Identification of Vibrio harveyi using PCR amplification of the toxR gene

Genes expression in Penaeus monodon of Bangladesh; challenged with AHPND-causing Vibrio parahaemolyticus

Vibrio parahaemolyticus, the causative agent of Acute hepatopancreatic necrosis disease (AHPND), was discovered in 2013 as a unique isolate that produces toxins and kills penaeid shrimps in devasting nature in Bangladesh and causes severe economic losses. This research aimed to understand the expressions of immune genes in different stages of the host species, Penaeus monodon, against virulence and toxin genes upon being challenged with V. parahaemolyticus. Healthy post-larvae (PL) samples were collected from southwestern of Bangladesh from July 2021 to August 2022. The tryptic soy agar with 1.5% sodium chloride (NaCl) was used to inoculate the cells of V. parahaemolyticus, and the tryptic soy broth (TSB) with 1.5% NaCl was used to transfer the colonies. The spectrophotometry measured bacteria density. PCR, qPCR, SDS-PAGE, and Western blot measured gene expression and survivability after the immersion challenge. The 1 × 105CFU/mL of V. parahaemolyticus was used for 144 h.p.i (hours post-infection) challenge to six stages of post-larvae (PL) of P. monodon (PL20, PL25, PL30, PL35, PL40, and PL45), PL30 and PL35 showed 100% mortality by day 72 (h.p.i.) after exposure that indicated most vulnerable to V. parahaemolyticus. The expression of immune and toxic genes was confirmed by qPCR. The immune genes toll-like receptors (TLR), prophenoloxidase (ProPO), lysozyme (lyso), and penaeidin (PEN) of PL20 and PL25 of P. monodon were expressed robustly up-trends. PL30 and PL35 showed the lowest gene expression at the end of 72 (h.p.i.). At the end of the 144 (h.p.i.) exposure, the immune genes TLR, ProPO, lyso, and PEN expressed highest in PL45 than other post-larvae stages of P. monodon. The toxic genes (pirA, ToxR, ToxA, ToxB, tlh, tdh, and trh) in PL30 and PL35 of P. monodon after exposure of V. parahaemolyticus were expressed highest at the end of the 72 (h.p.i.). The lowest toxic genes expressions were revealed in PL20 and PL45 at the end of the 144 (h.p.i.). The SDS-PAGE analysis of proteins from the bacterium revealed identical protein profiles with toxic genes, and those toxins were further confirmed by Western blot. The 20 kDa, 78 kDa (ToxR), 20 kDa, 25 kDa (ToxA), 25 kDa (ToxB), 20 kDa, 27 kDa, 75 kDa (tdh), and 20 kDa, 27 kDa, 75 kDa, and 78 kDa (trh) proteins were strong responses in Western blot, indicating the crucial involvement of these immune-related genes in the defense and recovery of the first-line defense mechanisms during V. parahaemolyticus infection to shrimp. The all-toxic genes showed a unique homology and those derived from the common ancestor compared with V. parahaemolyticus (NCBI accession no. AP014859.1). All clades were derived with different traits with very low genetic distance, where the overall mean distance was 3.18 and showed a very uniform and homogenous pattern among the lineages. The V. parahaemolyticus infection process in different PL stages in P. monodon revealed novel insights into the immune responses. The responses may lead to the subsequent production of a DNA vaccine, enhancing shrimp health management to minimize the economic losses due to AHPND experiencing an outbreak of early mortality syndrome (EMS) toward sustainable production P. monodon (shrimp).

Evaluation of Flathead Grey Mullets (Mugil cephalus) Immunization and Long-Term Protection against Vibrio harveyi Infection Using Three Different Vaccine Preparations

In recent years, flathead grey mullets (Mugil cephalus) cultured in Eilat (Israel) have been highly affected by Vibrio harveyi, showing neurological signs such as uncoordinated circular swimming followed by high mortality rates. Despite the advances in and different approaches to control vibriosis associated with Vibrio harveyi, including commercial vaccines, most of them have not succeeded in long-term protection. In this study, we evaluated the effectiveness, long-term protection, and antibody production of three vaccine preparations: heat-killed bacteria (HKB), membrane proteins denaturation (BME PROT), and internal proteins (INT PROT) developed specifically against Vibrio harveyi for grey mullets. Our results show that fish immunized with heat-killed bacteria emulsified with adjuvant presented the most effective and long-lasting protection against the bacterium, and a cross-protection against other bacteria from the harveyi clade. The effectiveness of each immunization treatment correlated with the levels of specific antibody production against Vibrio harveyi in the serum of the immunized fish.

Virulence and antimicrobial resistance characteristics assessment of Vibrio isolated from shrimp (Penaeus vannamei) breeding system in south China

The diseases caused by Vibrio during shrimp breeding program have the risk of spreading in different aquatic areas through larvae transportation between different regions. Therefore, the population distribution and the virulence and antibiotic resistance risk of 5 pathogenic Vibrio in shrimp (Penaeus vannamei) breeding system in China were evaluated for the first time. A total of 418 isolates were recovered from shrimp, breeding water and biological baits samples, and 312 isolates were identified as Vibrio genus based on 16s rDNA, among which V. alginolyticus, V. harveyi, V. parahaemolyticus, V. cholerae and V. campbellii were the dominant species. And 10/20 kinds of virulence genes (chiA, luxR, vhh, tlh, chxA, sepro, flaA, vch, VAC and rpoS) were detected among the 5 Vibrio species. Multiple antibiotic resistance (MAR) index of the 5 dominant Vibrio isolates were 0.13-0.88 %, and 36.5 % isolates with MAR < 0.2. But the antibiotic resistance pattern abundance (ARPA) index ranged from 0.25 to 0.56, which indicated the antibiotic phenotypes of Vibrio species in the shrimp breeding system in China were homogeneity. Furthermore, resistance quotients (RQs) calculation results displayed that the dominant Vibrio species in the shrimp breeding system in China showed no or low selection pressure for resistance to cefoperazone/sulbactam, enrofloxacin, ciprofloxacin, fluoroquine, florfenicol, tetracycline and doxycycline. But only 5 resistance genes were detected, which were strA (43.8 %), strB (11.7 %), QnrVC (2.9 %), sul2 (8.8 %) and Int4 (8.8 %), respectively, and the antimicrobial resistance genotypes were not previously correlated with their phenotypes. The relevant research results provide theoretical basis for epizootic tracking in aquatic system in China, and targeting its final risk in aquatic ecosystem and public health perspectives.

