A selective medium and a specific probe for detection of Vibrio vulnificus

Development of Recombinase Polymerase Amplification Combined with Lateral Flow Dipstick Assay To Detect Hemolysin Gene of Vibrio vulnificus in Oysters

ABSTRACT

Vibrio vulnificus inhabits estuarine waters around the world and can cause severe infections in people who eat contaminated raw or undercooked oysters. Although current detection methods are sensitive and specific, there are continuous demands for the development of rapid and accurate methods without a trained operator and equipment in the field conditions. Herein, we developed a simple and rapid method by detecting the hemolysin (vvh) gene of V. vulnificus by using recombinase polymerase amplification (RPA) combined with a lateral flow dipstick (LFD). The RPA-LFD could detect 100 fg of DNA (P < 0.05) and 20 CFU of V. vulnificus per reaction within 30 min (P < 0.01) and showed the result with incubation temperature ranges from 30 to 45°C (P < 0.001). The test was specific only to V. vulnificus and was not responsive to 10 other closely related Vibrio species and 18 foodborne pathogenic bacteria. Compared with PCR, quantitative PCR, and colony hybridization assays by using naturally contaminated oyster samples, our RPA-LFD showed the same detection ability as quantitative PCR assay. Therefore, the current RPA-LFD would be a valuable tool to detect V. vulnificus in oysters, especially in field conditions.

HIGHLIGHTS

A multiplex PCR for the detection of Vibrio vulnificus hazardous to human and/or animal health from seafood

Vibrio vulnificus is a zoonotic pathogen linked to aquaculture that is spreading due to climate change. The pathogen can be transmitted to humans and animals by ingestion of raw shellfish or seafood feed, respectively. The aim of this work was to design and test a new procedure to detect V. vulnificus hazardous to human and/or animal health in food/feed samples. For this purpose, we combined a pre-enrichment step with multiplex PCR using primers for the species and for human and animal virulence markers. In vitro assays with mixed DNA from different Vibrio species and Vibrio cultures showed that the new protocol was 100 % specific with a detection limit of 10 cfu/mL. The protocol was successfully validated in seafood using artificially contaminated live shrimp and proved useful also in pathogen isolation from animals and their ecosystem. In conclusion, this novel protocol could be applied in health risk studies associated with food/feed consumption, as well as in the routine identification and subtyping of V. vulnificus from environmental or clinical samples.

Wild eel microbiome reveals that skin mucus of fish could be a natural niche for aquatic mucosal pathogen evolution

BACKGROUND

Fish skin mucosal surfaces (SMS) are quite similar in composition and function to some mammalian MS and, in consequence, could constitute an adequate niche for the evolution of mucosal aquatic pathogens in natural environments. We aimed to test this hypothesis by searching for metagenomic and genomic evidences in the SMS-microbiome of a model fish species (Anguilla Anguilla or eel), from different ecosystems (four natural environments of different water salinity and one eel farm) as well as the water microbiome (W-microbiome) surrounding the host.

RESULTS

Remarkably, potentially pathogenic Vibrio monopolized wild eel SMS-microbiome from natural ecosystems, Vibrio anguillarum/Vibrio vulnificus and Vibrio cholerae/Vibrio metoecus being the most abundant ones in SMS from estuary and lake, respectively. Functions encoded in the SMS-microbiome differed significantly from those in the W-microbiome and allowed us to predict that successful mucus colonizers should have specific genes for (i) attachment (mainly by forming biofilms), (ii) bacterial competence and communication, and (iii) resistance to mucosal innate immunity, predators (amoeba), and heavy metals/drugs. In addition, we found several mobile genetic elements (mainly integrative conjugative elements) as well as a series of evidences suggesting that bacteria exchange DNA in SMS. Further, we isolated and sequenced a V. metoecus strain from SMS. This isolate shares pathogenicity islands with V. cholerae O1 from intestinal infections that are absent in the rest of sequenced V. metoecus strains, all of them from water and extra-intestinal infections.

CONCLUSIONS

We have obtained metagenomic and genomic evidence in favor of the hypothesis on the role of fish mucosal surfaces as a specialized habitat selecting microbes capable of colonizing and persisting on other comparable mucosal surfaces, e.g., the human intestine.

