Detection of Vibrio vulnificus biotypes 1 and 2 in eels and oysters by PCR amplification

Climate change and Vibrio vulnificus dynamics: A blueprint for infectious diseases

Climate change is having increasingly profound effects on human health, notably those associated with the occurrence, distribution, and transmission of infectious diseases. The number of disparate ecological parameters and pathogens affected by climate change are vast and expansive. Disentangling the complex relationship between these variables is critical for the development of effective countermeasures against its effects. The pathogen Vibrio vulnificus, a naturally occurring aquatic bacterium that causes fulminant septicemia, represents a quintessential climate-sensitive organism. In this review, we use V. vulnificus as a model organism to elucidate the intricate network of interactions between climatic factors and pathogens, with the objective of identifying common patterns by which climate change is affecting their disease burden. Recent findings indicate that in regions native to V. vulnificus or related pathogens, climate-driven natural disasters are the chief contributors to their disease outbreaks. Concurrently, climate change is increasing the environmental suitability of areas non-endemic to their diseases, promoting a surge in their natural populations and transmission dynamics, thus elevating the risk of new outbreaks. We highlight potential risk factors and climatic drivers aggravating the threat of V. vulnificus transmission under both scenarios and propose potential measures for mitigating its impact. By defining the mechanisms by which climate change influences V. vulnificus disease burden, we aim to shed light on the transmission dynamics of related disease-causing agents, thereby laying the groundwork for early warning systems and broadly applicable control measures.

Identification of a highly specific DNA aptamer for Vibrio vulnificus using systematic evolution of ligands by exponential enrichment coupled with asymmetric PCR

Vibrio vulnificus is an important bacterial pathogen that causes serious infections in fish and is also highly pathogenic to humans. Many effective detection methods targeting this pathogen have previously been designed, but many of these methods are time-consuming, complicated and expensive. Thus, these approaches cannot be widely used by small aqacultural concerns. Although DNA aptamers have been used to detect pathogenic bacteria, these have not been applied to marine bacteria, including V. vulnificus. Therefore, we developed a highly specific DNA aptamer for V. vulnificus detection using systematic evolution of ligands by exponential enrichment (SELEX), coupled with asymmetric PCR. After 13 rounds of cross-selection, we identified a novel DNA aptamer (Vapt2). We evaluated the affinity, specificity and limit of detection (LOD) of this aptamer for V. vulnificus. We found that Vapt2 had a high affinity for V. vulnificus (K_d  = 26.8 ± 5.3 nM) and detected this pathogen at a wide range of concentrations (8-2.0 × 10⁸  cfu/ml). Vapt2 bound to V. vulnificus with high selectivity in the presence of other pathogenic bacteria. Our study increases our knowledge of the possible applications of aptamers with respect to marine bacteria. Moreover, our work might provide a framework for the rapid detection of pathogenic bacteria and water pollution.

Vibrio vulnificus outbreaks in Dutch eel farms since 1996: strain diversity and impact

Vibrio vulnificus is a potentially zoonotic bacterial pathogen of fish, which can infect humans (causing necrotic fasciitis). We analysed 24 V. vulnificus isolates (from 23 severe eel disease outbreaks in 8 Dutch eel farms during 1996 to 2009, and 1 clinical strain from an eel farmer) for genetic correlation and zoonotic potential. Strains were typed using biotyping and molecular typing by high-throughput multilocus sequence typing (hiMLST) and REP-PCR (Diversilab®). We identified 19 strains of biotype 1 and 5 of biotype 2 (4 from eels, 1 from the eel farmer), that were subdivided into 8 MLST types (ST) according to the international standard method. This is the first report of V. vulnificus biotype 1 outbreaks in Dutch eel farms. Seven of the 8 STs, of unknown zoonotic potential, were newly identified and were deposited in the MLST database. The REP-PCR and the MLST were highly concordant, indicating that the REP-PCR is a useful alternative for MLST. The strains isolated from the farmer and his eels were ST 112, a known potential zoonotic strain. Antimicrobial resistance to cefoxitin was found in most of the V. vulnificus strains, and an increasing resistance to quinolones, trimethoprim + sulphonamide and tetracycline was found over time in strain ST 140. Virulence testing of isolates from diseased eels is recommended, and medical practitioners should be informed about the potential risk of zoonotic infections by V. vulnificus from eels for the prevention of infection especially among high-risk individuals. Additional use of molecular typing methods such as hiMLST and Diversilab® is recommended for epidemiological purposes during V. vulnificus outbreaks.