Characterization of a Vibrio-infecting bacteriophage, VPMCC5, and proposal of its incorporation as a new genus in the Zobellviridae family

Vibrio harveyi is a Gram-negative pathogenic bacterium responsible for luminous vibriosis in shrimp and causes mass mortality of shrimp that leads to economic losses. Considering the emergence of multi-drug resistant bacteria, there is always a need for an alternative to antibiotics. In this study, we have aimed to characterize the Vibrio-infecting bacteriophage VPMCC5 (isolated from an environmental sample by using V. harveyi S2A) and evaluate its efficacy in controlling the pathogen. The bacteriophage exhibited an isometric head and short non-contractile tail. The latent period of the bacteriophage was 10 min and the burst size was 20. The genome of the bacteriophage was 48938 bp long with 40.7 mol% G+C content. A total of 71 ORFs were identified and no tRNA and antibiotic-associated genes were detected. Comparative genomic analyses (CLANS, dot plot, progressiveMauve alignment, and phylogenetic tree) strongly suggest that the bacteriophage VPMCC5 might be a new genus in the family of Zobellviridae. A distinguishing feature of this bacteriophage among the other reported Vibrio-infecting bacteriophages is the presence of putative alginate lyase family protein-coding open reading frame. The bacteriophage was found to be surviving at pH 3-9 and in a wide range of temperatures (4-45 ᵒC). In liquid culture inhibition, the bacteriophage could completely lyse the host bacteria after 3 h. This bacteriophage might be used as a biocontrol agent in the extreme environment of shrimp culture.

First description of Vibrio harveyi as the causative agent of morbidity and mortality in farmed flathead grey mullet Mugil cephalus

Flathead grey mullet Mugil cephalus is an important species in the aquaculture industry in the Mediterranean basin and throughout the world. During the last 10 yr, M. cephalus breeding stocks, larvae, and juveniles cultured in Eilat (Israel) have shown neurological signs such as uncoordinated circular swimming, while also presenting oral hemorrhages. Death follows days after the onset of the clinical signs, and mortality rates may reach 80% in some cases, causing high economical losses. Bacteriology isolations from different organs, including the brain, and a Koch's postulate experiment, confirmed Vibrio harveyi as the causative agent. Histological analyses showed the presence of the bacterium in different organs. However, in the brain, the bacterium was observed only within blood vessels and meninges. In some samples, mild to severe brain tissue damage was seen. In order to understand the virulence and lethality of V. harveyi, a median lethal dose was calculated, and the result was 106 colony-forming units fish-1. To the best of our knowledge, this is the first report that describes V. harveyi isolated from the brain of M. cephalus and validates it as an etiological agent causing neurological signs in this fish species.

Biochemical and molecular characterization of three serologically different Vibrio harveyi strains isolated from farmed Dicentrarchus labrax from the Adriatic Sea

Vibrio harveyi is recognized as one of the major causes of vibriosis, a disease that threatens the long-term sustainability of aquaculture. Current research shows that the Mediterranean strains of V. harveyi are serologically heterogeneous, though research comparing the traits of different strains is scarce. This study aims to describe the biochemical, physiological and genetic characteristics of three serologically different strains of V. harveyi isolated from farmed European Sea bass (Dicentrarchus labrax) from the Adriatic Sea. A total of 32 morphological and biochemical markers were examined and, the susceptibility to 13 antimicrobials tested, and then compared the results of high-throughput sequencing and in silico analyses. This study also presents the first whole genome sequences of V. harveyi isolated from European sea bass. A large number of nonsynonymous variations were detected among sequences of the three strains. The prediction analysis of resistance genes did not correspond with the in vitro antimicrobial susceptibility tests. Six virulence genes previously unrelated to virulence of vibrios were detected in all three studied strains. The results show that differences were detected at every level of comparison among the three studied strains isolated from the same fish species originating from a small geographic area.

Inactivated Whole Vaccine Inhibits Lethal Vibrio harveyi Infection in Oplegnathus punctatus

Aquaculture plays a key role in food production globally and provides a valuable source of protein and nutrition, addressing a worldwide growing demand. Oplegnathus punctatus (spotted knifejaw) is an economically important fish species with a high market value and demand. Previous studies on O. punctatus focused mainly on gonadal development, chromosomal microstructure, selective breeding, characterization of immune genes, and viral diseases. There is no published scientific research regarding vibriosis in this fish species. In this study, two potential pathogenic bacteria, Vibrio harveyi and Enterococcus gallinarum, were isolated from moribund cultured O. punctatus. The sequence of the universal 16S rDNA gene was used to identify potential pathogenic bacteria isolated from the moribund O. punctatus, and morphological assessments and API20E tests of the bacterial isolates were conducted to verify the identity and biochemical characteristics of the isolates. Injection of E. gallinarum did not lead to mortality in O. punctatus during the 21 days of observation. In contrast, fish died overnight when challenged with V. harveyi at 1.25 × 105 CFU/g body weight, suggesting that the cause of death of the cultured O. punctatus was V. harveyi infection. Antimicrobial sensitivity analyses revealed that the V. harveyi strain NTOU is sensitive to flumequine, doxycycline, oxolinic acid, and amoxycillin. Importantly, we demonstrated for the first time that intraperitoneal administration of an inactivated V. harveyi whole-cell vaccine resulted in a high level of protection against V. harveyi infection in O. punctatus.

Occurrence and antibiogram signatures of some Vibrio species recovered from selected rivers in South West Nigeria

Vibrio species, widely distributed in water environments, has emerged as a prominent cause of water and food-related disease outbreaks posing significant risk to human and animal health worldwide. About 40% of presumptive isolates recovered from four selected rivers in Southwest Nigeria and, established as Vibrio species genus through polymerase chain reaction techniques., were subjected to antibiotic susceptibility testing against a panel of 18 commonly used antibiotics. The relative prevalence of key Vibrio species (V. parahaemolyticus, V. vulnificus, V. mimicus, V. harveyi, and V. cholerae) was in the order 17%, 13.3%, 4.4%, 2.2%, and 2.2% respectively. Antibiotic resistance by all Vibrio species was mostly observed against doxycycline (71-89%), erythromycin (86-100%), tetracycline (71-89%), rifampicin (86-100%), and sulfamethoxazole (87-100%), though susceptibility to meropenem (86-100%), cephalothin (60-100%), norfloxacin (93-100%), ciprofloxacin (88-100%), amikacin (64-100%), gentamicin (57-74%), and trimethoprim/sulfamethoxazole (57-81%) was equally observed in all species. Vibrio mimicus expressed highest resistance against streptomycin and chloramphenicol (64%), while V. vulnificus (52%) and V. cholerae (57%) had the highest resistance against cephalothin. High resistance against ampicillin (57%) and amoxicillin (50%) was exhibited by V. cholerae and V. mimicus respectively. Indexes of multiple antibiotic resistances (MARI) among Vibrio species ranged between 0.11 and 0.72 with the highest MAR index of 0.72 observed in one isolate of V. vulnificus. This study reveals high prevalence of Vibrio species in the selected rivers as well as elevated resistance against some first-line antibiotics, which suggests possible inappropriate antimicrobial usage around study communities. We conclude that the freshwater resources investigated are unfit for domestic, industrial, and recreational uses without treatment prior to use and are potential reservoirs of antibiotic-resistant Vibrio species in this environment.

