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

Genome sequence and pathogenicity of Vibrio vulnificus strain MCCC 1A08743 isolated from contaminated prawns

Vibrio vulnificus is an opportunistic pathogen that naturally inhabits sea water globally and is responsible for most vibriosis-related deaths. The consumption of V. vulnificus contaminated seafood and exposure of wounds to Vibrio can result in systemic infection, with increased risks of amputation and extremely high rates of mortality. However, the pathogenicity and virulence factors of V. vulnificus are not fully understood. The genomic characterization of V. vulnificus will be helpful to extend our understanding on V. vulnificus at a genomic level. In this manuscript, the genome of V. vulnificus strain MCCC 1A08743 isolated from contaminated prawns from Zhanjiang, China, was sequenced using Illumina HiSeq X Ten system and annotated through multiple databases. The strain MCCC 1A08743 genome included 4371 protein-coding genes and 117 RNA genes. Average nucleotide identity analysis and core genome phylogenetic analysis revealed that MCCC 1A08743 was most closely related to strains from clinical samples from the United States. Pathogenicity annotation of the MCCC 1A08743 genome, using Virulence Factor Database and Pathogen-Host Interactions database, predicted the pathogenicity of the strain, and this was confirmed using mice infection experiments, which indicated that V. vulnificus strain MCCC 1A08743 could infect C57BL/6J mice and cause liver lesions.

This article has an associated First Person interview with the first author of the paper.

Summary:Vibrio vulnificus strain MCCC 1A08743 was newly isolated, sequenced and tested for its pathogenicity in mice.

Effect of luxS encoding a synthase of quorum-sensing signal molecule AI-2 of Vibrio vulnificus on mouse gut microbiome

Autoinducer-2 (AI-2), a quorum-sensing signal molecule from the human pathogen Vibrio vulnificus, was assessed for its effect on the gut microbiome of mice. For this, we employed 16S rRNA sequencing to compare the gut microbiome of mice infected with either wild-type V. vulnificus or with the isotype ΔluxS that has a deletion in luxS which encodes the biosynthetic function of AI-2. The relative ratio of wild-type Vibrio species in the jejunum and ileum of mice infected with the wild type was significantly higher than that in mice infected with ΔluxS, suggesting that AI-2 plays an important role in the colonization of V. vulnificus in the small intestine. The bacterial composition in the gut of mice infected with ΔluxS comprises a higher proportion of Firmicutes, composed mainly of Lactobacillus, compared to the mice infected with wild-type cells. In the large intestine, Vibrio species were barely detected regardless of genetic background. Three Lactobacillus spp. isolated from fecal samples from mice infected with ΔluxS manifested significant antibacterial activities against V. vulnificus. Culture supernatants from these three species were dissolved by HPLC, and a substance in fractions showing inhibitory activity against V. vulnificus was determined to be lactic acid. Our results suggest that luxS in V. vulnificus affects not only the ability of the species to colonize the host gut but also its susceptibility to the growth-inhibiting activity of commensal bacteria including Lactobacillus. KEY POINTS: • Gut microbiomes of ΔluxS-infected and WT Vibrio-infected mice differed greatly. • Difference was most prominent in the jejunum and ileum compared to the duodenum or large intestine. • In the small and large intestines of mice, the relative proportions of Vibrio and Lactobacillus species showed a negative relationship. • Effector molecules produced by Lactobacillus in mouse gut inhibit Vibrio growth.

Biochemical and Virulence Characterization of Vibrio vulnificus Isolates From Clinical and Environmental Sources

Vibrio vulnificus is a deadly human pathogen for which infections occur via seafood consumption (foodborne) or direct contact with wounds. Virulence is not fully characterized for this organism; however, there is evidence of biochemical and genotypic correlations with virulence potential. In this study, biochemical profiles and virulence genotype, based on 16S rRNA gene (rrn) and virulence correlated gene (vcg) types, were determined for 30 clinical and 39 oyster isolates. Oyster isolates were more biochemically diverse than the clinical isolates, with four of the 20 tests producing variable (defined as 20–80% of isolates) results. Whereas, for clinical isolates only mannitol fermentation, which has previously been associated with virulence potential, varied among the isolates. Nearly half (43%) of clinical isolates were the more virulent genotype (rrnB/vcgC); this trend was consistent when only looking at clinical isolates from blood. The majority (64%) of oyster isolates were the less virulent genotype (rrnA or AB/vcgE). These data were used to select a sub-set of 27 isolates for virulence testing with a subcutaneously inoculated, iron-dextran treated mouse model. Based on the mouse model data, 11 isolates were non-lethal, whereas 16 isolates were lethal, indicating a potential for human infection. Within the non-lethal group there were eight oyster and three clinical isolates. Six of the non-lethal isolates were the less virulent genotype (rrnA/vcgE or rrnAB/vcgE) and two were rrnB/vcgC with the remaining two of mixed genotype (rrnAB/vcgC and rrnB/vcgE). Of the lethal isolates, five were oysters and 11 were clinical. Eight of the lethal isolates were the less virulent genotype and seven the more virulent genotype, with the remaining isolate a mixed genotype (rrnA/vcgC). A discordance between virulence genotype and individual mouse virulence parameters (liver infection, skin infection, skin lesion score, and body temperature) was observed; the variable most strongly associated with mouse virulence parameters was season (warm or cold conditions at time of strain isolation), with more virulent strains isolated from cold conditions. These results indicate that biochemical profiles and genotype are not significantly associated with virulence potential, as determined by a mouse model. However, a relationship with virulence potential and seasonality was observed.

DIDS inhibits Vibrio vulnificus cytotoxicity by interfering with TolC-mediated RtxA1 toxin secretion

Vibrio vulnificus (V. vulnificus) infection, frequently resulting in fatal septicemia, has become a growing health concern worldwide. The present study aimed to explore the potential agents that could protect against V. vulnificus cytotoxicity, and to analyze the possible underlying mechanisms. First, we observed that 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid disodium salt hydrate (DIDS) significantly suppressed V. vulnificus cytotoxicity to host cells by using a lactate dehydrogenase (LDH) assay. DIDS did not exhibit any effect on host cell viability, bacterial growth, microbial adhesion and swarming motility. DIDS effectively lowered V. vulnificus RtxA1 toxin-induced calcium influx into host mitochondria and RtxA1 binding to host cells. To further elucidate the underlying mechanism, the synthesis and secretion of RtxA1 toxin were investigated by Western blotting. Intriguingly, DIDS selectively inhibited the secretion of RtxA1 toxin, but did not influence its synthesis. Consequently, the outer membrane portal TolC, a key conduit for RtxA1 export coupled with tripartite efflux pumps, was examined by RT-PCR and Western blotting. We found that DIDS significantly reduced the expression of TolCV1 protein at the transcriptional level. Taken together, these results suggest that DIDS is a promising new paradigm as an antimicrobial drug that targets TolC-mediated toxin.