Role of extracellular matrix protein CabA in resistance of Vibrio vulnificus biofilms to decontamination strategies

Biofilms are recalcitrant and raise safety problems in the food industry. In this study, the role of CabA, an extracellular matrix protein, in the resistance of the biofilms of Vibrio vulnificus, a foodborne pathogen, to decontamination strategies was investigated. Biofilms of the cabA mutant revealed reduced resistance to detachment by vibration and disinfection by sodium hypochlorite compared to the biofilms of the parental wild type in vitro. The reduced resistance of the cabA mutant biofilms was complemented by introducing a recombinant cabA, indicating that the reduced resistance of the cabA mutant biofilms is caused by the inactivation of cabA. The expression of cabA was induced in cells bound to oyster, the primary vehicle of the pathogen. The cabA mutant biofilms on oyster are defective in biomass and resistance to detachment and disinfection. The bacterial cells in the wild-type biofilms are clustered by filaments which are not apparent in the cabA mutant biofilms. The combined results indicated that CabA contributes to the structural integrity of V. vulnificus biofilms possibly by forming filaments in the matrix and thus rendering the biofilms robust, suggesting that CabA could be a target to control V. vulnificus biofilms on oyster.

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

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

VibrioBase: A MALDI-TOF MS database for fast identification of Vibrio spp. that are potentially pathogenic in humans

Mesophilic marine bacteria of the family Vibrionaceae, specifically V. cholerae, V. parahaemolyticus and V. vulnificus, are considered to cause severe illness in humans. Due to climate-change-driven temperature increases, higher Vibrio abundances and infections are predicted for Northern Europe, which in turn necessitates environmental surveillance programs to evaluate this risk. We propose that whole-cell matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) profiling is a promising tool for the fast and reliable species classification of environmental isolates. Because the reference database does not contain sufficient Vibrio spectra we generated the VibrioBase database in this study. Mass spectrometric data were generated from 997 largely environmental strains and filed in this new database. MALDI-TOF MS clusters were assigned based on the species classification obtained by analysis of partial rpoB (RNA polymerase beta-subunit) sequences. The affiliation of strains to species-specific clusters was consistent in 97% of all cases using both approaches, and the extended VibrioBase generated more specific species identifications with higher matching scores compared to the commercially available database. Therefore, we have made the VibrioBase database freely accessible, which paves the way for detailed risk assessment studies of potentially pathogenic Vibrio spp. from marine environments.

IscR is a global regulator essential for pathogenesis of Vibrio vulnificus and induced by host cells

A mutant that exhibited less cytotoxic activity toward INT-407 human intestinal epithelial cells than the wild type was screened from a random transposon mutant library of Vibrio vulnificus, and an open reading frame encoding an Fe-S cluster regulator, IscR, was identified using a transposon-tagging method. A mutational analysis demonstrated that IscR contributes to mouse mortality as well as cytotoxicity toward the INT-407 cells, indicating that IscR is essential for the pathogenesis of V. vulnificus. A whole-genome microarray analysis revealed that IscR influenced the expression of 67 genes, of which 52 were upregulated and 15 were downregulated. Among these, 12 genes most likely involved in motility and adhesion to host cells, hemolytic activity, and survival under oxidative stress of the pathogen during infection were selected and experimentally verified to be upregulated by IscR. Accordingly, the disruption of iscR resulted in a significant reduction in motility and adhesion to INT-407 cells, in hemolytic activity, and in resistance to reactive oxygen species (ROS) such as H2O2 and tert-butyl hydroperoxide (t-BOOH). Furthermore, the present study demonstrated that iscR expression was induced by exposure of V. vulnificus to the INT-407 cells, and the induction appeared to be mediated by ROS generated by the host cells during infection. Consequently, the combined results indicated that IscR is a global regulator that contributes to the overall success in the pathogenesis of V. vulnificus by regulating the expression of various virulence and survival genes in addition to Fe-S cluster genes.