Vibrio harveyi isolated from marine aquaculture species in eastern China and virulence to the large yellow croaker (Larimichthys crocea)

AIMS

Diseases of maricultured species caused by Vibrio harveyi are increasing in China and other regions. This study examined the genetic diversity, antimicrobial susceptibility, plasmid profiles and virulence potential of the V. harveyi isolated from marine organisms farmed in two provinces in eastern China between 2014 and 2019.

METHODS AND RESULTS

A total of 54 V. harveyi were obtained from seven marine species. Enterobacterial repetitive intergenic consensus (ERIC)-PCR fingerprinting revealed substantial genetic heterogeneity among the V. harveyi isolates. There was no significant correlation between ERIC-PCR genotypes and host origins or fish farms. All the isolates were resistant to amoxicillin and ampicillin, and 79·6% to kanamycin. We found that 61·1% of the V. harveyi isolates had plasmid(s) and there were 14 different plasmid profiles. Most isolates from fish hosts (76·5%) contained plasmids; however, 75% of isolates from nonfish hosts lacked plasmids. Experimental infection results showed that isolates with plasmid(s) were more virulent to large yellow croaker than isolates lacking plasmids (P < 0·05).

CONCLUSIONS

This study confirmed that V. harveyi isolates obtained from animals farmed in the coastal region of east China were genetically diverse. Our results suggest that the virulence of various V. harveyi strains to fish is associated with the plasmids they carry.

SIGNIFICANCE AND IMPACT OF THE STUDY

More than 50% of the V. harveyi isolates carried one to 11 plasmids. The plasmid-borne traits of V. harveyi strains might be important for host adaptation and virulence, but they were not associated with susceptibility to the tested antibiotics.

Impact of a reduced water salinity on the composition of Vibrio spp. in recirculating aquaculture systems for Pacific white shrimp (Litopenaeus vannamei) and its possible risks for shrimp health and food safety

Tropical shrimp, like Litopenaeus vannamei, in land-based recirculating aquaculture systems (RAS) are often kept at low water salinities to reduce costs for artificial sea salt and the amount of salty wastewater. Although these shrimp are tolerant against low salinities, innate immunity suppression and changes in the microbial composition in the water can occur. As especially Vibrio spp. are relevant for shrimp health, alterations in the species composition of the Vibrio community were analysed in water from six RAS, run at 15‰ or 30‰. Additionally, pathogenicity factors including pirA/B, VPI, toxR, toxS, vhh, vfh, tdh, trh, flagellin genes and T6SS1/2 of V. parahaemolyticus were analysed. The Vibrio composition differed significantly depending on water salinity. In RAS at 15‰, higher numbers of the potentially pathogenic species V. parahaemolyticus, V. owensii and V. campbellii were detected, and especially in V. parahaemolyticus, various pathogenicity factors were present. A reduced salinity may therefore pose a higher risk of disease outbreaks in shrimp RAS. Because some of the detected pathogenicity factors are relevant for human health, this might also affect food safety. In order to produce healthy shrimp as a safe food for human consumption, maintaining high water salinities seems to be recommendable.

Delineating the key virulence factors and intraspecies divergence of Vibrio harveyi via whole-genome sequencing

Vibrio harveyi is one of the major pathogens in aquaculture. To identify the key virulence factors affecting pathogenesis of V. harveyi towards fish, we conducted a field investigation for three representative fish farms infected with V. harveyi. Multilocus sequence typing (MLST) and whole-genome sequencing were conducted to delineate the phylogenetic relationship and genetic divergence of V. harveyi. A total of 25 V. harveyi strains were isolated from the diseased fish and groundwater and were subtyped into 12 sequence types by MLST. Five virulence genes, mshB, pilA, hutR, ureB, and ureG, were variably presented in the sequenced strains. The virulence gene profiles strongly correlated with the distinct pathogenicity of V. harveyi strains, with a strain harboring all five genes exhibiting the highest virulence towards fish. Phenotype assay confirmed that reduced virulence correlated with decreased motility and biofilm formation ability. Additionally, three types of type VI secretion system, namely T6SS1, T6SS2, and T6SS3, were identified in V. harveyi strains, which can be classified into six, four, and 12 subtypes, respectively. In conclusion, the results indicated that the virulence level of V. harveyi is mainly determined by the above virulence genes, which may play vital roles in environmental adaptation for V. harveyi.

Aeromonas schubertii, a novel bacterium recovered from AHPND affected farm is lethal to whiteleg shrimp, Penaeus vannamei

Retrospective diagnosis of a bacterial collection (n = 31) originated from five farms reportedly affected by early mortality syndrome (EMS) in Southeast Asia in 2016 revealed that 9/31 isolates from two farms tested positive for V. parahaemolyticus causing acute hepatopancreatic necrosis disease (VP_AHPND). Molecular analysis of the 22 remaining isolates showed that 21 isolates belong to Vibrio species including VP_non-AHPND, V. vulnificus, V. cholerae, V. owensii and V. alginolyticus. One isolate from an AHPND farm was preliminarily identified as Aeromonas schubertii based on 99.43% nucleotide identity of 16S rRNA to the reference strain ATCC 43700 (X60416). Diseases caused by Vibrio bacteria have been well-studied in shrimp while pathogenic potential of non-Vibrio species has been relatively overlooked. Since the description of A. schubertii present in shrimp farms is rare, this study therefore focused on species identification and its pathogenic potential to shrimp based on a combination of multiple approaches i.e. multilocus sequence analysis (MLSA), challenge test, histopathology and in situ hybridization (ISH). Based on MLSA of 2464 bp derived from 16S rRNA (1346 bp), gyrB (568 bp) and rpoB (550 bp), this isolate was confirmed as A. schubertii. Immersion challenge using three successive 10-fold serial dilutions (2 × 10⁴ to 2 × 10⁶ CFU/mL) revealed that A. schubertii was pathogenic to shrimp and cumulative mortalities were dose-dependent (45-70%). The diseased shrimp exhibited gross sign of reddish body and remarkable histopathological lesion of collapsed hepatopancreatic tubules and typical encapsulation. ISH using A. schubertii-specific probe confirmed localization of bacteria in the hepatopancreas of the infected shrimp. In summary, this study reported a novel pathogenic, non-Vibrio species, A. schubertii recovered from an AHPND-affected farm causing up to 70% mortality in immersion challenge. Since A. schubertii is relatively new to shrimp, this may pose a potential risk for low salinity shrimp farming areas, active surveillance of this pathogen, therefore, should not be overlooked.