Differential expression of vvhA and CPS operon allele 1 genes in Vibrio vulnificus under biofilm and planktonic conditions

Examination of genes encoding for the virulence factors, hemolysin/cytolysin (vvhA) and capsular polysaccharide (CPS allele 1), during biofilm formation revealed that their expression was influenced by the maturity of the biofilm as well as by temperature. At 24 °C, expression of vvhA during biofilm formation was low between 4 and 12 h but increased 10-fold by 24 h to (5.1 × 10⁴ ± 6.3 × 10³mRNA copies/ml) as the biofilm matured. Compared to planktonic cells, expression of vvhA during biofilm formation at 24 °C was initially up-regulated at 4 h (1.07 ± 0.00-fold) but then was down-regulated almost four-fold during the intermediate and mature stages of biofilm formation. In contrast, vvhA expression at 37 °C was up-regulated almost four-fold in the early stages (4 and 6 h) of biofilm formation and remained two-fold up-regulated by 24 h even as the biofilm was deteriorating. CPS allele 1 expression at 24 °C during biofilm formation was up-regulated (1.50 ± 0.18-fold) during the initial attachment phase of the cells but was strongly down-regulated during the intermediate phases at 8 and 10 h (74.42 ± 42.16-fold and 453.76 ± 193.32-fold, respectively), indicating that capsular polysaccharide (CPS) is not important to intermediate biofilm architecture. Interestingly, as the biofilm matured by 24 h, expression of CPS allele 1 was again up-regulated (1.88 ± 1.07), showing that CPS plays a role in mature biofilm. At 37 °C, CPS allele 1 expression was significantly up-regulated (up to 10⁵) during biofilm formation, indicating that the biofilm form of V. vulnificus may be preferred over the planktonic form in the human host.

Characterization of temperature-dependent hemin uptake receptors HupA and HvtA in Vibrio vulnificus

The Gram-negative pathogen Vibrio vulnificus produces several iron-sequestration systems including a hemin uptake system in response to iron limitation as a means to acquire this essential element. Strains of this organism are capable of causing serious septicemia in humans and eels, where hemin is abundant and an advantageous source of iron. Vibrio vulnificus hemin uptake systems consist of HupA, a well studied outer membrane protein, and a recently identified HvtA protein receptor. In this study, we confirmed that the expression of the hvtA gene is iron-regulated in a fur-dependent manner. When analyzed for virulence in a hemin-overloaded murine model system, the hupA gene was more important for establishing infection than the hvtA gene. Transcriptional profiling of these genes using strains of two different biotypes, biotype 1 (human pathogen) and biotype 2 (eel pathogen), showed that the expression of the two receptors was also regulated in response to temperature. The expression of hupA was highly induced in elevated temperatures in the human pathogenic strain when tested in iron-depleted conditions. Conversely, hvtA expression was induced significantly in the eel pathogenic strain at a lower temperature, a condition where the hupA locus was relatively repressed. Our results indicate that although both hupA and hvtA are involved for optimal hemin uptake in V. vulnificus, their expression is dually regulated by the environmental cues of iron concentration and temperature. Together, these data suggest that the virulence genes hupA and hvtA are tightly regulated and strictly induced during iron limitation combined with the physiological temperature of the host organism.

The geographical region of origin determines the phagocytic vulnerability of Lichtheimia strains

Mucormycoses are life-threatening infections that affect patients suffering from immune deficiencies. We performed phagocytosis assays confronting various strains of Lichtheimia species with alveolar macrophages, which form the first line of defence of the innate immune system. To investigate 17 strains from four different continents in a comparative fashion, transmitted light and confocal fluorescence microscopy was applied in combination with automated image analysis. This interdisciplinary approach enabled the objective and quantitative processing of the big volume of image data. Applying machine-learning supported methods, a spontaneous clustering of the strains was revealed in the space of phagocytic measures. This clustering was not driven by measures of fungal morphology but rather by the geographical origin of the fungal strains. Our study illustrates the crucial contribution of machine-learning supported automated image analysis to the qualitative discovery and quantitative comparison of major factors affecting host-pathogen interactions. We found that the phagocytic vulnerability of Lichtheimia species depends on their geographical origin, where strains within each geographic region behaved similarly, but strongly differed amongst the regions. Based on this clustering, we were able to also classify clinical isolates with regard to their potential geographical origin.

Quantitative PCR enumeration of vcgC and 16S rRNA type A and B genes as virulence indicators for environmental and clinical strains of Vibrio vulnificus in Galveston Bay oysters

Oysters from a reef in Galveston Bay, Texas, USA, were screened for more virulent clinical strains versus less virulent environmental strains of Vibrio vulnificus using a combination of quantitative PCR assays for the virulence correlating gene (clinical variant, vcgC) and 16S rRNA types A and B (type A = environmental, type B = clinical). The combination of vcgC and 16S rRNA type B loci to determine clinical type strains was suitable, as indicated by the strong correlation (R² = 0.98; p < 0.001) between these gene counts over time and their relative proportion (up to 93.8% and 94.3%, respectively) to vvhA genes used to quantify all strains of V. vulnificus. A strong seasonal shift of V. vulnificus strain types was observed. Environmental strains (16S rRNA type A) predominated from April to mid-June as salinities increased from 22 to 27 PSU (practical salinity unit) and temperatures rose 20 to 28 °C, with peak gene quantities of 16 812 ± 56 CFU/g. As temperatures increased to ≥30 °C from mid-June to September and salinities rose above 27 PSU, clinical strains (16S rRNA type B; vcgC) predominated with peak quantities 31 868 ± 287 and 32 360 ± 178 CFU/g, respectively.

Fatal case of necrotising fasciitis due to Vibrio vulnificus in a patient with alcoholic liver disease and diabetes mellitus

Vibrio vulnificus is a halophilic Vibrio found globally. They are thought to be normal microbiome in the estuaries along the coasts associated with seawater and seashells. Infection usually results from consumption of raw oysters or shellfish or exposure of broken skin or open wounds to contaminated salt or brackish water. Clinical manifestations range from gastroenteritis to skin and subcutaneous infection and primary sepsis. Pathogen has the ability to cause infections with significant mortality in high-risk populations, including patients with chronic liver disease, immunodeficiency, diabetes mellitus and iron storage disorders. There is often a lack of clinical suspicion in cases due to Vibrio vulnificus leading to delay in treatment and subsequent mortality. Herein we report a case of necrotising fasciitis in a diabetic patient with alcoholic liver disease caused by Vibrio vulnificus which ended fatally.

Uncovering the geographical and host impacts on the classification of Vibrio vulnificus

Vibrio vulnificus causes human sickness throughout the world via the consumption of undercooked seafood or exposure to contaminated water. Previous attempts at phylogenetic analyses of V. vulnificus have proven unsuccessful, mainly due to the poorly understood impact of factors on its divergence. In this study, we used advanced statistical and phylogenetic methods to strengthen the classification of V. vulnificus. This updated classification included the impact of geographical and host factors. The results demonstrate the existence of hierarchies and multidimensional effects in the classification of V. vulnificus, from the molecular level using biotypes, to the distributional level using geographical location, to the adaptational level through host immune response. These findings have implications for the classification of bacteria, bacterial evolution, and public health.