Vibrio vulnificus bacteriophage SSP002 as a possible biocontrol agent

A novel Vibrio vulnificus-infecting bacteriophage, SSP002, belonging to the Siphoviridae family, was isolated from the coastal area of the Yellow Sea of South Korea. Host range analysis revealed that the growth inhibition of phage SSP002 is relatively specific to V. vulnificus strains from both clinical and environmental samples. In addition, a one-step growth curve analysis and a bacteriophage stability test revealed a latent period of 65 min, a burst size of 23 ± 2 PFU, as well as broad temperature (20°C to 60°C) and pH stability (pH 3 to 12) ranges. A Tn5 random transposon mutation of V. vulnificus and partial DNA sequencing of the inserted Tn5 regions revealed that the flhA, flhB, fliF, and fleQ mutants are resistant to SSP002 phage infection, suggesting that the flagellum may be the host receptor for infection. The subsequent construction of specific gene-inactivated mutants (flhA, flhB, fliF, and fleQ) and complementation experiments substantiated this. Previously, the genome of phage SSP002 was completely sequenced and analyzed. Comparative genomic analysis of phage SSP002 and Vibrio parahaemolyticus phage vB_VpaS_MAR10 showed differences among their tail-related genes, supporting different host ranges at the species level, even though their genome sequences are highly similar. An additional mouse survival test showed that the administration of phage SSP002 at a multiplicity of infection of 1,000 significantly protects mice from infection by V. vulnificus for up to 2 months, suggesting that this phage may be a good candidate for the development of biocontrol agents against V. vulnificus infection.

A novel agar formulation for isolation and direct enumeration of Vibrio vulnificus from oyster tissue

A new selective and differential medium, Vibrio vulnificus X-Gal (VVX), was developed for direct enumeration of V. vulnificus (Vv) from oyster samples. This agar utilizes cellobiose and lactose as carbon sources, and the antibiotics colistin and polymyxin B as selective agents. Hydrolysis of 5-bromo-4-chloro-3-indolyl- beta-d-galactopyranoside (x-gal), used in the agar as a lactose analog, produces an insoluble blue dye that makes lactose positive colonies easily distinguishable from any non-lactose fermenting bacteria. Various bacterial species were spot plated onto thiosulfate-citrate-bile salts-sucrose agar (TCBS), and CHROMagar Vibrio, two vibrio-specific selective agars, non-selective agar, and VVX to compare selectivity of VVX to other widely used media. A V. vulnificus pure culture was serially diluted on VVX and non-selective agar to determine the VVX percent recovery. Water and oyster samples were spread plated on VVX agar and allowed to incubate for 16-18 h at 33 °C. Blue and white colonies from VVX agar were picked and screened by end point PCR for the Vv hemolysin vvhA. VVX agar showed a significant improvement over TCBS and CHROMagar at preventing non-target growth. There was an 87.5% recovery compared to non-selective plating and a 98% positivity rate of blue colonies picked from oyster tissue plating. The findings suggest that this new agar is a fast, distinctive, and accurate method for enumeration of V. vulnificus from the environment.

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.

Enzymatic substrates in microbiology

Enzymatic substrates are powerful tools in biochemistry. They are widely used in microbiology to study metabolic pathways, to monitor metabolism and to detect, enumerate and identify microorganisms. Synthetic enzymatic substrates have been customized for various microbial assays, to detect an expanding range of both new enzymatic activities and target microorganisms. Recent developments in synthetic enzymatic substrates with new spectral, chemical and biochemical properties allow improved detection, enumeration and identification of food-borne microorganisms, clinical pathogens and multi-resistant bacteria in various sample types. In the past 20 years, the range of synthetic enzymatic substrates used in microbiology has been markedly extended supporting the development of new multi-test systems (e.g., Microscan, Vitek 2, Phoenix) and chromogenic culture media. The use of such substrates enables an improvement in time to detection and specificity over conventional tests that employ natural substrates. In the era of intense developments in molecular biology, phenotypic tests involving enzymatic substrates remain useful to analyse both simple and complex samples. Such tests are applicable to diagnostic and research laboratories all over the world.

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.

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.

Identification of a cellobiose utilization gene cluster with cryptic beta-galactosidase activity in Vibrio fischeri

Cellobiose utilization is a variable trait that is often used to differentiate members of the family Vibrionaceae. We investigated how Vibrio fischeri ES114 utilizes cellobiose and found a cluster of genes required for growth on this beta-1,4-linked glucose disaccharide. This cluster includes genes annotated as a phosphotransferase system II (celA, celB, and celC), a glucokinase (celK), and a glucosidase (celG). Directly downstream of celCBGKA is celI, which encodes a LacI family regulator that represses cel transcription in the absence of cellobiose. When the celCBGKAI gene cluster was transferred to cellobiose-negative strains of Vibrio and Photobacterium, the cluster conferred the ability to utilize cellobiose. Genomic analyses of naturally cellobiose-positive Vibrio species revealed that V. salmonicida has a homolog of the celCBGKAI cluster, but V. vulnificus does not. Moreover, bioinformatic analyses revealed that CelG and CelK share the greatest homology with glucosidases and glucokinases in the phylum Firmicutes. These observations suggest that distinct genes for cellobiose utilization have been acquired by different lineages within the family Vibrionaceae. In addition, the loss of the celI regulator, but not the structural genes, attenuated the ability of V. fischeri to compete for colonization of its natural host, Euprymna scolopes, suggesting that repression of the cel gene cluster is important in this symbiosis. Finally, we show that the V. fischeri cellobioase (CelG) preferentially cleaves beta-d-glucose linkages but also cleaves beta-d-galactose-linked substrates such as 5-bromo-4-chloro-3-indolyl-beta-d-galactoside (X-gal), a finding that has important implications for the use of lacZ as a marker or reporter gene in V. fischeri.