Bactericidal effect on skin mucosa of dietary guava (Psidium guajava L.) leaves in hybrid tilapia (Oreochromis niloticus × O. mossambicus)

ETHNOPHARMACOLOGICAL RELEVANCE

Due to the intensification practices in global aquaculture, fish are often confined in small volumes, which can results in outbreak diseases. In this context, the use of antibiotics is very usual. Thus, looking for natural substance able to reduce the use of the antibiotics is imperative. Among them, there is a great interest at present in the study of medicinal plants such as guava (Psidium guajava L.). These plants could help to develop a more sustainable aquaculture all over the world. The application of guava in traditional medicine dates for centuries and it is widely used in tropical countries for the treatment of diseases in human and animals.

AIM OF THE STUDY

The purpose of this work was to study the effects of the dietary administration of dried leaves of Psidium guajava on the skin mucosal immunity of hybrid tilapia (Oreochromis niloticus × O. mossambicus). Furthermore, the ability of this plant to inhibit the bacterial load in different tissues after an experimental infection with Vibrio harveyi was studied.

MATERIALS AND METHODS

P. guajava leaves collection and the experimentation was carried out in Dominican Republic. Fish were fed with a commercial diet supplemented with guava leaf at different concentrations (0%, 1.5% and 3%) for 21 days before being intraperitoneally injected with V. harveyi (1 × 10⁴ cells mL^-1). Thereafter, several immune activities were measured in fish skin mucus and after 48 h of injection, the skin, spleen and liver were collected to analyse the bactericidal activity of guava leaf and the gene expression of some immune related genes.

RESULTS

The administration of P. guajava leaves significantly modulated some immune-related enzymes (protease, antiprotease and peroxidase) in the skin mucus of hybrid tilapia. In addition, the bacterial load after V. harveyi infection in skin, spleen and liver significantly reduced in fish supplemented with guava leaves. Finally, the expression profile of hepcidin gene in skin and liver was modulated in fish feed with control diet after V. harveyi infection.

CONCLUSION

These results demonstrate that the dietary intake of guava leaves increases the skin mucosal barrier defences of hybrid tilapia and confers protection against V. harveyi colonization.

Vibrio mediterranei, a potential emerging pathogen of marine fauna: investigation of pathogenicity using a bacterial challenge in Pinna nobilis and development of a species-specific PCR

AIMS

Extreme mortality events affecting Pinna nobilis, some associated to Vibrio mediterranei, have depleted many populations of this bivalve. The objective of this study was to demonstrate pathogenicity of V. mediterranei in the host P. nobilis by performing a bacterial challenge in P. nobilis to understand if V. mediterranei has specific virulence in this host. To assist this objective, a secondary objective was to develop a species-specific DNA diagnostic test.

METHODS AND RESULTS

Pinna nobilis collected from local bays were used in a challenge experiment with V. mediterranei (strain IRTA18-108). The virulence in the host background of P. nobilis was demonstrated at doses of 10³ CFUs per animal. An alignment of published Vibrio sp. atpA sequences was used to design V. mediterranei-specific primers. Furthermore, data mining of published literature and V. mediterranei genomes identified multiple virulence-related genes (vir genes) from which specific primers were designed for PCR detection of selected genes.

CONCLUSION

Vibrio mediterranei strain IRTA18-108 is pathogenic in the host P. nobilis. The virulence genes sod, rtx and mshA were identified in this strain. Temperatures of 24°C or higher appear to trigger onset of virulence. Sensitivity and specificity of the Vm atpA PCR is useful for diagnosis of Vibriosis in shellfish.

SIGNIFICANCE AND IMPACT OF THE STUDY

The presence of previously described virulence genes have been confirmed in this strain. The specific Vm atpA PCR assay will aid management of future epizootics of this emerging pathogen of aquatic fauna, and improve surveillance capabilities for mortality events where Vibrios are suspect.

Application of Bacteriophages to Control Vibrio alginolyticus Contamination in Oyster (Saccostrea glomerata) Larvae

Mortalities of bivalve larvae and spat linked with Vibrio spp. infection have been described in hatcheries since 1959, causing potential development of resistant bacteria. A reliable and sustainable solution to this problem is yet to be developed. Potential treatment of bacterial infection with bacteriophages is gaining interest in aquaculture as a more sustainable option for managing Vibrio spp. infection. This study assessed the effectiveness of bacteriophages (Φ-5, Φ-6, and Φ-7) against pathogenic Vibrio isolates (USC-26004 and USC-26005). These phage isolates were found to belong to the Myoviridae viral family. A total of 212 ORFs of Φ-5 were identified and annotated. The genome of this phage contained putative thymidine kinase and lysin enzyme. During infections with phages, the OD values of the isolates USC-26005 and USC-26004 remained stable at a much lower reading compared to the control after 9 h of incubation. Mortality rate of oyster (Saccostrea glomerata) larvae was 28.2 ± 3.5% in the bacteriophage treatment group, compared to 77.9 ± 9.1% in the bacterial treatment group after 24 h incubation. Findings of this study indicate that lytic phages might be utilized as potential bio-control agents of luminescent bacterial disease in oyster hatcheries.

Genotypic diversity, and molecular and pathogenic characterization of Photobacterium damselae subsp. piscicida isolated from different fish species in Taiwan

Photobacteriosis, caused by Photobacterium damselae subsp. piscicida (Phdp), is a serious disease in marine fish species worldwide. To date, the epidemiological characterization of this pathogen in Taiwan remains limited. In this study, we collected 39 Phdp isolates obtained from different farmed fish for phenotypic and genotypic analysis. Phenotype bioassays using API-20E and API-20NE systems showed that the Phdp is a homogeneous group. However, genotyping using the pulsed-field gel electrophoresis (PFGE) technique revealed genetic variability among Phdp isolates when 13 and 11 different PFGE band patterns were obtained with SmaI and NotI as restriction enzymes, respectively. Phylogenetic analysis using 16S rDNA and the Fur gene clustered Taiwanese isolates and other species of P. damselae in the same clade. In contrast, the ToxR phylogenetic tree, a powerful discriminatory marker, separated the two subspecies. Furthermore, the virulence-associated genes, AIP56, P55, PDP_0080, Sod and Irp1, were detected from all isolates. Virulence testing with nine representative isolates in cobia (Rachycentron canadum) and Asian sea bass (Lates calcarifer) showed that some were highly pathogenic with 80%-100% mortality rates. This study provides epidemiological data of Phdp infections in farmed fish in Taiwan, which is necessary to develop comprehensive prevention and control strategies for the disease.