Identification and Validation of an Antivirulence Agent Targeting HlyU-Regulated Virulence in Vibrio vulnificus

Antimicrobial resistance (AMR) in pathogens is the result of indiscriminate use of antibiotics and consequent metabolic/genetic modulation to evolve survival strategies and clonal-selection in AMR strains. As an alternative to antibiotic treatment, antivirulence strategies are being developed, not only to combat bacterial pathogenesis, but also to avoid emerging antibiotic resistance. Vibrio vulnificus is a foodborne pathogen that causes gastroenteritis, necrotizing wound infections, and sepsis with a high rate of mortality. Here, we developed an inhibitor-screening reporter platform to target HlyU, a master transcriptional regulator of virulence factors in V. vulnificus by assessing rtxA1 transcription under its control. The inhibitor-screening platform includes wild type and ΔhlyU mutant strains of V. vulnificus harboring the reporter construct P_rtxA1::luxCDABE for desired luminescence signal detection and control background luminescence, respectively. Using the inhibitor-screening platform, we identified a small molecule, fursultiamine hydrochloride (FTH), that inhibits the transcription of the highly invasive repeat-in-toxin (rtxA1) and hemolysin (vvhA) along with other HlyU regulated virulence genes. FTH has no cytotoxic effects on either host cells or pathogen at the tested concentrations. FTH rescues host cells from the necrotic cell-death induced by RtxA1 and decreases the hemolytic activity under in vitro conditions. The most important point is that FTH treatment does not induce the antivirulence resistance. Current study validated the antivirulence strategy targeting the HlyU virulence transcription factor and toxin-network of V. vulnificus and demonstrated that FTH, exhibits a potential to inhibit the pathogenesis of deadly, opportunistic human pathogen, V. vulnificus without inducing AMR.

Antimicrobial peptide-loaded gold nanoparticle-DNA aptamer conjugates as highly effective antibacterial therapeutics against Vibrio vulnificus

Vibrio vulnificus causes fatal infections in humans, and antibiotics are commonly used in treatment regimens against V. vulnificus infection. However, the therapeutic effects of antibiotics are limited by multidrug resistance. In this study, we demonstrated that an antimicrobial peptide (AMP), HPA3P^His, loaded onto a gold nanoparticle-DNA aptamer (AuNP-Apt) conjugate (AuNP-Apt-HPA3P^His) is an effective therapeutic tool against V. vulnificus infection in vivo in mice. HPA3P^His induced bacterial cell death through the disruption of membrane integrity of V. vulnificus. The introduction of AuNP-Apt-HPA3P^His into V. vulnificus-infected HeLa cells dramatically reduced intracellular V. vulnificus by 90%, leading to an increase in the viability of the infected cells. Moreover, when V. vulnificus-infected mice were intravenously injected with AuNP-Apt-HPA3P^His, a complete inhibition of V. vulnificus colonization was observed in the mouse organs, leading to a 100% survival rate among the treated mice, whereas all the control mice died within 40 hours of being infected. Therefore, this study demonstrated the potential of an AMP delivered by AuNP-Apt as an effective and rapid treatment option against infection caused by a major pathogen in humans and aquatic animals.

Cross-protection against Vibrio cholerae infection by monoclonal antibodies against Vibrio vulnificus RtxA1/MARTXVv

Gram-negative Vibrio species secrete multifunctional autoprocessing repeats-in-toxin (MARTX) toxins associated with bacterial pathogenesis. Here, the cross-reactivity and cross-protectivity of mAbs against V. vulnificus RtxA1/MARTX_Vv was evaluated. Passive administration of any of these mAbs (21RA, 24RA, 46RA, 47RA and 50RA) provided strong protection against lethal V. cholerae infection. Interestingly, 24RA and 46RA, which map to the cysteine protease domain of V. cholerae MARTX_Vc , inhibited CPD autocleavage in vitro; this process is involved in V. cholerae pathogenesis. These results generate new insight into the development of broadly protective mAbs and/or vaccines against Vibrio species with MARTX toxins.

Vibrio Iron Transport: Evolutionary Adaptation to Life in Multiple Environments

Iron is an essential element for Vibrio spp., but the acquisition of iron is complicated by its tendency to form insoluble ferric complexes in nature and its association with high-affinity iron-binding proteins in the host. Vibrios occupy a variety of different niches, and each of these niches presents particular challenges for acquiring sufficient iron. Vibrio species have evolved a wide array of iron transport systems that allow the bacteria to compete for this essential element in each of its habitats. These systems include the secretion and uptake of high-affinity iron-binding compounds (siderophores) as well as transport systems for iron bound to host complexes. Transporters for ferric and ferrous iron not complexed to siderophores are also common to Vibrio species. Some of the genes encoding these systems show evidence of horizontal transmission, and the ability to acquire and incorporate additional iron transport systems may have allowed Vibrio species to more rapidly adapt to new environmental niches. While too little iron prevents growth of the bacteria, too much can be lethal. The appropriate balance is maintained in vibrios through complex regulatory networks involving transcriptional repressors and activators and small RNAs (sRNAs) that act posttranscriptionally. Examination of the number and variety of iron transport systems found in Vibrio spp. offers insights into how this group of bacteria has adapted to such a wide range of habitats.

Hepcidin-induced hypoferremia is a critical host defense mechanism against the siderophilic bacterium Vibrio vulnificus

Hereditary hemochromatosis, an iron overload disease caused by a deficiency in the iron-regulatory hormone hepcidin, is associated with lethal infections by siderophilic bacteria. To elucidate the mechanisms of this susceptibility, we infected wild-type and hepcidin-deficient mice with the siderophilic bacterium Vibrio vulnificus and found that hepcidin deficiency results in increased bacteremia and decreased survival of infected mice, which can be partially ameliorated by dietary iron depletion. Additionally, timely administration of hepcidin agonists to hepcidin-deficient mice induces hypoferremia that decreases bacterial loads and rescues these mice from death, regardless of initial iron levels. Studies of Vibrio vulnificus growth ex vivo show that high iron sera from hepcidin-deficient mice support extraordinarily rapid bacterial growth and that this is inhibited in hypoferremic sera. Our findings demonstrate that hepcidin-mediated hypoferremia is a host defense mechanism against siderophilic pathogens and suggest that hepcidin agonists may improve infection outcomes in patients with hereditary hemochromatosis or thalassemia.

Development and characterization of a catalytically inactive cysteine protease domain of RtxA1/MARTXVv as a potential vaccine for Vibrio vulnificus

Recent studies have defined several virulence factors as vaccine candidates against Vibrio vulnificus. However, most of these factors have the potential to cause pathogenic effects in the vaccinees or induce incomplete protection. To overcome these drawbacks, a catalytically inactive form, CPDVv (C3725S), of the well-conserved cysteine protease domain (CPD) of V. vulnificus multifunctional autoprocessing repeats-in-toxin (MARTXVv /RtxA1) was recombinantly generated and characterized. Notably, active and passive immunization with CPDVv (C3725S) conferred protective immunity against V. vulnificus strains. These results may provide a novel framework for developing safe and efficient subunit vaccines and/or therapeutics against V. vulnificus that target the CPD of MARTX toxins.