Multiplex PCR assays for simultaneous detection and characterization of Vibrio vulnificus strains

AIMS

Vibrio vulnificus is a major cause of seafood-related deaths in the United States. Several biomarkers, e.g. the virulence-correlated gene (vcg), 16S rRNA, and the capsular polysaccharide operon (CPS) have been used to differentiate virulent- from nonvirulent-type V. vulnificus strains. In this study, we combined the use of these biomarkers with a species-specific V. vulnificus cytolysin/haemolysin gene (vvhA) to develop two pairs of multiplex PCR assays that simultaneously detect and characterize V. vulnificus strains.

METHODS AND RESULTS

The first multiplex PCR pair amplified four genes (vvhA, vcg, 16S rRNA, and CPS), with one for virulent-type and the other one for nonvirulent-type V. vulnificus strains, while the second pair targeted three of those genes excluding CPS. Primer concentration and annealing temperature were optimized for the four multiplex PCR assays. When testing ten V. vulnificus reference strains and 80 field oyster isolates, results from each multiplex PCR matched 100% with known strain characteristics for these target genes.

CONCLUSIONS

The optimized multiplex PCR assays were capable of simultaneously detecting and characterizing V. vulnificus with high specificity and speed.

SIGNIFICANCE AND IMPACT OF THE STUDY

Multiplex PCR assays designed in this study are valuable tools for microbial ecology and epidemiology studies. They may facilitate better control of V. vulnificus risks in oysters, thereby reducing the number of illnesses and deaths because of V. vulnificus in the long run.

Evaluation of a loop-mediated isothermal amplification assay for detecting Vibrio vulnificus in raw oysters

Human consumption of raw or undercooked seafood, particularly oysters, may lead to severe infections due to the presence of Vibrio vulnificus. In this study, a sensitive and specific loop-mediated isothermal amplification (LAMP) assay was developed to detect this pathogen in raw oysters. Two outer and two inner primers were designed to specifically recognize the V. vulnificus cytolysin/hemolysin gene (vvhA), and the reaction could be completed in 1 hour at 63 degrees C. Direct visual observation of the LAMP amplicons was achieved with the aid of SYBR Green I fluorescent dye. The assay specificity was determined using 50 bacterial strains, including multiple Vibrio spp. and bacteria of other genera. No false-positive or false-negative results were observed. The detection limit of the LAMP assay was approximately 20 colony-forming units (CFU) in pure cultures, 10-fold more sensitive than a conventional polymerase chain reaction (PCR). When directly applied in oyster homogenate, the LAMP assay had a detection limit of approximately 10(7) CFU/g. After 5-hour enrichment, LAMP was capable of detecting 7 CFU of V. vulnificus per gram of oyster tissue without lengthy DNA extraction steps. This level of detection was 1000-fold more sensitive than PCRs included for comparison. Because of its isothermal format and unique amplicon detection technique, this rapid and sensitive LAMP assay holds potential for future field applications.

Sensitive and rapid identification of Vibrio vulnificus by loop-mediated isothermal amplification

Vibrio vulnificus is a serious bacterial pathogen for humans and aquatic animals. We developed a rapid, sensitive and specific identification method for V. vulnificus using loop-mediated isothermal amplification (LAMP) technique. A set of primers, composed of two outer primers and two inner primers, was designed based on the cytolysin gene sequence of V. vulnificus. The LAMP reaction was processed in a heat block at 65 degrees C for 60 min. The amplification products were detected by visual inspection using SYBR Green I, as well as by electrophoresis on agarose gels. Our results showed that the LAMP reaction was highly specific to V. vulnificus. This method was 10-fold more sensitive than conventional PCR. In conclusion, the LAMP assay was extremely rapid, simple, cost-effective, sensitive and specific for the rapid identification of V. vulnificus.