Refined medium for direct isolation of Vibrio vulnificus from oyster tissue and seawater

We have developed a new medium for the direct isolation of Vibrio vulnificus from water and oyster samples. The medium was shown in laboratory and field studies to be highly selective without providing preferential isolation of either V. vulnificus genotype.

Multiplex PCR assay for detection of Vibrio vulnificus biotype 2 and simultaneous discrimination of serovar E strains

In the present work we develop a multiplex PCR assay for the detection and identification of the fish pathogen Vibrio vulnificus biotype 2 with discriminating potential for zoonotic strains (serovar E). The PCR assay allowed the identification of two new biotype 2 serovar E human isolates from culture collections. Finally, the multiplex was successfully applied to both diagnosis and carrier detection in field samples.

Vibrio navarrensis biotype pommerensis: a new biotype of V. navarrensis isolated in the German Baltic Sea

The determination of phenotypic and genotypic traits of a group of closely related Vibrio strains from the Baltic Sea did not allow species designation. DNA-DNA hybridization and fatty acid analysis revealed them as Vibrio navarrensis. Therefore we suggest these Vibrios to represent a new biotype, named V. navarrensis biotype pommerensis.

Occurrence of drug-resistant bacteria in two European eel farms

The occurrence of strains that are resistant to oxolinic acid, oxytetracycline, sulfamethoxazole-trimethoprim, and nitrofurantoin among heterotrophic bacteria, including human and fish pathogens, in two freshwater eel farms was investigated. High levels of individual- and multiple-drug-resistant bacteria were detected, although sampling events were not correlated with clinical outbreaks and drug therapy.

Methods for isolation and confirmation of Vibrio vulnificus from oysters and environmental sources: a review

The gram-negative bacterium Vibrio vulnificus is a natural inhabitant of estuarine waters and poses a significant health threat to humans who suffer from immune disorders, liver disease, or hemochromatosis (iron overload). V. vulnificus enters human hosts via wound infections or consumption of raw shellfish (primarily oysters), and infections frequently progress to septicemia and death in susceptible individuals. Prevalence in waters and shellfish is not correlated with fecal indicator organisms; therefore, species-specific detection and enumeration of V. vulnificus in the environment has become a priority for agencies that are responsible for shellfish safety. The many selective-differential media developed for isolation of Vibrio spp., and specifically for V. vulnificus detection, are reviewed here; however, none of the media developed to date combines the sensitivity to low numbers with the specificity necessary to inhibit growth of other organisms. Therefore, immunological and molecular protocols are needed for confirmation of the identity of the organism and are discussed in detail. Methods under development that hold promise for rapid, accurate, and sensitive detection and enumeration of the organism include multiplex and real-time PCR. Developing technologies that have proven useful for detection and investigation of other pathogens such as biosensors, spectroscopy and microarrays may provide the next generation of tools for investigation of the prevalence and ecology of V. vulnificus.

Protocol for specific isolation of virulent strains of Vibrio vulnificus serovar E (biotype 2) from environmental samples

The eel pathogen Vibrio vulnificus biotype 2 comprises at least three serovars, with serovar E being the only one involved in both epizootics of eel vibriosis and sporadic cases of human infections. The virulent strains of this serovar (VSE) have only been recovered from clinical (mainly eel tissue) sources. The main objective of this work was to design and validate a new protocol for VSE-specific isolation from environmental samples. The key element of the new protocol is the broth used for the first step (saline eel serum broth [SEB]), which contains eel serum as a nutritive and selective component. This approach takes advantage of the ability of VSE cells to grow in eel serum and thus to separate themselves from the pool of competitors. The growth yield in SEB after 8 h of incubation was 1,000 times higher for VSE strains than for their putative competitors (including biotype 1 strains of the species). The selective and differential agar Vibrio vulnificus medium (VVM) was selected from five selective media for the second step because it gave the highest plating efficiency not only for the VSE group but also for other V. vulnificus groups, including biotype 3. The entire protocol was validated by field studies, with alkaline peptone water plus VVM as a control. V. vulnificus was isolated by both protocols, but serovar E was only recovered by the new method described here. All selected serovar E isolates were identified as VSE since they were virulent for both eels and iron-overloaded mice and resisted the bactericidal action of eel and iron-overloaded human sera. In conclusion, this new protocol is a suitable method for the isolation of VSE strains from environmental samples and is recommended for epidemiological studies of the pathogenic serovar E.