Monoclonal antibodies against Vibrio vulnificus RtxA1 elicit protective immunity through distinct mechanisms

Vibrio vulnificus causes rapidly progressing septicemia with an extremely high mortality rate (≥50%), even with aggressive antibiotic treatment. The bacteria secrete multifunctional autoprocessing repeats-in-toxin (MARTX) toxins, which are involved in the pathogenesis of Gram-negative Vibrio species. Recently, we reported that immunization with the C-terminal region of V. vulnificus RtxA1/MARTXVv, RtxA1-C, elicits a protective immune response against V. vulnificus through a poorly defined mechanism. In this study, we generated a panel of new monoclonal antibodies (MAbs) against V. vulnificus RtxA1-C and investigated their protective efficacies and mechanisms in a mouse model of infection. Prophylactic administration of seven MAbs strongly protected mice against lethal V. vulnificus infection (more than 90% survival). Moreover, three of these MAbs (21RA, 24RA, and 47RA) demonstrated marked efficacy as postexposure therapy. Notably, 21RA was therapeutically effective against lethal V. vulnificus infection by a variety of routes. Using Fab fragments and a neutropenic mouse model, we showed that 21RA and 24RA mediate protection from V. vulnificus infection through an Fc-independent and/or neutrophil-independent pathway. In contrast, 47RA-mediated protection was dependent on its Fc region and was reduced to 50% in neutropenic mice compared with 21RA-mediated and 24RA-mediated protection. Bacteriological study indicated that 21RA appears to enhance the clearance of V. vulnificus from the blood. Overall, these studies suggest that humoral immunity controls V. vulnificus infection through at least two different mechanisms. Furthermore, our panel of MAbs could provide attractive candidates for the further development of immunoprophylaxis/therapeutics and other therapies against V. vulnificus that target the MARTX toxin.

Vibrio vulnificus MO6-24/O lipopolysaccharide stimulates superoxide anion, thromboxane B₂, matrix metalloproteinase-9, cytokine and chemokine release by rat brain microglia in vitro

Although human exposure to Gram-negative Vibrio vulnificus (V. vulnificus) lipopolysaccharide (LPS) has been reported to result in septic shock, its impact on the central nervous system's innate immunity remains undetermined. The purpose of this study was to determine whether V. vulnificus MO6-24/O LPS might activate rat microglia in vitro and stimulate the release of superoxide anion (O₂⁻), a reactive oxygen species known to cause oxidative stress and neuronal injury in vivo. Brain microglia were isolated from neonatal rats, and then treated with either V. vulnificus MO6-24/O LPS or Escherichia coli O26:B6 LPS for 17 hours in vitro. O₂⁻ was determined by cytochrome C reduction, and matrix metalloproteinase-2 (MMP-2) and MMP-9 by gelatinase zymography. Generation of cytokines tumor necrosis factor alpha (TNF-α), interleukin-1 alpha (IL-1α), IL-6, and transforming growth factor-beta 1 (TGF-β1), chemokines macrophage inflammatory protein (MIP-1α)/chemokine (C-C motif) ligand 3 (CCL3), MIP-2/chemokine (C-X-C motif) ligand 2 (CXCL2), monocyte chemotactic protein-1 (MCP-1)/CCL2, and cytokine-induced neutrophil chemoattractant-2alpha/beta (CINC-2α/β)/CXCL3, and brain-derived neurotrophic factor (BDNF), were determined by specific immunoassays. Priming of rat microglia by V. vulnificus MO6-24/O LPS in vitro yielded a bell-shaped dose-response curve for PMA (phorbol 12-myristate 13-acetate)-stimulated O₂⁻ generation: (1) 0.1-1 ng/mL V. vulnificus LPS enhanced O₂⁻ generation significantly but with limited inflammatory mediator generation; (2) 10-100 ng/mL V. vulnificus LPS maximized O₂⁻ generation with concomitant release of thromboxane B2 (TXB2), matrix metalloproteinase-9 (MMP-9), and several cytokines and chemokines; (3) 1000-100,000 ng/mL V. vulnificus LPS, with the exception of TXB2, yielded both attenuated O₂⁻ production, and a progressive decrease in MMP-9, cytokines and chemokines investigated. Thus concentration-dependent treatment of neonatal brain microglia with V. vulnificus MO6-24/O LPS resulted in a significant rise in O₂⁻ production, followed by a progressive decrease in O₂⁻ release, with concomitant release of lactic dehydrogenase (LDH), and generation of TXB2, MMP-9, cytokines and chemokines. We hypothesize that the inflammatory mediators investigated may be cytotoxic to microglia in vitro, by an as yet undetermined autocrine mechanism. Although V. vulnificus LPS was less potent than E. coli LPS in vitro, inflammatory mediator release by the former was clearly more efficacious. Finally, we hypothesize that should V. vulnificus LPS gain entry into the CNS, it would be possible that microglia might become activated, resulting in high levels of O₂⁻ as well as neuroinflammatory TXB2, MMP-9, cytokines and chemokines.

An assay to screen bacterial adhesion to mucus biomolecules

AIMS

To develop an assay for rapid screening of bacterial adhesion to various groups of biomolecules present in fish mucus.

METHODS AND RESULTS

A novel assay was developed for investigation of bacterial adhesion to various groups of mucus biomolecules from fish. Lipid-, protein-, carbohydrate- and nucleic acid-rich constituents of mucus were separated using isopycnic density gradient centrifugation techniques. Separated mucus fractions were assayed for bacterial adhesion using a blotting apparatus. The assay was validated using Vibrio vulnificus and skin mucus from hybrid tilapia.

CONCLUSIONS

A novel assay was developed for the screening of bacterial adhesion to major groups of mucus biomolecules. Adhesion of V. vulnificus MLT403 positively correlated with lipid- and protein-rich mucus constituents and negatively correlated with carbohydrate-rich mucus constituents.

SIGNIFICANCE AND IMPACT OF THE STUDY

The assay can be used as an initial approach in a systematic identification of mucus constituent(s) exhibiting the most favourable adhesion properties for bacteria.

Quantitative PCR and in vivo efficacy of antibiotics in the treatment of Vibrio vulnificus infection in a mouse model

The Centers for Disease Control and Prevention (CDC) recommend oral or intravenous doxycycline plus a third-generation cephalosporin or fluoroquinolone alone for the treatment of Vibrio vulnificus infections. Until now, no study has compared oral with parenteral administered doxycycline with respect to their in vivo efficacy. In the present work, ICR mice infected with a high dose of V. vulnificus were administered ciprofloxacin, ceftriaxone, and doxycycline. The bacterial DNA copy number in surviving and non-surviving mice was determined using quantitative polymerase chain reaction (qPCR). In this setting, ciprofloxacin was the most effective monotherapeutic drug, but a higher survival rate (50%) was achieved using the combination therapy of intraperitoneal doxycycline plus ceftriaxone. The blood of non-surviving mice at 12 h post-infection contained at least 10(4) DNA copies/μL, in contrast to 10(2) to 10(3) DNA copies/μL in surviving mice. Thus, in the treatment of V. vulnificus infections in humans, when the intravenous form of doxycycline is unavailable, ciprofloxacin might be a better option than oral doxycycline to lower mortality. In addition, our results demonstrate that qPCR can be a useful tool for identifying the V. vulnificus load in infected patients, with the DNA copy number providing a marker of either disease severity or mortality.