A gyrB-based PCR for the detection of Vibrio vulnificus and its application for direct detection of this pathogen in oyster enrichment broths

A polymerase chain reaction (PCR) method based on the gyrB (encoding gyrase B or topoisomerase II) gene sequence was developed for the detection of Vibrio vulnificus in seafood. The gyrB primers detected all laboratory isolates of V. vulnificus and did not cross react with other Vibiro and non-Vibrio species examined in this study. The sensitivity of detection of V. vulnificus by gyrB PCR was 300 CFU/g in artificially seeded oyster homogenate without enrichment while, 30 CFU/g could be detected following 18 h enrichment in alkaline peptone water (APW). The gyrB-specific PCR was employed for the direct detection of V. vulnificus in oyster enrichment broths. The assay detected V. vulnificus in 75% of natural oyster samples after 18 h enrichment in APW. The gyrB-based PCR described here offers a simple and specific one step PCR method for the detection of V. vulnificus in seafood enrichment broths.

Development of a PCR assay for Streptococcus iniae based on the lactate oxidase (lctO) gene with potential diagnostic value

Streptococcus iniae is a well-known pathogen of both fish and humans that is difficult to identify by conventional biochemical tests. A PCR assay based on the lactate oxidase (lctO) gene of S. iniae was developed for the rapid and specific detection and identification of this pathogen from different sources. The PCR assay had a detection limit of 62-31 cells, and 25 pg of DNA per PCR reaction mixture. The PCR was also effective in detecting the bacterium from inoculated tissue homogenates, suggesting its potential use for a rapid and accurate diagnosis of S. iniae infections.

A rapid and improved method for the detection of Vibrio parahaemolyticus and Vibrio vulnificus strains grown on hydrophobic grid membrane filters

DNA probe-based detection methods were developed and characterized as an alternative to time-consuming and less specific conventional protocols. Digoxigenin-labeled probes were prepared by polymerase chain reaction amplification of the targeted sequences in the specific amplicons generated from genomic DNA. Specific probes with high yields were generated for the detection of the tlh gene of Vibrio parahaemolyticus and the cth gene of V. vulnificus. Colony (Southern) hybridization analyses were carried out using hydrophobic grid membrane filters (HGMFs) to allow biotype-specific differentiation of the two species. Eight strains of V. vulnificus and five strains of V. parahaemolyticus, including one standard (ATCC) strain of each biotype, were examined. Colony lysis, hybridization, and nonradioactive detection parameters were optimized for identification of the target biotypes arranged on the same HGMF and also on a conventional nylon membrane, thereby confirming the specificity of the probes and the comparative usefulness of the HGMFs. The experimental procedure presented here can be completed in 1 day. The protocol was designed specifically to identify the target Vibrio spp. and could potentially be used for the enumeration and differentiation of V. parahaemolyticus and V. vulnificus in foods.

Detection of Flavobacterium psychrophilum from fish tissue and water samples by PCR amplification

Rainbow trout fry syndrome and cold-water disease, caused by Flavobacterium psychrophilum, are important diseases in farmed salmonids. Some of the presently available techniques for the detection of Fl. psychrophilum are either time consuming or lack sufficient sensitivity. In the present investigation, the possible detection of Fl. psychrophilum from fish tissue and water samples was examined using nested PCR with DNA probes against a sequence of the 16S rRNA genes. The DNA was extracted using Chelex(R) 100 chelating resin. The primers, which were tested against strains isolated from diseased fish, healthy fish, fish farm environments and reference strains, proved to be specific for Fl. psychrophilum. The obtained detection limit of Fl. psychrophilum seeded into rainbow trout brain tissue was 0.4 cfu in the PCR tube, corresponding to 17 cfu mg-1 brain tissue. The PCR-assay proved to be more sensitive than agar cultivation of tissue samples from the brain of rainbow trout injected with Fl. psychrophilum. In non-sterile fresh water seeded with Fl. psychrophilum the detection limit of the PCR-assay was 1.7 cfu in the PCR tube, corresponding to 110 cfu ml-1 water. The PCR-assay detected Fl. psychrophilum in water samples taken from a rainbow trout farm, but Fl. psychrophilum could not be isolated using inoculation on selective agar. The method presented here has the potential to detect low levels of Fl. psychrophilum in fish tissue and in water samples, and the technique can be a useful tool for understanding the epidemiology of Fl. psychrophilum.

