contribute to cause serious systemic diseases in humans.

A K+ yptake protein, TrkA, is required for serum, protamine, and polymyxin B resistance in Vibrio vulnificus

Vibrio vulnificus, a highly virulent marine bacterium, is the causative agent of both serious wound infections and fatal septicemia in many areas of the world. To identify the genes required for resistance to human serum, we constructed a library of transposon mutants of V. vulnificus and screened them for hypersensitivity to human serum. Here we report that one of the isolated serum-susceptible mutants had a mutation in an open reading frame identified as trkA, a gene encoding an amino acid sequence showing high identity to that of TrkA of Vibrio alginolyticus, a protein required for the uptake of potassium. A trkA isogenic mutant was constructed via insertional inactivation, and it was significantly more easily killed by human serum, protamine, or polymyxin B than was the wild type. At K+ concentrations of 1 to 20 mM, this isogenic mutant showed attenuated growth compared to the wild-type strain. In addition, infection experiments demonstrated virulence attenuation when this mutant was administered intraperitoneally or subcutaneously to both normal and iron-treated mice, indicating that TrkA may modulate the transport of potassium and resistance to host innate defenses and that it is important for virulence in mice.

RpoS-dependent stress response and exoenzyme production in Vibrio vulnificus

Vibrio vulnificus is an estuarine bacterium capable of causing rapidly fatal infections through both ingestion and wound infection. Like other opportunistic pathogens, V. vulnificus must adapt to potentially stressful environmental changes while living freely in seawater, upon colonization of the oyster gut, and upon infection of such diverse hosts as humans and eels. In order to begin to understand the ability of V. vulnificus to respond to such stresses, we examined the role of the alternate sigma factor RpoS, which is important in stress response and virulence in many pathogens. An rpoS mutant of V. vulnificus strain C7184o was constructed by homologous recombination. The mutant strain exhibited a decreased ability to survive diverse environmental stresses, including exposure to hydrogen peroxide, hyperosmolarity, and acidic conditions. The most striking difference was a high sensitivity of the mutant to hydrogen peroxide. Albuminase, caseinase, and elastase activity were detected in the wild type but not in the mutant strain, and an additional two hydrolytic activities (collagenase and gelatinase) were reduced in the mutant strain compared to the wild type. Additionally, the motility of the rpoS mutant was severely diminished. Overall, these studies suggest that rpoS in V. vulnificus is important for adaptation to environmental changes and may have a role in virulence.

Comparative genome analysis of Vibrio vulnificus, a marine pathogen

The halophile Vibrio vulnificus is an etiologic agent of human mortality from seafood-borne infections. We applied whole-genome sequencing and comparative analysis to investigate the evolution of this pathogen. The genome of biotype 1 strain, V. vulnificus YJ016, was sequenced and includes two chromosomes of estimated 3377 kbp and 1857 kbp in size, and a plasmid of 48,508 bp. A super-integron (SI) was identified, and the SI region spans 139 kbp and contains 188 gene cassettes. In contrast to non-SI sequences, the captured gene cassettes are unique for any given Vibrio species and are highly variable among V. vulnificus strains. Multiple rearrangements were found when comparing the 5.3-Mbp V. vulnificus YJ016 genome and the 4.0-Mbp V. cholerae El Tor N16961 genome. The organization of gene clusters of capsular polysaccharide, iron metabolism, and RTX toxin showed distinct genetic features of V. vulnificus and V. cholerae. The content of the V. vulnificus genome contained gene duplications and evidence of horizontal transfer, allowing for genetic diversity and function in the marine environment. The genomic information obtained in this study can be applied to monitoring vibrio infections and identifying virulence genes in V. vulnificus.

Flagellar basal body flg operon as a virulence determinant of Vibrio vulnificus

Vibrio vulnificus, a halophilic estuarine bacterium causing a rapidly progressing fatal septicemia, is highly cytotoxic to eukaryotic cells. To identify new virulence factors associated with cytotoxicity, we constructed a mariner-based transposon (Tn Himar1) library of the highly virulent clinical isolate MO6-24/O having a double mutation in the hemolysin and protease genes. The Himar1 mutant library was extensively screened for the mutants showing decreased cytotoxicity to HeLa cells. We selected a cytotoxicity defective mutant having a Himar1 insertion in an open reading frame showing 96% identity to Vibrio parahaemolyticus FlgC, a flagella basal body rod protein. The Tn Himar1 insertion mutation also resulted in a significant decrease in motility, adhesion, cytotoxicity, and lethality to mice. This is the first report showing that flg genes, which are components of the flagellum biogenesis gene cluster, might play an important role in the virulence of V. vulnificus.