Pyrosequencing-based comparative genome analysis of Vibrio vulnificus environmental isolates

Between 1996 and 2006, the US Centers for Disease Control reported that the only category of food-borne infections increasing in frequency were those caused by members of the genus Vibrio. The Gram-negative bacterium Vibrio vulnificus is a ubiquitous inhabitant of estuarine waters, and is the number one cause of seafood-related deaths in the US. Many V. vulnificus isolates have been studied, and it has been shown that two genetically distinct subtypes, distinguished by 16S rDNA and other gene polymorphisms, are associated predominantly with either environmental or clinical isolation. While local genetic differences between the subtypes have been probed, only the genomes of clinical isolates have so far been completely sequenced. In order to better understand V. vulnificus as an agent of disease and to identify the molecular components of its virulence mechanisms, we have completed whole genome shotgun sequencing of three diverse environmental genotypes using a pyrosequencing approach. V. vulnificus strain JY1305 was sequenced to a depth of 33×, and strains E64MW and JY1701 were sequenced to lesser depth, covering approximately 99.9% of each genome. We have performed a comparative analysis of these sequences against the previously published sequences of three V. vulnificus clinical isolates. We find that the genome of V. vulnificus is dynamic, with 1.27% of genes in the C-genotype genomes not found in the E- genotype genomes. We identified key genes that differentiate between the genomes of the clinical and environmental genotypes. 167 genes were found to be specifically associated with environmental genotypes and 278 genes with clinical genotypes. Genes specific to the clinical strains include components of sialic acid catabolism, mannitol fermentation, and a component of a Type IV secretory pathway VirB4, as well as several other genes with potential significance for human virulence. Genes specific to environmental strains included several that may have implications for the balance between self-preservation under stress and nutritional competence.

A conserved GTPase YchF of Vibrio vulnificus is involved in macrophage cytotoxicity, iron acquisition, and mouse virulence

Vibrio vulnificus, a highly virulent marine bacterium, causes serious wound infections and fatal septicemia in many areas of the world. To identify V. vulnificus genes required for killing macrophages, we made an insertional mutant library of V. vulnificus and screened it for reduced macrophage cytotoxicity. One mutant defective in macrophage cytotoxicity had an insertion in ychF, a gene encoding a putative GTPase. In addition to reduced cytotoxicity, this mutant had attenuated growth in iron-limited medium and reduced virulence in iron-overloaded mice. The ychF mutation also down-regulated the transcription level of the rtxA1 gene. RtxA1 mutants significantly decreased cytotoxicity to macrophages compared to wild-type bacteria. Overall, these results show that YchF elicits macrophage cytotoxicity through an rtxA1 pathway and is important for mouse virulence.

Genotype is correlated with but does not predict virulence of Vibrio vulnificus biotype 1 in subcutaneously inoculated, iron dextran-treated mice

Vibrio vulnificus is the leading cause of reported deaths from infections related to consumption of seafood in the United States. Affected predisposed individuals frequently die rapidly from sepsis. Otherwise healthy people can experience severe wound infection, which can lead to sepsis and death. A question is why, with so many people consuming contaminated raw oysters, the incidence of severe V. vulnificus disease is low. Molecular typing systems have shown associations of V. vulnificus genotypes and the environmental or clinical source of the strains, suggesting that different genotypes possess different virulence potentials. We examined 69 V. vulnificus biotype 1 strains that were genotyped by several methods and evaluated them for virulence in a subcutaneously inoculated iron dextran-treated mouse model. By examining the relationships between skin infection, systemic liver infection, and presumptive death (a decrease in body temperature), we determined that liver infection is predicated on severe skin infection and that death requires significant liver infection. Although most strains caused severe skin infection, not every strain caused systemic infection and death. Strains with polymorphisms at multiple loci (rrn, vcg, housekeeping genes, and repetitive DNA) designated profile 2 were more likely to cause lethal systemic infection with more severe indicators of virulence than were profile 1 strains with different polymorphisms at these loci. However, some profile 1 strains were lethal and some profile 2 strains did not cause systemic infection. Therefore, current genotyping schemes cannot strictly predict the virulence of V. vulnificus strains and further investigation is needed to identify virulence genes as markers of virulence.

Vibrio vulnificus DNA load and mortality

We determined the association between DNA load and mortality in patients with Vibrio vulnificus infection. Real-time PCR performed on sera of 27 culture-positive patients showed a significantly higher median DNA load in nonsurvivors than in survivors. Hence, real-time PCR can be used as an early prognostic factor in V. vulnificus septicemia.

SOLiD sequencing of four Vibrio vulnificus genomes enables comparative genomic analysis and identification of candidate clade-specific virulence genes

BACKGROUND

Vibrio vulnificus is the leading cause of reported death from consumption of seafood in the United States. Despite several decades of research on molecular pathogenesis, much remains to be learned about the mechanisms of virulence of this opportunistic bacterial pathogen. The two complete and annotated genomic DNA sequences of V. vulnificus belong to strains of clade 2, which is the predominant clade among clinical strains. Clade 2 strains generally possess higher virulence potential in animal models of disease compared with clade 1, which predominates among environmental strains. SOLiD sequencing of four V. vulnificus strains representing different clades (1 and 2) and biotypes (1 and 2) was used for comparative genomic analysis.

RESULTS

Greater than 4,100,000 bases were sequenced of each strain, yielding approximately 100-fold coverage for each of the four genomes. Although the read lengths of SOLiD genomic sequencing were only 35 nt, we were able to make significant conclusions about the unique and shared sequences among the genomes, including identification of single nucleotide polymorphisms. Comparative analysis of the newly sequenced genomes to the existing reference genomes enabled the identification of 3,459 core V. vulnificus genes shared among all six strains and 80 clade 2-specific genes. We identified 523,161 SNPs among the six genomes.

CONCLUSIONS

We were able to glean much information about the genomic content of each strain using next generation sequencing. Flp pili, GGDEF proteins, and genomic island XII were identified as possible virulence factors because of their presence in virulent sequenced strains. Genomic comparisons also point toward the involvement of sialic acid catabolism in pathogenesis.

Biosafety level 2 model of pneumonic plague and protection studies with F1 and Psa

Attenuated Yersinia pestis pgm strains, such as KIM5, lack the siderophore yersiniabactin. Strain KIM5 does not induce significant pneumonia when delivered intranasally. In this study, mice were found to develop pneumonia after intranasal challenge with strain KIM5 when they were injected intraperitoneally with iron dextran, though not with iron sulfate. KIM5-infected mice treated daily with 4 mg iron dextran died in 3 days with severe pneumonia. Pneumonia was less severe if 4 mg iron dextran was administered only once before infection. The best-studied experimental vaccine against plague currently consists of the Yersinia pestis capsular antigen F1 and the type 3 secreted protein LcrV. The F1 antigen was shown to be protective against KIM5 infections in mice administered iron dextran doses leading to light or severe pneumonia, supporting the use of an iron dextran-treated model of pneumonic plague. Since F1 has been reported to be incompletely protective in some primates, and bacterial isolates lacking F1 are still virulent, there has been considerable interest in identifying additional protective subunit immunogens. Here we showed that the highly conserved Psa fimbriae of Y. pestis (also called pH 6 antigen) are expressed in murine organs after infection through the respiratory tract. Studies with iron dextran-treated mice showed that vaccination with the Psa fimbrial protein together with an adjuvant afforded incomplete but significant protection in the mouse model described. Therefore, further investigations to fully characterize the protective properties of the Psa fimbriae are warranted.