Biodiversity of vibrios

Vibrios are ubiquitous and abundant in the aquatic environment. A high abundance of vibrios is also detected in tissues and/or organs of various marine algae and animals, e.g., abalones, bivalves, corals, fish, shrimp, sponges, squid, and zooplankton. Vibrios harbour a wealth of diverse genomes as revealed by different genomic techniques including amplified fragment length polymorphism, multilocus sequence typing, repetetive extragenic palindrome PCR, ribotyping, and whole-genome sequencing. The 74 species of this group are distributed among four different families, i.e., Enterovibrionaceae, Photobacteriaceae, Salinivibrionaceae, and Vibrionaceae. Two new genera, i.e., Enterovibrio norvegicus and Grimontia hollisae, and 20 novel species, i.e., Enterovibrio coralii, Photobacterium eurosenbergii, V. brasiliensis, V. chagasii, V. coralliillyticus, V. crassostreae, V. fortis, V. gallicus, V. hepatarius, V. hispanicus, V. kanaloaei, V. neonatus, V. neptunius, V. pomeroyi, V. pacinii, V. rotiferianus, V. superstes, V. tasmaniensis, V. ezurae, and V. xuii, have been described in the last few years. Comparative genome analyses have already revealed a variety of genomic events, including mutations, chromosomal rearrangements, loss of genes by decay or deletion, and gene acquisitions through duplication or horizontal transfer (e.g., in the acquisition of bacteriophages, pathogenicity islands, and super-integrons), that are probably important driving forces in the evolution and speciation of vibrios. Whole-genome sequencing and comparative genomics through the application of, e.g., microarrays will facilitate the investigation of the gene repertoire at the species level. Based on such new genomic information, the taxonomy and the species concept for vibrios will be reviewed in the next years.

Determination of Vibrio scophthalmi and its phenotypic diversity in turbot larvae

The association of Vibrio scophthalmi with turbot larvae was assessed, by molecular methods with a species-specific probe, in the rearing stages of turbot (Scophthalmus maximus) larvae using a routine batch of production at a fish farm. The phenotypic diversity of this bacterial species was also studied to identify predominant phenotypes at successive stages of larval development. Vibrio scophthalmi was detected in all turbot larvae samples except in the sample from day 0 after hatching. The percentage of V. scophthalmi in the intestinal microbiota increased throughout larval development. Vibrio scophthalmi was also detected in live food (brine shrimps) and water from the tanks, but not in the sediment. All turbot larvae, 15-57 day old, showed several V. scophthalmi phenotypes, and a pattern of successive waves of phenotypes was observed during successive larval stages. This indicates that certain strains may colonize the intestine more efficiently and thus maintain their population for longer than other strains. Vibrio scophthalmi populations from turbots of different origin were very similar, suggesting that irrespective of geographical area, turbot populations share similar V. scophthalmi strains. Vibrio scophthalmi strain was not isolated from other cultured fish, only turbot larvae, at the same hatchery receiving water from the same supply.

Detection and identification of Vibrio scophthalmi in the intestinal microbiota of fish and evaluation of host specificity

AIMS

To develop a species-specific probe (VSV3) for the detection of Vibrio scophthalmi in fish intestine and to apply this probe to study the host specificity of V. scophthalmi.

METHODS AND RESULTS

A specific probe (VSV3) based on the variable region V3 of the 16S rRNA gene (rDNA) was designed. Its specificity was tested by DNA-DNA hybridization and by colony hybridization. No cross-hybridization was found. The sensitivity of the probe was tested both by DNA-DNA hybridization and by colony hybridization. The detection limit of V. scophthalmi 16S rDNA was 150 pg or 10 cfu. Vibrio scophthalmi cells were detected in experimental samples constituted by mixed cultures when present in proportions of 1 : 10 and 1 : 100. The VSV3 probe also proved to be reliable for the detection of V. scophthalmi in samples of fish intestine.