Use of Bacteriophages to Control Vibrio Contamination of Microalgae Used as a Food Source for Oyster Larvae During Hatchery Culture

Cultured microalgae are the primary food source for oyster larvae during hatchery culture and are a potential vector for Vibrio spp. infection of larval cultures. Bacteriophages have shown potential for controlling contamination of Vibrio spp. in aquaculture systems and their application could be an effective biological control method to eliminate such bacterial contamination of microalgae. This study investigated whether Vibrio-free microalgae sources could be ensured via the application of Vibrio specific phages. As a first step, four different Vibrio bacteriophages (belonging to the Myoviridae viral family) were isolated from marine waters in Queensland, Australia and used in challenge tests against a Vibrio host species, previously isolated from New South Wales oyster hatchery and found to be closely related to V. alginolyticus (ATCC 17749). The genome sequence of one of the four isolated bacteriophages, Vibrio Φ-2, that displayed strongest virulence against the host was determined. The 242446 bp genome of this bacteriophage was predicted to encode 217 proteins with an average GC content of 43.91%, containing putative thymidine kinases and a lysin enzyme. Application of these bacteriophages to pathogenic Vibrio spp. contaminating microalgae suspensions resulted in significant decreases in their numbers within 2 h. Findings indicated that direct application of bacteriophages to microalgae suspensions could be an effective method of reducing the occurrence of vibriosis in oyster hatcheries.

Vibrio harveyi: a serious pathogen of fish and invertebrates in mariculture

Vibrio harveyi, which belongs to family Vibrionaceae of class Gammaproteobacteria, includes the species V. carchariae and V. trachuri as its junior synonyms. The organism is a well-recognized and serious bacterial pathogen of marine fish and invertebrates, including penaeid shrimp, in aquaculture. Diseased fish may exhibit a range of lesions, including eye lesions/blindness, gastro-enteritis, muscle necrosis, skin ulcers, and tail rot disease. In shrimp, V. harveyi is regarded as the etiological agent of luminous vibriosis in which affected animals glow in the dark. There is a second condition of shrimp known as Bolitas negricans where the digestive tract is filled with spheres of sloughed-off tissue. It is recognized that the pathogenicity mechanisms of V. harveyi may be different in fish and penaeid shrimp. In shrimp, the pathogenicity mechanisms involved the endotoxin lipopolysaccharide, and extracellular proteases, and interaction with bacteriophages. In fish, the pathogenicity mechanisms involved extracellular hemolysin (encoded by duplicate hemolysin genes), which was identified as a phospholipase B and could inactivate fish cells by apoptosis, via the caspase activation pathway. V. harveyi may enter the so-called viable but nonculturable (VBNC) state, and resuscitation of the VBNC cells may be an important reason for vibriosis outbreaks in aquaculture. Disease control measures center on dietary supplements (including probiotics), nonspecific immunostimulants, and vaccines and to a lesser extent antibiotics and other antimicrobial compounds.

Draft Genome Sequence of a Harveyi Clade Bacterium Isolated from Lolliguncula brevis Squid

Vibrio species of the Harveyi clade are commonly found in free-living and host-associated marine habitats. Here, we report the draft genome sequence for a Harveyi clade bacterium, Vibrio sp. strain LB10LO1, which was isolated from the Atlantic brief squid Lolliguncula brevis.

A real-time recombinase polymerase amplification assay for the rapid detection of Vibrio harveyi

Vibrio harveyi is a pathogen that infects fish and shellfish worldwide, causing severe economic losses for the aquaculture industry. As the early diagnosis of V. harveyi infection is crucial to disease surveillance and prevention in cultured marine animals, a fast and accurate method to detect V. harveyi is required. Here, we performed recombinase polymerase amplification (RPA) using novel primers specifically designed to recognize the V. harveyi toxR gene, which encodes a transmembrane protein, and then hybridized this gene with a carboxy fluorescein (FAM)-labeled probe. The optimal conditions for the real-time RPA assay were a probe concentration of 90 nM and a 20 min incubation at 37 °C. The sensitivity of our real-time RPA assay was 50 copies of the standard plasmid, while that of real-time PCR was 500 copies. In V. harveyi-spiked Pseudosciaena crocea samples, the sensitivity of our real-time RPA was 60 CFUs per reaction, while that of PCR was 600 CFUs per reaction. SPSS probit regression analysis indicated that the limit of detection (LOD) of our RPA assay, with 95% probability, was 18 copies. The LOD was reached within 20 min and was highly reproducible across eight independent assays. Our novel RPA method successfully differentiated V. harveyi from all other tested Vibrio species, including some that were closely related. Our real-time RPA assay, in combination with a rapid DNA extraction protocol, is a fast and accurate tool for the detection of V. harveyi and for monitoring disease outbreaks. This tool will be valuable for the aquaculture industry.

The management and exploitation of naturally light-emitting bacteria as a flexible analytical tool: A tutorial

Conventional detection of toxic contaminants on surfaces, in food, and in the environment takes time. Current analytical approaches to chemical detection can be of limited utility due to long detection times, high costs, and the need for a laboratory and trained personnel. A non-specific but easy, rapid, and inexpensive screening test can be useful to quickly classify a specimen as toxic or non toxic, so prompt appropriate measures can be taken, exactly where required. The bioluminescent bacteria-based tests meet all these characteristics. Bioluminescence methods are extremely attractive because of their high sensitivity, speed, ease of implementation, and statistical significance. They are usually sensitive enough to detect the majority of pollutants toxic to humans and mammals. This tutorial provides practical guidelines for isolating, cultivating, and exploiting marine bioluminescent bacteria as a simple and versatile analytical tool. Although mostly applied for aqueous phase sample and organic extracts, the test can also be conducted directly on soil and sediment samples so as to reflect the true toxicity due to the bioavailability fraction. Because tests can be performed with freeze-dried cell preparations, they could make a major contribution to field screening activity. They can be easily conducted in a mobile environmental laboratory and may be adaptable to miniaturized field instruments and field test kits.

Isolation and Characterization of Two Lytic Bacteriophages, φSt2 and φGrn1; Phage Therapy Application for Biological Control of Vibrio alginolyticus in Aquaculture Live Feeds

Bacterial infections are a serious problem in aquaculture since they can result in massive mortalities in farmed fish and invertebrates. Vibriosis is one of the most common diseases in marine aquaculture hatcheries and its causative agents are bacteria of the genus Vibrio mostly entering larval rearing water through live feeds, such as Artemia and rotifers. The pathogenic Vibrio alginolyticus strain V1, isolated during a vibriosis outbreak in cultured seabream, Sparus aurata, was used as host to isolate and characterize the two novel bacteriophages φSt2 and φGrn1 for phage therapy application. In vitro cell lysis experiments were performed against the bacterial host V. alginolyticus strain V1 but also against 12 presumptive Vibrio strains originating from live prey Artemia salina cultures indicating the strong lytic efficacy of the 2 phages. In vivo administration of the phage cocktail, φSt2 and φGrn1, at MOI = 100 directly on live prey A. salina cultures, led to a 93% decrease of presumptive Vibrio population after 4 h of treatment. Current study suggests that administration of φSt2 and φGrn1 to live preys could selectively reduce Vibrio load in fish hatcheries. Innovative and environmental friendly solutions against bacterial diseases are more than necessary and phage therapy is one of them.