Genetic characterization of Vibrio vulnificus strains from tilapia aquaculture in Bangladesh

Outbreaks of Vibrio vulnificus wound infections in Israel were previously attributed to tilapia aquaculture. In this study, V. vulnificus was frequently isolated from coastal but not freshwater aquaculture in Bangladesh. Phylogenetic analyses showed that strains from Bangladesh differed remarkably from isolates commonly recovered elsewhere from fish or oysters and were more closely related to strains of clinical origin.

Identification of a c-di-GMP-regulated polysaccharide locus governing stress resistance and biofilm and rugose colony formation in Vibrio vulnificus

As an etiological agent of bacterial sepsis and wound infections, Vibrio vulnificus is unique among the Vibrionaceae. Its continued environmental persistence and transmission are bolstered by its ability to colonize shellfish, form biofilms on various marine biotic surfaces, and generate a morphologically and physiologically distinct rugose (R) variant that yields profuse biofilms. Here, we identify a c-di-GMP-regulated locus (brp, for biofilm and rugose polysaccharide) and two transcription factors (BrpR and BrpT) that regulate these physiological responses. Disruption of glycosyltransferases within the locus or either regulator abated the inducing effect of c-di-GMP on biofilm formation, rugosity, and stress resistance. The same lesions, or depletion of intracellular c-di-GMP levels, abrogated these phenotypes in the R variant. The parental and brp mutant strains formed only scant monolayers on glass surfaces and oyster shells, and although the R variant formed expansive biofilms, these were of limited depth. Dramatic vertical expansion of the biofilm structure was observed in the parental strain and R variant, but not the brp mutants, when intracellular c-di-GMP levels were elevated. Hence, the brp-encoded polysaccharide is important for surface colonization and stress resistance in V. vulnificus, and its expression may control how the bacteria switch from a planktonic lifestyle to colonizing shellfish to invading human tissue.

Septic arthritis as the initial manifestation of fatal Vibrio vulnificus septicemia in a patient with thalassemia and iron overload

Vibrio vulnificus infection is an uncommon but potentially fatal disease in children such that prompt recognition has prognostic implications. We describe here the case of a 9-year-old female with thalassemia and iron overload who presented with septic arthritis as an atypical initial manifestation of fatal V. vulnificus septicemia. This report underscores the possibility of septic arthritis as an early manifestation of V. vulnificus septicemia. Pediatricians should be alert to this extremely invasive disease, especially in children with iron overload.

Ecology of Vibrio vulnificus and Vibrio parahaemolyticus in brackish environments of the Sada River in Shimane Prefecture, Japan

While there are several studies on the ecology of Vibrio vulnificus and Vibrio parahaemolyticus in estuarine water environments around the world, there is little information on the distribution of both organisms during the cold-weather months. Thus, we conducted a multi-year study on the ecology of both organisms in brackish environments of the Sada River, a drainage canal flowing slowly into the Japan Sea from Lake Shinji in Shimane Prefecture, Japan. Water samples were collected twice a month at five sites from August 2000 to May 2002. Both organisms were enumerated in 10 l water, 100 g sediment and 10 g shellfish by the most probable number (MPN) procedure. Isolates were confirmed as V. vulnificus using hemolysin gene PCR. During the last 7 months (including winter) of the study, water and sediment samples were also analyzed for the presence of both organisms. V. parahaemolyticus was isolated from river mouths and coastal environments of average salinity > or = 4.4+/-2.0 ppt throughout the year at cell concentrations of 10(-3) to 10(1) MPN ml(-1). Similar concentrations of V. vulnificus were isolated from coastal environments of average salinity 24.0+/-5.4 ppt, except for two times when water moved to the upper reaches due to high tide and V. vulnificus was rifted to the upper reaches. These findings suggest that both organisms are continuously distributed in the Sada estuary throughout the year regardless of water temperature.

Roles of RseB, sigmaE, and DegP in virulence and phase variation of colony morphotype of Vibrio vulnificus

Vibrio vulnificus is an estuarine bacterium capable of causing serious and often fatal wound infections and primary septicemia. We used alkaline phosphatase insertion mutagenesis to identify genes necessary for the virulence of this pathogen. One mutant had an in-frame fusion of 'phoA to the gene encoding RseB, a periplasmic negative regulator of the alternative sigma factor sigma(E). sigma(E) controls an extensive regulon involved in responding to cell envelope stresses. Colonies of the rseB mutant were less opaque than wild-type colonies and underwent phase variation between translucent and opaque morphologies. rseB mutants were attenuated for virulence in subcutaneously inoculated iron-dextran-treated mice. To obtain insight into the role of rseB and the extracytoplasmic stress response in V. vulnificus, mutants with defined mutations in rseB and two important members of the extracytoplasmic stress regulon, rpoE and degP, were constructed for analysis of virulence, colony morphology, and stress-associated phenotypes. Deletion of rseB caused reversible phase variation in the colony morphotype that was associated with extracellular polysaccharides. Translucent and transparent morphotype strains were attenuated for virulence. rpoE and degP deletion mutants were sensitive to membrane-perturbing agents and heat but were not significantly attenuated for V. vulnificus virulence in mice. These results reveal complex relationships between regulation of the extracytoplasmic stress response, exopolysaccharides, and the virulence of V. vulnificus.

USER friendly cloning coupled with chitin-based natural transformation enables rapid mutagenesis of Vibrio vulnificus

Vibrio vulnificus is a bacterial contaminant of shellfish and causes highly lethal sepsis and destructive wound infections. A definitive identification of virulence factors using the molecular version of Koch's postulates has been hindered because of difficulties in performing molecular genetic analysis of this opportunistic pathogen. For example, conjugation is required to introduce plasmid DNA, and allelic exchange suicide vectors that rely on sucrose sensitivity for counterselection are not efficient. We therefore incorporated USER friendly cloning techniques into pCVD442-based allelic exchange suicide vectors and other expression vectors to enable the rapid and efficient capture of PCR amplicons. Upstream and downstream DNA sequences flanking genes targeted for deletion were cloned together in a single step. Based on results from Vibrio cholerae, we determined that V. vulnificus becomes naturally transformable with linear DNA during growth on chitin in the form of crab shells. By combining USER friendly cloning and chitin-based transformation, we rapidly and efficiently produced targeted deletions in V. vulnificus, bypassing the need for two-step, suicide vector-mediated allelic exchange. These methods were used to examine the roles of two flagellin loci (flaCDE and flaFBA), the motAB genes, and the cheY-3 gene in motility and to create deletions of rtxC, rtxA1, and fadR. Additionally, chitin-based transformation was useful in moving antibiotic resistance-labeled mutations between V. vulnificus strains by simply coculturing the strains on crab shells. The methods and genetic tools that we developed should be of general use to those performing molecular genetic analysis and manipulation of other gram-negative bacteria.