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

A selective medium (VVM) and a specific 16S rRNA gene (rDNA) probe (V3VV) for the detection of Vibrio vulnificus were developed. The medium contains D-(+)-cellobiose as the main carbon source and electrolytes (MgCl(2)-6H(2)O and KCl), which stimulate bacterial growth. Polymyxin B, colistin, and moderate alkalinity and salinity provide selectivity properties. V. vulnificus grows on VVM as flat, bright yellow colonies. Other Vibrio species tested either did not grow or showed green-bluish colonies, with the exception of V. campbelli, V. carchariae, and V. navarrensis. There is a higher colony count on VVM agar than on cellobiose-colistin agar or on modified cellobiose-polymyxin B-colistin agar. The specific probe was evaluated by colony hybridization and dot blot hybridization with PCR-amplified 16S rDNA using collection strains and environmental isolates. No strain studied other than V. vulnificus showed positive hybridization with this oligonucleotide. The combined use of VVM agar and the V3VV probe provided the recovery of V. vulnificus from mixed bacterial suspensions and spiked mussels.

Development of a PCR assay for detection of Yersinia ruckeri in tissues of inoculated and naturally infected trout

A PCR-based method was developed for the specific detection of Yersinia ruckeri in tissues of inoculated trout and naturally infected trout. No amplification products were obtained with other yersiniae, bacterial fish pathogens, or phylogenetically related bacteria (n = 34). The sensitivity of PCR detection was 60 to 65 bacterial cells per PCR tube, which was decreased to 10 to 20 cells by hybridization with a nonradioactive probe. The PCR assay proved to be as reliable as and faster than the conventional culture method for the detection of Y. ruckeri in infected trout tissues.

A comparison of strategies for the detection and recovery of Vibrio vulnificus from marine samples of the western Mediterranean coast

We have compared the effectiveness of culture-based methods and a DNA-based method for the detection, of Vibrio vulnificus from a seawater and three types of shellfish collected from the costal waters of Valencia, Spain. For culture-based method, we used two selective media, thiosulphate-citrate-salts-sucrose (TCBS), and cellobiose-polymyxin B-colistin (CPC) agars with and without previous enrichment in alkaline-saline-peptone-water (APWS). Presumptive colonies were confirmed as V. vulnificus by the polymerase chain reaction (PCR) using previously described 23S rRNA V. vulficus-specific sequences as primers (Dvu 9V and Dvu 45R). Direct detection was accomplished by a nested-PCR procedure developed for environmental samples, with the above mentioned primers for the second amplification. Of 32 seawater samples, only one yielded positive results by direct detection by PCR, whereas five were positive by culture methods. Of the 32 bivalve samples, two were positive by PCR and five by culture methods. From a total of 675 presumptive colonies selected on the two media, only 48 (20 from seawater and 28 from bivalves) were confirmed as V. vulnificus by PCR. Forty-six V. vulnificus isolates were obtained after enrichment and only two after direct inoculation of CPC. Except for one sampling, positive results by direct detection did not correlate with confirmed strains obtained from culture media. API 20E profiles were recorded for all isolates previously identified as V. vulnificus, revealing that around 20% of the strains were sucrose-positive. For our samples, the best strategy consisted in the combination of culture based methods (3 h enrichment in APWs at 40 degrees C followed by CPC at the same temperature) and DNA-based procedures (specific PCR amplification of the presumptive colonies with primers Dvu 9V and Dvu 45 R), which allowed the detection and accurate identification of V. vulnificus in less than 48h. This is the first report on the detection of cells of V. vulnificus naturally present in seawater and edible shellfish in the Spanish Mediterranean coast.

An enzyme-linked immunosorbent assay for detection of Vibrio vulnificus biotype 2: development and field studies

Vibrio vulnificus biotype 2 is a primary eel pathogen which constitutes a lipopolysaccharide (LPS)-based homogeneous O serogroup within the species. In the present work, we have developed an enzyme-linked immunosorbent assay (ELISA) based on the specificity of LPS for the detection of this pathogen. The ELISA specificity was confirmed after testing 36 biotype 2 strains from laboratory cultures and environmental samples, 31 clinical and environmental biotype 1 isolates, and several strains of Vibrio, Aeromonas, and Yersinia species, including the fish pathogens V. anguillarum, V. furnissii, A. hydrophila, and Y. ruckerii. The detection limits for biotype 2 cells were around 10(4) to 10(5) cells/well, and the immunoassay was also able to detect cells in the nonculturable state. Artificially infected eels and environmental samples were analyzed, and the immunodetection was confirmed by cultural methods (isolation on selective and nonselective media before and after broth enrichment). With this methodology, V. vulnificus biotype 2 was successfully detected in infected eels and asymptomatic carriers, which suggests that eels can act as a reservoir for this pathogen.