Dactylogyrus intermedius parasitism enhances Flavobacterium columnare invasion and alters immune-related gene expression in Carassius auratus

The monogenean Dactylogyrus intermedius and the bacterium Flavobacterium columnare are 2 common pathogens in aquaculture. The objective of the present study was to examine the effect of prior parasitism by D. intermedius on the susceptibility of goldfish to F. columnare and to explore the potential immune mechanisms related to the parasite infection. A F. columnare challenge trial was conducted between D. intermedius-parasitized and non-parasitized goldfish. The F. columnare load in gill, kidney, spleen and liver were compared. The expression of immune-related genes (IL-1β2, TNF-α1, TGF-β, iNOS-a, C3 and Lyz) in gill and kidney of D. intermedius-only infected and uninfected control fish were evaluated. D. intermedius-parasitized goldfish exhibited higher mortality and significantly higher loads (3051 to 537,379 genome equivalents [GEs] mg(-1)) of F. columnare, which were 1.13 to 50.82-fold higher than non-parasitized fish (389 to 17,829 GEs mg(-1)). Furthermore, the immune genes IL-1β2, TNF-α1, iNOS-a and Lyz were up-regulated while the TGF-β and C3 were down-regulated in the gill and kidney of parasite-infected fish compared to the non-parasitized controls. The down-regulation TGF-β and C3 was especially noteworthy, as this might indicate the suppression of the host immune functions due to the parasitism by D. intermedius. Taken together, these data demonstrate that parasite infection can enhance bacterial invasion and presents a hypothesis, based on gene expression data, that modulation of host immune response could play a role.

An immunochromatographic test strip for rapid detection of fish pathogen Edwardsiella tarda

Background

Edwardsiella tarda, the etiologic agent of edwardsiellosis, is a devastating fish pathogen prevailing in worldwide aquaculture industries and accounting for severe economic losses. There is a raising concern about E. tarda being a significant zoonotic pathogen, and it is urgent to develop a rapid detection of this pathogen. This is the first study to develop a test strip for rapid detection of E. tarda in turbot.

Results

Mouse monoclonal antibodies (MAbs) and rabbit polyclonal antibody (PAb) against E. tarda were generated from immunization of mice and rabbits with a virulent isolate of E. tarda EIB202. Two MAbs specific to isolates of E. tarda were obtained, and one of them (25C1) was selected to conjugate with colloidal gold as the detector antibody. Rabbit PAb was used as the capture antibody. It was found the strip had no cross-reactivity with non-E. tarda bacterial microbes and the limit of detection (LOD) was 1 × 105 colony-forming units (CFU)/ml. The detection could be visually observed by the naked eye within 5 min. This test strip was verified with a similar detection limit and much less analysis time compared with a dot blot immunoassay (1 × 105 CFU/ml for LOD and 120 min for reaction time). When the samples were mixed with turbot tissue homogenates, strong immunoreactivity was observed over 105 CFU/ml, which suggested that the turbot tissue homogenates did not affect the detection of the strip. Pre-enrichment with homogenized turbot tissue for 12 h could increase the detection limit of the E. tarda present in the sample up to 1 to 10 CFU/ml. In practice, in detecting 20 turbot ascite samples infected by E. tarda, the immunochromatographic test strip showed a high accuracy (100% positive).

Conclusions

The immunochromatographic test strip offers great promise for a rapid, simple, and economical method of E. tarda on-site detection, and with different antibodies, it might be used to detect other aquatic pathogens.

Population dynamics of Vibrio and Pseudomonas species isolated from farmed Tasmanian Atlantic salmon (Salmo salar L.): a seasonal study

Vibrio and Pseudomonas species have been shown to be part of the normal microbiota of Atlantic salmon (Salmo salar L.), with some strains causing disease in fish. The factors affecting their prevalence and persistence in the salmon gut, however, have not been well studied. In this study, we collected 340 Vibrio and 150 Pseudomonas isolates from the hindgut of farmed Tasmanian Atlantic salmon, fed with two commercially available diets. Samples were collected every 6-8 weeks between July 2011 and May 2012. Isolates from selective agar were initially identified using biochemical tests and confirmed using genus-specific primers and 16S ribosomal RNA (16S rRNA) sequencing. Random amplified polymorphic DNA (RAPD) PCR was used to type both Pseudomonas and Vibrio; the latter was further typed using a biochemical fingerprinting method (PhP-RV plates). We observed low species diversity with strains comprising Vibrio ichthyoenteri/Vibrio scophthalmi, Vibrio crassostreae/Vibrio splendidus, Aliivibrio finisterrensis, Photobacterium phosphoreum and Pseudomonas fragi. Out of 340 Vibrio isolates, 238 (70 %) belonged to 21 clonal types and were found predominantly during summer when water temperatures reached 15 to 21 °C. Of these, the four major clonal types were found in multiple samples (70 %). P. fragi, on the other hand, was only found during the colder water temperatures and belonged to 18 clonal types. The presence of both groups of bacteria and their clonal types were independent of the fish diets used, suggesting that the water temperature was the main factor of the prevalence and persistence of these bacteria in the gut of Atlantic salmon.

Multipurpose assessment for the quantification of Vibrio spp. and total bacteria in fish and seawater using multiplex real-time polymerase chain reaction

BACKGROUND

This study describes the first multiplex real-time polymerase chain reaction assay developed, as a multipurpose assessment, for the simultaneous quantification of total bacteria and three Vibrio spp. (V. parahaemolyticus, V. vulnificus and V. anguillarum) in fish and seawater. The consumption of raw finfish as sushi or sashimi has been increasing the chance of Vibrio outbreaks in consumers. Freshness and quality of fishery products also depend on the total bacterial populations present.

RESULTS

The detection sensitivity of the specific targets for the multiplex assay was 1 CFU mL⁻¹ in pure culture and seawater, and 10 CFU g⁻¹ in fish. While total bacterial counts by the multiplex assay were similar to those obtained by cultural methods, the levels of Vibrio detected by the multiplex assay were generally higher than by cultural methods of the same populations. Among the natural samples without Vibrio spp. inoculation, eight out of 10 seawater and three out of 20 fish samples were determined to contain Vibrio spp.

CONCLUSION

Our data demonstrate that this multiplex assay could be useful for the rapid detection and quantification of Vibrio spp. and total bacteria as a multipurpose tool for surveillance of fish and water quality as well as diagnostic method.