Proteomic analysis of Vibrio vulnificus M2799 grown under iron-repleted and iron-depleted conditions

Vibrio vulnificus is an opportunistic marine bacterium that causes a serious, often fatal, infection in human. An important factor that determines the survival of V. vulnificus in the human body is the ability to acquire iron. The differential expression of proteins in whole-cell lysates of V. vulnificus M2799, a clinical isolate, was evaluated under iron-repleted and iron-depleted conditions during the early, mid and late logarithmic growth phases. A total of 32, 53 and 42 iron-regulated spots were detected by two-dimensional differential gel electrophoresis (2D-DIGE) in the early, mid and late logarithmic growth phases, respectively. Of these, 18 (early logarithmic growth phase), 31 (mid logarithmic growth phase) and 26 (late logarithmic growth phase) proteins were subsequently identified by matrix-assisted laser desorption/ionization-time of flight analysis. These proteins were classified into 10 functional categories, including inorganic ion transport and metabolism, carbohydrate transport and metabolism, and amino acid transport and metabolism. Based on this classification, the expression of proteins involved in the iron acquisition system increased from the early to the mid logarithmic growth phases, while that of proteins involved in other metabolic pathways increased from the mid to the late logarithmic growth phases. Furthermore, when the protein expression profile of the wild type bacterium was compared with that of the fur mutant grown under the iron-repleted condition, the expression of 18 proteins was found to be regulated by iron and Fur.

Regulation of fatty acid metabolism by FadR is essential for Vibrio vulnificus to cause infection of mice

The opportunistic bacterial pathogen Vibrio vulnificus causes severe wound infection and fatal septicemia. We used alkaline phosphatase insertion mutagenesis in a clinical isolate of V. vulnificus to find genes necessary for virulence, and we identified fadR, which encodes a regulator of fatty acid metabolism. The fadR::mini-Tn5Km2phoA mutant was highly attenuated in a subcutaneously inoculated iron dextran-treated mouse model of V. vulnificus disease, was hypersensitive to the fatty acid synthase inhibitor cerulenin, showed aberrant expression of fatty acid biosynthetic (fab) genes and fatty acid oxidative (fad) genes, produced smaller colonies on agar media, and grew slower in rich broth than did the wild-type parent. Deletion of fadR essentially recapitulated the phenotypes of the insertion mutant, and the DeltafadR mutation was complemented in trans with the wild-type gene. Further characterization of the DeltafadR mutant showed that it was not generally hypersensitive to envelope stresses but had decreased motility and showed an altered membrane lipid profile compared to that of the wild type. Supplementation of broth with the unsaturated fatty acid oleate restored wild-type growth in vitro, and infection with oleate in the inoculum increased the ability of the DeltafadR mutant to infect mice. We conclude that fadR and regulation of fatty acid metabolism are essential for V. vulnificus to be able to cause disease in mammalian hosts.

Global gene expression as a function of the iron status of the bacterial cell: influence of differentially expressed genes in the virulence of the human pathogen Vibrio vulnificus

Vibrio vulnificus multiplies rapidly in host tissues under iron-overloaded conditions. To understand the effects of iron in the physiology of this pathogen, we performed a genome-wide transcriptional analysis of V. vulnificus growing at three different iron concentrations, i.e., iron-limiting [Trypticase soy broth with 1.5% NaCl (TSBS) plus ethylenediamine-di-(o-hydroxyphenylacetic) acid (EDDA)], low-iron (1 microg Fe/ml; TSBS), and iron-rich (38 microg Fe/ml; TSBS plus ferric ammonium citrate) concentrations. A few genes were upregulated under the last two conditions, while several genes were expressed differentially under only one of them. A gene upregulated under both conditions encodes the outer membrane porin, OmpH, while others are related to the biosynthesis of amino sugars. An ompH mutant showed sensitivity to sodium dodecyl sulfate (SDS) and polymyxin B and also had a reduced competitive index compared with the wild type in the iron-overloaded mice. Under iron-limiting conditions, two of the TonB systems involved in vulnibactin transport were induced. These genes were essential for virulence in the iron-overloaded mice inoculated subcutaneously, underscoring the importance of active iron transport in infection, even under the high-iron conditions of this animal model. Furthermore, we demonstrated that a RyhB homologue is also essential for virulence in the iron-overloaded mouse. This novel information on the role of genes induced under iron limitation in the iron-overloaded mouse model and the finding of new genes with putative roles in virulence that are expressed only under iron-rich conditions shed light on the many strategies used by this pathogen to multiply rapidly in the susceptible host.

Nonfoodborne Vibrio infections: an important cause of morbidity and mortality in the United States, 1997-2006

BACKGROUND

Infections due to Vibrio species cause an estimated 8000 illnesses annually, often through consumption of undercooked seafood. Like foodborne Vibrio infections, nonfoodborne Vibrio infections (NFVI) also result in serious illness, but awareness of these infections is limited.

METHODS

We analyzed illnesses occuring during the period 1997-2006 that were reported to the Centers for Disease Control and Prevention's Cholera and Other Vibrio Illness Surveillance system. The diagnosis of NFVI required isolation of Vibrio species from a patient with contact with seawater.

RESULTS

Of 4754 Vibrio infections reported, 1210 (25%) were NFVIs. Vibrio vulnificus infections were the most common (accounting for 35% of NFVIs), with 72% of V. vulnificus infections reported from residents of Gulf Coast states. Infections due to V. vulnificus resulted in fever (72% of cases), cellulitis (85%), amputation (10%), and death (17%). V. vulnificus caused 62 NFVI-associated deaths (78%). Recreational activities accounted for 70% of exposures for patients with NFVIs associated with all species. Patients with liver disease were significantly more likely to die as a result of infection (odds ratio, 7.8; 95% confidence interval, 2.8-21.9). Regardless of pre-existing conditions, patients were more likely to die when hospitalization occurred >2 days after symptom onset (odds ratio, 2.9; 95% confidence interval, 1.8-4.8).

CONCLUSION

NFVIs, especially those due to V. vulnificus, demonstrate high morbidity and mortality. Persons with liver disease should be advised of the risks associated with seawater exposure if a wound is already present or is likely to occur. Clinicians should consider Vibrio species as an etiologic agent in infections occurring in persons with recent seawater exposure, even if the individual was only exposed during recreational marine activities. Immediate antibiotic treatment with aggressive monitoring is advised in suspected cases.