CONCLUSIONS

The VSV3 probe can be used for the detection of V. scophthalmi in colony hybridization or DNA-DNA hybridization of amplified 16S rDNA. Preliminary results indicate that V. scophthalmi may present certain host specificity for turbot.

SIGNIFICANCE AND IMPACT OF THE STUDY

The VSV3 probe provides a useful tool for ecological studies.

Shell disease syndrome in the edible crab, Cancer pagurus--isolation, characterization and pathogenicity of chitinolytic bacteria

Chitinolytic bacteria are believed to be the primary aetiological agents of shell disease syndrome in marine crustaceans. The disease principally results from the breakdown of their chitinous exoskeletons by the shell disease pathogens, but pathogenicity may also manifest internally should a breach of the carapace occur. The current study looks at the pathogenicity of a number of bacteria (predominantly from the genus Vibrio) isolated from the edible crab, Cancer pagurus. All chitinase-producing bacteria investigated were capable of growth in a minimal medium consisting of chitin powder from crab shells, but differed in their speed of growth and nature of chitinolytic activity, suggesting that they have different roles within the lesion community. Two isolates (designated I4 and I7) were chosen for studies on internal pathogenicity, which included the effect of the pathogen on crab tissues, the ability of the host to remove the bacteria from circulation and the antibacterial activity of crab blood. Initially, I4 was rapidly removed from circulation, but began to reappear in the blood after 24 h. By 100 h, 100% of crabs were moribund. The septicaemic effects of the isolate were reflected in the low levels of its killing by blood-cell lysate and serum. By contrast, I7 was only slowly removed from circulation and caused the rapid mortality of all crabs in <3 h. A large decline in the number of circulating blood cells following injection of I7 was mirrored by an accumulation of these cells in the gills. Initial experiments suggest that the death of the crabs following injection with I7 may be caused by toxic extracellular bacterial products that exert their effects on the blood cells and nervous system of the crabs.

Transmission to eels, portals of entry, and putative reservoirs of Vibrio vulnificus serovar E (biotype 2)

Vibrio vulnificus serovar E (formerly biotype 2) is the etiologic agent that is responsible for the main infectious disease affecting farmed eels. Although the pathogen can theoretically use water as a vehicle for disease transmission, it has not been isolated from tank water during epizootics to date. In this work, the mode of transmission of the disease to healthy eels, the portals of entry of the pathogen into fish, and their putative reservoirs have been investigated by means of laboratory and field experiments. Results of the experiments of direct and indirect host-to-host transmission, patch contact challenges, and oral-anal intubations suggest that water is the prime vehicle for disease transmission and that gills are the main portals of entry into the eel body. The pathogen mixed with food can also come into the fish through the gastrointestinal tract and develop the disease. These conclusions were supported by field data obtained during a natural outbreak in which we were able to isolate this microorganism from tank water for the first time. The examination of some survivors from experimental infections by indirect immunofluorescence and scanning electron microscopy showed that V. vulnificus serovar E formed a biofilm-like structure on the eel skin surface. In vitro assays demonstrated that the ability of the pathogen to colonize both hydrophilic and hydrophobic surfaces was inhibited by glucose. The capacity to form biofilms on eel surface could constitute a strategy for surviving between epizootics or outbreaks, and coated survivors could act as reservoirs for the disease.

Comparison of selective media for the detection of Vibrio vulnificus in environmental samples

AIMS

To compare two selective agars, cellobiose-colistin (CC) agar and a modification of the Vibrio vulnificus medium (VVMc agar), for the isolation of Vibrio vulnificus from environmental samples.

METHODS AND RESULTS

The efficiencies of recovery of V. vulnificus collection strains on CC, VVM, VVMc and on thiosulphate-citrate-bile salts-sucrose (TCBS) agar were compared and similar efficiencies were obtained. A slightly higher recovery was observed on VVMc agar. The detection of V. vulnificus in environmental samples (eels and water) was performed by combining culture-based methods (CC and VVMc agars) with DNA-based methods using species-specific probes based on the cytolysin-haemolysin and the 16S rDNA genes. A lower accompanying microbiota was found on CC agar than on VVMc agar.

CONCLUSION

The comparison between CC and VVMc agars confirms that both are useful for the detection of V. vulnificus in environmental samples. However, the use of any of these media should be combined with a species-specific probe.

SIGNIFICANCE AND IMPACT OF THE STUDY

The combined use of a selective medium and a specific probe provides a feasible method for the detection of V. vulnificus for epidemiological and ecological studies.