Identification and characterisation of an ostreid herpesvirus-1 microvariant (OsHV-1 µ-var) in Crassostrea gigas (Pacific oysters) in Australia

Between November 2010 and January 2011, triploid Crassostrea gigas (Pacific oysters) cultivated in the Georges River, New South Wales, experienced >95% mortality. Mortalities also occurred in wild diploid C. gigas in the Georges River and shortly thereafter in the adjacent Parramatta River estuary upstream from Sydney Harbour. Neighbouring Saccostrea glomerata (Sydney rock oysters) did not experience mortalities in either estuary. Surviving oysters were collected to investigate the cause of mortalities. Histologically all oysters displayed significant pathology, and molecular testing revealed a high prevalence of ostreid herpesvirus-1 (OsHV-1). Quantitative PCR indicated that many C. gigas were carrying a high viral load at the time of sampling, while the load in S. glomerata was significantly lower (p < 0.001). Subsequent in situ hybridisation experiments confirmed the presence of a herpesvirus in C. gigas but not S. glomerata tissues, suggesting that S. glomerata is not susceptible to infection with OsHV-1. Naïve sentinel triploid C. gigas placed in the Georges River estuary in January 2011 quickly became infected and experienced nearly 100% mortality within 2 wk of exposure, indicating the persistence of the virus in the environment. Phylogenetic analysis of sequences derived from the C2/C6 region of the virus revealed that the Australian strain of OsHV-1 belongs to the microvariant (µ-var) cluster, which has been associated with severe mortalities in C. gigas in other countries since 2008. Environmental data revealed that the Woolooware Bay outbreaks occurred during a time of considerable environmental disturbance, with increased water temperatures, heavy rainfall, a toxic phytoplankton bloom and the presence of a pathogenic Vibrio sp. all potentially contributing to oyster stress. This is the first confirmed report of OsHV-1 µ-var related C. gigas mortalities in Australia.

Diversity of Vibrio spp. isolated at ambient environmental temperature in the Eastern English Channel as determined by pyrH sequencing

AIMS

To describe the diversity of the culturable mesophilic and potentially pathogenic vibrios isolated at 22 and 37°C on TCBS medium, in September 2009 from seawater and surface sediments.

METHODS AND RESULTS

q-PCR assays previously selected for the identification of bacterial strains isolated at 37°C were used in combination with the partial sequencing of two housekeeping genes, pyrH and toxR, to identify 315 strains isolated at 22°C. The great majority of the 37°C strains was identified by q-PCR assays, (five of the six species) with the predominance of Vibrio alginolyticus (85·9%) and V. harveyi (10·7%). The human pathogens V. parahaemolyticus and V. cholerae were rarely detected (two strains each). The 22°C strains were successfully identified by the phylogeny analysis of pyrH and toxR genes, revealing 20 Vibrio species, with the predominance of the clam pathogen V. celticus (36·8%). The Splendidus and the Harveyi groups represented the main Vibrio group at 22°C (80%) and 37°C (99·5%), respectively.

CONCLUSIONS

The combination of q-PCR assays and the sequencing of pyrH and toxR genes highlighted two different Vibrio communities at 22 and 37°C both dominated by pathogenic species for marine organisms.

SIGNIFICANCE AND IMPACT OF THE STUDY

The sequencing of the pyrH gene revealed to be a valuable tool to identify environmental Vibrio spp. strains isolated at 22°C, as 92·3% of them were identified in this study.

Characterization of ISR region and development of a PCR assay for rapid detection of the fish pathogen Tenacibaculum soleae

The aims of this work were to characterize the 16S-23S internal spacer region of the fish pathogen Tenacibaculum soleae and to develop a PCR assay for its identification and detection. All T. soleae strains tested displayed a single internal spacer region class, containing tRNA(I) (le) and tRNA(A) (la) genes; nevertheless, a considerable intraspecific heterogeneity was observed. However, this region proved to be useful for differentiation of T. soleae from related and non-related species. Species-specific primers were designed targeting the 16S rRNA gene and the internal spacer region region, yielding a 1555-bp fragment. Detection limit was of 1 pg DNA per reaction (< 30 bacterial cells) when using pure cultures. The detection level in the presence of DNA from fish or other bacteria was lower; however, 10 pg were detected at a target/background ratio of 1 : 10(5) . The PCR assay proved to be more sensitive than agar cultivation for the detection of T. soleae from naturally diseased fish, offering a useful tool for diagnosis and for understanding the epidemiology of this pathogen.

Vibriosis induced by experimental cohabitation in Crassostrea gigas: evidence of early infection and down-expression of immune-related genes

The understanding of reciprocal interactions between Crassostrea gigas and Vibrio sp., whether these be virulent or avirulent, is vital for the development of methods to improve the health status of cultured oysters. We describe an original non-invasive experimental infection technique using cohabitation, designed to explore these interactions. Using real-time PCR techniques we examined the dynamics of virulent and avirulent Vibrio sp. in oyster hemolymph and tank seawater, and made a parallel study of the expression of four genes involved in oyster immune defense: Cg-BPI, Cg-EcSOD, Cg-IκB, Cg-TIMP. No mortality occurred in control animals, but oysters put in cohabitation for 2-48 h with animals previously infected by two Vibrio pathogens suffered mortalities from 2 to 16 days post-cohabitation. Our results show that virulent Vibrio infect healthy individuals after only 2 h of cohabitation, with values ranging from 4.5 x 10² to 2 x 10⁴ cells ml⁻¹ hemolymph. Simultaneously, an approximate ten-fold increase of the total Vibrio population was observed in control animals, with a 6.6-78.5-fold up-expression of targeted genes. In contrast, oysters exposed to harmful bacteria had mean expression levels strongly down-regulated by a factor of 9.2-29 (depending on the gene) compared with control animals. Although oysters were still found to be infected by virulent Vibrio after 6-48 h of cohabitation, no significant differences were noted when comparing levels of each transcript in control and infected oysters at the same sampling times during this period: the important differences were noted before 6 h cohabitation. Taken together, our data support (1) the hypothesis that virulent Vibrio disturbs the immune response of this invertebrate host both rapidly and significantly, although this occurs specifically during an early and transient period during the first 6 h of cohabitation challenge, and that (2) expression of targeted genes is not correlated with vibriosis resistance.

Simple and direct detection of Aeromonas hydrophila infection in the goldfish, Carassius auratus (L.), by dot blotting using specific monoclonal antibodies

A combination of eight isolates of Aeromonas hydrophila was used to produce monoclonal antibodies (MAbs). Ten different groups of MAbs specific to Aeromonas were selected. The first five groups of MAbs demonstrated high specificity and bound to only one or two isolates of A. hydrophila. The sixth and the seventh groups of MAbs were A. hydrophila specific. They recognized seven of eight A. hydrophila isolates (AH1, 2, 3, 4, 5, 6, 8); however, the MAb in the seventh group also showed cross-reactivity to one isolate of Aeromonas caviae (AC3). The eighth MAb group recognized two isolates of A. hydrophila (AH2 and AH5) and demonstrated cross-reactivity to one isolate of Aeromonas sobria (AS1) and one isolate of A. caviae (AC3). The tenth group of MAbs bound to all isolates of Aeromonas spp. tested (AH1-8, AS1-6, AC1-5, Aeromonas veronii and Aeromonas jandaei) without cross-reactivity to any of the other bacteria tested. MAbs in the ninth group showed similar specificity to those in the tenth group but did not recognize two isolates of A. sobria (AS4 and AS6) or A. jandaei. All the MAbs could be used to identify Aeromonas by dot blotting with a sensitivity ranging from 10⁵ to 10⁷ CFU mL⁻¹. However, the sensitivity of detection was increased to 10²-10³ CFU mL⁻¹ after inoculation of the sample in tryptic soy broth for 3-6 h before performing the dot blotting. The dot blot method can be used for the direct detection of A. hydrophila infection in symptomatic and asymptomatic goldfish. This study demonstrated a convenient immunological tool that can be used for the direct detection of A. hydrophila and Aeromonas infections in a complex sample without the requirement for separation of the bacteria or isolation and biochemical tests.