Phage therapy of local and systemic disease caused by Vibrio vulnificus in iron-dextran-treated mice

Vibrio vulnificus is a gram-negative bacterium that contaminates filter-feeding shellfish such as oysters. After ingestion of contaminated oysters, predisposed people may experience highly lethal septicemia. Contamination of wounds with the bacteria can result in devastating necrotizing fasciitis, which can progress to septicemia. The extremely rapid progression of these diseases can render antibiotic treatment ineffective, and death is a frequent outcome. In this study, we examined the potential use of bacteriophages as therapeutic agents against V. vulnificus in an iron-dextran-treated mouse model of V. vulnificus infection. Mice were injected subcutaneously with 10 times the lethal dose of V. vulnificus and injected intravenously, either simultaneously or at various times after infection, with phages. Treatment of mice with phages could prevent death; systemic disease, as measured by CFU per gram of liver and body temperature; and local disease, as measured by CFU per gram of lesion material and histopathologic analysis. Two different phages were effective against three different V. vulnificus strains with various degrees of virulence, while a third phage that required the presence of seawater to lyse bacteria in vitro was ineffective at treating mice. Optimum protection required that the phages be administered within 3 h of bacterial inoculation at doses as high as 10(8) PFU. One of the protective phages had a half-life in blood of over 2 h. These results demonstrate that bacteriophages have therapeutic potential for both localized and systemic infections caused by V. vulnificus in animals. This model should be useful in answering basic questions regarding phage therapy.

The InhA2 metalloprotease of Bacillus thuringiensis strain 407 is required for pathogenicity in insects infected via the oral route

The entomopathogenic bacterium Bacillus thuringiensis is known to secrete a zinc metalloprotease (InhA) that specifically cleaves antibacterial peptides produced by insect hosts. We identified a second copy of the inhA gene, named inhA2, in B. thuringiensis strain 407 Cry(-). The inhA2 gene encodes a putative polypeptide showing 66.2% overall identity with the InhA protein and harboring the zinc-binding domain (HEXXH), which is characteristic of the zinc-requiring metalloproteases. We used a transcriptional inhA2'-lacZ fusion to show that inhA2 expression is induced at the onset of the stationary phase and is overexpressed in a Spo0A minus background. The presence of a reverse Spo0A box in the promoter region of inhA2 suggests that Spo0A directly regulates the transcription of inhA2. To determine the role of the InhA and InhA2 metalloproteases in pathogenesis, we used allelic exchange to isolate single and double mutant strains for the two genes. Spores and vegetative cells of the mutant strains were as virulent as those of the parental strain in immunized Bombyx mori larvae infected by the intrahemocoelic route. Exponential phase cells of all the strains displayed the same in vitro potential for colonizing the vaccinated hemocoel. We investigated the synergistic effect of the mutant strain spores on the toxicity of Cry1C proteins against Galleria mellonella larvae infected via the oral pathway. The spores of DeltainhA2 mutant strain were ineffective in providing synergism whereas those of the DeltainhA mutant strain were not. These results indicate that the B. thuringiensis InhA2 zinc metalloprotease has a vital role in virulence when the host is infected via the oral route.

Specificity of a heme-assimilating system of Vibrio vulnificus to synthetic heme compounds

Vibrio vulnificus strain L-180, a clinical isolate, can obtain iron from a synthetic heme, iron-tetra(4-sulfonatophenyl)porphyrin (Fe-TPPS), as well as from a natural heme, protoheme. This assimilation of iron bound to TPPS was demonstrated to be a common property of V. vulnificus by testing a total of 27 strains isolated from both clinical and environmental sources. Strain L-180 could also utilize Fe-TCPP, but not Fe-TMPyP, as a sole iron source. TPPS or its complex with a metal ion reduced bacterial multiplication in the broth containing a minimum dose of Fe-TPPS. When inoculated into human serum supplemented with Fe-TCPP, L-180 could grow only in the presence of a protease from the same bacterium. In both TPPS and TCPP, each side chain of a porphyrin ring has a negative charge. Therefore, this negative charge may be important for interaction with an outer membrane receptor involving in a heme-assimilating system of V. vulnificus.

Isolation and characterization of a Vibrio vulnificus mutant deficient in both extracellular metalloprotease and cytolysin

We isolated a Vibrio vulnificus mutant that was deficient in both metalloprotease and cytolysin by allelic exchange. The virulence of this mutant in mice and its cytotoxicity for HEp-2 cells were comparable to those of the wild-type strain, indicating that neither factor was essential for these properties. The cytolysin, but not the protease, seemed to be important for causing damage in the alimentary tract of the mice.