Cyclic-di-GMP regulates extracellular polysaccharide production, biofilm formation, and rugose colony development by Vibrio vulnificus

Vibrio vulnificus is a human and animal pathogen that carries the highest death rate of any food-borne disease agent. It colonizes shellfish and forms biofilms on the surfaces of plankton, algae, fish, and eels. Greater understanding of biofilm formation by the organism could provide insight into approaches to decrease its load in filter feeders and on biotic surfaces and control the occurrence of invasive disease. The capsular polysaccharide (CPS), although essential for virulence, is not required for biofilm formation under the conditions used here. In other bacteria, increased biofilm formation often correlates with increased exopolysaccharide (EPS) production. We exploited the translucent phenotype of acapsular mutants to screen a V. vulnificus genomic library and identify genes that imparted an opaque phenotype to both CPS biosynthesis and transport mutants. One of these encoded a diguanylate cyclase (DGC), an enzyme that synthesizes bis-(3'-5')-cyclic-di-GMP (c-di-GMP). This prompted us to use this DGC, DcpA, to examine the effect of elevated c-di-GMP levels on several developmental pathways in V. vulnificus. Increased c-di-GMP levels induced the production of an EPS that was distinct from the CPS and dramatically enhanced biofilm formation and rugosity in a CPS-independent manner. However, the EPS could not compensate for the loss of CPS production that is required for virulence. In contrast to V. cholerae, motility and virulence appeared unaffected by elevated levels of c-di-GMP.

A widespread deferoxamine-mediated iron-uptake system in Vibrio vulnificus

Vibrio vulnificus can use the standard iron chelator deferoxamine (Desferal) for efficient iron-uptake via the specific receptor DesA, which is encoded by desA. We investigated the ubiquity of the deferoxamine-mediated iron-uptake system in V. vulnificus strains and the potential risk of the system. By polymerase chain reaction (PCR), desA was found in 10 of 10 clinical strains and in 9 of 10 environmental strains, and their growth was stimulated by deferoxamine. By reverse-transcriptase PCR, desA was expressed only under iron-limited conditions containing deferoxamine. V. vulnificus growth in the presence of deferoxamine was suppressed by desA mutation, and the suppressed growth was recovered by desA complementation. Deferoxamine stimulated V. vulnificus growth in iron-limited in vitro and ex vivo backgrounds containing transferrin-bound iron. Overall, V. vulnificus can use transferrin-bound iron via the widespread deferoxamine-mediated iron-uptake system; this cautions that deferoxamine therapy in patients with iron overload may increase the risk of fatal infections caused by V. vulnificus.

Role of iron in human serum resistance of the clinical and environmental Vibrio vulnificus genotypes

We recently reported a simple PCR procedure that targets a sequence variation of the virulence-correlated gene locus vcg. It was found that 90% of all clinical isolates possessed the vcgC sequence variant, while 93% of all environmental isolates possessed the vcgE sequence variant. Here we report that the clinical genotype of Vibrio vulnificus is significantly better able to survive in human serum than is the environmental genotype. The presence of a siderophore-encoding gene, viuB, influenced serum survivability among all isolates of V. vulnificus tested. Those strains positive for viuB (all C-type strains but very few E-type strains) showed greater serum survivability than those lacking viuB (most E-type strains). The addition of iron (in the form of ferric ammonium citrate) to human serum restored the survival of E-type strains lacking viuB to levels not significantly different from those of C-type and E-type strains that possess viuB. These findings suggest that viuB may dictate serum survival in both C- and E-type strains of V. vulnificus and may explain why some strains (C- and E-type strains) are pathogenic and others (predominately E-type strains) are not. Additionally, C-type strains exhibited a cross-protective response against human serum, not exhibited by E-type strains, after incubation under nutrient and osmotic downshift conditions that mimicked estuarine waters. This suggests that the nutrient/osmotic environment may influence the survival of V. vulnificus following entry into the human body, leading to selection of the C genotype over the E genotype.

Emergence of a virulent clade of Vibrio vulnificus and correlation with the presence of a 33-kilobase genomic island

Vibrio vulnificus is a ubiquitous inhabitant of the marine coastal environment, and an important pathogen of humans. We characterized a globally distributed sample of environmental isolates from a range of habitats and hosts and compared these with isolates recovered from cases of human infection. Multilocus sequence typing data using six housekeeping genes divided 63 of the 67 isolates into the two main lineages previously noted for this species, and this division was also confirmed using the 16S rRNA and open reading frame VV0401 markers. Lineage I was comprised exclusively of biotype 1 isolates, whereas lineage II contained biotype 1 and all biotype 2 isolates. Four isolates did not cluster within either lineage: two biotype 3 and two biotype 1 isolates. The proportion of isolates recovered from a clinical setting was noted to be higher in lineage I than in lineage II. Lineage I isolates were also associated with a 33-kb genomic island (region XII), one of three regions identified by genome comparisons as unique to the species. Region XII contained an arylsulfatase gene cluster, a sulfate reduction system, two chondroitinase genes, and an oligopeptide ABC transport system, all of which are absent from the majority of lineage II isolates. Arylsulfatases and the sulfate reduction system, along with performing a scavenging role, have been hypothesized to play a role in pathogenic processes in other bacteria. Our data suggest that lineage I may have a higher pathogenic potential and that region XII, along with other regions, may give isolates a selective advantage either in the human host or in the aquatic environment or both.

Vibrio vulnificus damages macrophages during the early phase of infection

Vibrio vulnificus is an estuarine bacterium that can cause primary septicemia as well as serious wound infections. Generally, clinical isolates have a high lethal effect compared with environmental isolates. However, little is known about the mechanisms by which V. vulnificus causes disease. In this study, we compared the pathogenicity of a clinical isolate, strain M2799, with that of an environmental isolate, strain JCM3731. The clinical isolate showed 100 times higher lethality in mice than the environmental isolate. In strain M2799-inoculated mice, the number of macrophages decreased significantly, whereas there was no appreciable change in the number of macrophages in strain JCM3731-inoculated mice. The clinical isolate showed high cytotoxic activity, especially to macrophages, compared with the environmental isolate in vitro. The growth of the clinical isolate was almost completely inhibited in the presence of macrophages. Moreover, the survival rate of the clinical isolate-inoculated mice increased by recruitment of macrophages. These results indicate that V. vulnificus infection progresses by damage to macrophages during the early phase of infection.

The HlyU protein is a positive regulator of rtxA1, a gene responsible for cytotoxicity and virulence in the human pathogen Vibrio vulnificus

Vibrio vulnificus is an opportunistic human pathogen that preferentially infects compromised iron-overloaded patients, causing a fatal primary septicemia with very rapid progress, resulting in a high mortality rate. In this study we determined that the HlyU protein, a virulence factor in V. vulnificus CMCP6, up-regulates the expression of VV20479, a homologue of the Vibrio cholerae RTX (repeats in toxin) toxin gene that we named rtxA1. This gene is part of an operon together with two other open reading frames, VV20481 and VV20480, that encode two predicted proteins, a peptide chain release factor 1 and a hemolysin acyltransferase, respectively. A mutation in rtxA1 not only contributes to the loss of cytotoxic activity but also results in a decrease in virulence, whereas a deletion of VV20481 and VV20480 causes a slight decrease in virulence but with no effect in cytotoxicity. Activation of the expression of the rtxA1 operon by HlyU occurs at the transcription initiation level by binding of the HlyU protein to a region upstream of this operon.