In vitro and in vivo expression of virulence genes in Vibrio isolates belonging to the Harveyi clade in relation to their virulence towards gnotobiotic brine shrimp (Artemia franciscana)

Vibrios belonging to the Harveyi clade are pathogenic marine bacteria affecting both vertebrates and invertebrates, thereby causing a severe threat to the aquaculture industry. In this study, the expression of haemolysin, metalloprotease, serine protease, the quorum sensing master regulator LuxR and the virulence regulator ToxR in different Harveyi clade isolates was measured with reverse transcriptase real-time PCR with specific primers. There was relatively low variation in the in vitro expression levels of the quorum sensing master regulator luxR (sevenfold), whereas for the other genes, the difference in expression between the isolates showing lowest and highest expression levels was over 25-fold. Furthermore, there was a significant correlation between expression levels of toxR and luxR and between the expression levels of these regulators and the protease genes. The expression levels of luxR, toxR and haemolysin were negatively correlated with the survival of brine shrimp larvae challenged with the isolates. Finally, a non-virulent, a moderately virulent and a strongly virulent isolate were selected to study in vivo expression of the virulence genes during infection of gnotobiotic brine shrimp larvae. The in vivo gene expression study showed a clear difference in virulence gene expression between both virulent isolates and the non-virulent isolate.

Evaluation of a loop-mediated isothermal amplification method for the rapid detection of Vibrio harveyi in cultured marine shellfish

AIMS

The purpose of this study was to establish a loop-mediated isothermal amplification (LAMP) method for the rapid, sensitive detection of Vibrio harveyi in mariculture shellfish.

METHODS AND RESULTS

A set of four primers, two outer and two inner primers, were designed from the toxR gene sequence of V. harveyi. The LAMP reaction was conducted at 65 degrees C for 60 min. There were no cross-reactions with other bacterial strains indicating a high specificity of the LAMP. The detection sensitivity of the LAMP assay for V. harveyi with both of pure cultures and added shellfish cultures is about 10(-5) dilution level (equivalent to 17.2 cells per reaction). The amplification products were detected by visual inspection using SYBR Green I. The detection sensitivity using the LAMP method was 10 times higher than that of conventional PCR.

CONCLUSIONS

The LAMP assay established in this study is an extremely specific, sensitive and rapid for identification of V. harveyi in mariculture shellfish.

SIGNIFICANCE AND IMPACT OF THE STUDY

This LAMP technique provides an important detecting tool for the detection of V. harveyi infection both in the laboratory and field. This technique is recommended as an applied protocol for health management programme and disease surveillance of in hatcheries as well as in grow-out pond, to prevent the disease outbreak.

Identification of vhhP2, a novel genetic marker of Vibrio harveyi, and its application in the quick detection of V. harveyi from animal specimens and environmental samples

AIMS

To investigate the species-specific prevalence of vhhP2 among Vibrio harveyi isolates and the applicability of vhhP2 in the specific detection of V. harveyi from crude samples of animal and environmental origins.

METHODS AND RESULTS

A gene (vhhP2) encoding an outer membrane protein of unknown function was identified from a pathogenic V. harveyi isolate. vhhP2 is present in 24 V. harveyi strains isolated from different geographical locations but is absent in 24 strains representing 17 different non-V. harveyi species, including V. parahaemolyticus and V. alginolyticus. A simple polymerase chain reaction method for the identification of V. harveyi was developed based on the conserved sequence of vhhP2. This method was demonstrated to be applicable to the quick detection of V. harveyi from crude animal specimens and environmental samples. The specificity of this method was tested by applying it to the examination of two strains of V. campbellii, which is most closely related to V. harveyi. One of the V. campbellii strains was falsely identified as V. harveyi.

CONCLUSIONS

vhhP2 is ubiquitously present in the V. harveyi species and is absent in most of the non-V. harveyi species; this feature enables vhhP2 to serve as a genetic marker for the rapid identification of V. harveyi. However, this method can not distinguish some V. campbellii strains from V. harveyi.

SIGNIFICANCE AND IMPACT OF THE STUDY

the significance of our study is the identification of a novel gene of V. harveyi and the development of a simple method for the relatively accurate detection of V. harveyi from animal specimens and environmental samples.

Real-time PCR assay for rapid detection and quantification of Vibrio aestuarianus in oyster and seawater: a useful tool for epidemiologic studies

Because Vibrio aestuarianus is known to cause serious infections in Pacific oyster Crassostrea gigas, a real-time PCR assay was developed targeting the dnaJ gene of this bacterium. Only V. aestuarianus strains isolated from C. gigas mortality events in different geographic areas and the reference strain tested positive, whereas no amplification products was obtained with type strains belonging to 23 other species of Vibrio. Sensitivity and reproducibility of the method were assessed using either seawater or oyster homogenate samples spiked with one V. aestuarianus strain. All these samples were stored at -20 degrees C in order to mimic retrospective or grouped natural sample analysis without quantification bias due to prolonged freezing. Analysis of standard curves revealed excellent correlation values between light microscopy cell enumerations and PCR Threshold Cycle (Ct) values, and acceptable PCR reaction efficiencies for all type of samples. Quantification curves of both sample types were equivalent, with a detection level as low as 1.6 V. aestuarianus cells in the PCR reaction tube, corresponding to 1.6 x 10(2) cells ml(-1) and 1.6 x 10(2) cells mg(-1) in seawater and entire oyster samples, respectively, taking into account the dilution factor used for appropriate template DNA preparation. Comparison of PCR assay reproducibility according to the complexity of samples revealed that seawater samples gave more reproducible quantification measures than samples from oyster homogenate, with precision of measured Ct values inferior to 0.4 and 0.6 respectively at 99% confidence. Use of the real-time PCR assay allowed us to monitor V. aestuarianus load in oysters naturally infected with this pathogen. Furthermore, we were able to detect V. aestuarianus in samples of seawater in which oysters had been reared and in algal cultures used for feeding oysters. Because of the rapidity and reliability of the real-time PCR assay method used in this study, just a few hours are needed compared with the two days required using the classic culture method, this technique will be particularly valuable in mollusc pathology laboratories, for monitoring the source and course of infections by V. aestuarianus in pathogenesis and epidemiologic studies, as well as for designing appropriate prophylactic control measures.