Differential expression of Vibrio vulnificus elastase gene in a growth phase-dependent manner by two different types of promoters

Elastase activity of Vibrio vulnificus was highly dependent on growth phase, reached a maximum during the stationary phase, and was regulated at the level of transcription. The stationary phase production of elastase in crp or rpoS mutants, which were constructed by allelic exchanges, decreased about 3- and 10-fold, respectively. However, the promoter activity of vvpE encoding elastase was unaffected by those mutations in the log phase when analyzed using a vvpE-lux fusion. A primer extension analysis revealed that the transcription of vvpE begins at two different sites, consisting of putative promoter L (PL) and promoter S (PS). The PL activity was constitutive through the log and stationary phases, lower than the PS activity, and unaffected by the crp or rpoS mutations. The transcription of PS, induced only in the stationary phase, was dependent on RpoS. The mutation in crp reduced the activity of PS; however, the additional inactivation of crp did not influence the PS activity in the rpoS mutant, indicating that CRP exerted its effects through PS requiring RpoS. These results demonstrate that vvpE expression is differentially directed by PL and PS depending on the growth phase and elevated by RpoS and CRP in the stationary phase.

Regulation of metalloprotease gene expression in Vibrio vulnificus by a Vibrio harveyi LuxR homologue

Expression of the Vibrio vulnificus metalloprotease gene, vvp, was turned up rapidly when bacterial growth reached the late log phase. A similar pattern of expression has been found in the metalloprotease gene of Vibrio cholerae, and this has been shown to be regulated by a Vibrio harveyi LuxR-like transcriptional activator. To find out whether a LuxR homologue exists in V. vulnificus, a gene library of this organism was screened by colony hybridization using a probe derived from a sequence that is conserved in various luxR-like genes of vibrios. A gene containing a 618-bp open reading frame was identified and found to be identical to the smcR gene of V. vulnificus reported previously. An isogenic SmcR-deficient (RD) mutant was further constructed by an in vivo allelic exchange technique. This mutant exhibited an extremely low level of vvp transcription compared with that of the parent strain. On the other hand, the cytolysin gene, vvhA, was expressed at a higher level in the RD mutant than in the parent strain during the log phase of growth. These data suggested that SmcR might not only be a positive regulator of the protease gene but might also be involved in negative regulation of the cytolysin gene. Virulence of the RD mutant in either normal or iron-overloaded mice challenged by intraperitoneal injection was comparable to that of the parent strain, indicating that SmcR is not required for V. vulnificus virulence in mice.

Cloning and characterization of a periplasmic nuclease of Vibrio vulnificus and its role in preventing uptake of foreign DNA

We have cloned a nuclease gene, vvn, from Vibrio vulnificus, an estuarine bacterium that causes wound infections and septicemia in humans and eels. The gene contained a 696-bp open reading frame encoding 232 amino acids (aa), including a signal sequence of 18 aa. The deduced amino acid sequence of the mature nuclease predicted a molecular mass of 25 kDa, which was confirmed by vital stain, and a pI of 8.6. Vvn was produced in the periplasm of either V. vulnificus or recombinant Escherichia coli strains and was active in the oxidized (but not the reduced) form. This nuclease was able to digest DNA and RNA, with differential thermostability in DNase and RNase activities. Expression of Vvn in E. coli DH5alpha reduced the frequencies of transformation with the divalent ion-treated cells and electroporation by about 6 and 2 logs, respectively. In addition, the transformation frequency of a Vvn-deficient V. vulnificus mutant (ND) was 10-fold higher than that of the parent strain. These data suggested that Vvn may be involved in preventing uptake of foreign DNA by transformation. However, Vvn expressed in the recipients had little effect on the conjugation frequency in either E. coli or V. vulnificus. Some other DNase(s) may be present in the periplasm and responsible for a residual DNase activity, which was about one-fourth of that of the parent strain, detected in the ND mutant. We also demonstrated that Vvn was not required for the virulence of V. vulnificus mice.

Pathogenesis of infection by clinical and environmental strains of Vibrio vulnificus in iron-dextran-treated mice

Vibrio vulnificus is an opportunistic pathogen that contaminates oysters harvested from the Gulf of Mexico. In humans with compromising conditions, especially excess levels of iron in plasma and tissues, consumption of contaminated seafood or exposure of wounds to contaminated water can lead to systemic infection and disfiguring skin infection with extremely high mortality. V. vulnificus-associated diseases are noted for the rapid replication of the bacteria in host tissues, with extensive tissue damage. In this study we examined the virulence attributes of three virulent clinical strains and three attenuated oyster or seawater isolates in mouse models of systemic disease. All six V. vulnificus strains caused identical skin lesions in subcutaneously (s.c.) inoculated iron dextran-treated mice in terms of numbers of recovered CFU and histopathology; however, the inocula required for identical frequency and magnitude of infection were at least 350-fold higher for the environmental strains. At lethal doses, all strains caused s. c. skin lesions with extensive edema, necrosis of proximate host cells, vasodilation, and as many as 10(8) CFU/g, especially in perivascular regions. These data suggest that the differences between these clinical and environmental strains may be related to growth in the host or susceptibility to host defenses. In non-iron dextran-treated mice, strains required 10(5)-fold-higher inocula to cause an identical disease process as with iron dextran treatment. These results demonstrate that s.c. inoculation of iron dextran-treated mice is a useful model for studying systemic disease caused by V. vulnificus.