Cloning and expression of gene encoding the thermostable direct hemolysin from Vibrio alginolyticus strain HY9901, the causative agent of vibriosis of crimson snapper (Lutjanus erythopterus)

Purification and Characterization of the Lecithin-Dependent Thermolabile Hemolysin Vhe1 from the Vibrio sp. Strain MA3 Associated with Mass Mortality of Pearl Oyster (Pinctada fucata)

In a previous study, we isolated a Vibrio sp. strain MA3 and its virulence factor, a hemolysin encoded by vhe1. This strain is associated with mass mortalities of the pearl oyster Pinctada fucata. In the present study, the vhe1 gene from strain MA3 was cloned and its encoded product was purified and characterized. Our results show that the vhe1 gene encodes a protein of 417 amino acids with an estimated molecular mass of 47.2 kDa and a pI of 5.14. The deduced protein, Vhe1, was found to contain the conserved amino acid sequence (GDSL motif) of the hydrolase/esterase superfamily and five conserved blocks characteristic of SGNH hydrolases. A BLAST homology search indicated that Vhe1 belongs the lecithin-dependent hemolysin/thermolabile hemolysin (LDH/TLH) family. In activity analyses, the optimal temperature for both the hemolytic and phospholipase activities of Vhe1 was 50 °C. Vhe1 hemolytic activity and phospholipase activity were highest at pH 8.5 and pH 8.0, respectively. However, both enzymatic activities sharply decreased at high temperature (> 50 °C) and pH < 7.0. Compared with previously reported hemolysins, Vhe1 appeared to be more thermal- and pH-labile. Both its hemolytic activity and phospholipase activity were significantly inhibited by CuCl2, CdCl2, ZnCl2, and NiCl2, and slightly inhibited by MnCl2 and CoCl2. Vhe1 showed higher phospholipase activity toward medium-chain fatty acids (C8-C12) than toward shorter- and longer-chain fatty acids. These results accumulate knowledge about the LDH/TLH of V. alginolyticus, which detailed characterization has not been reported, and contribute to solving of the mass mortality of pearl oyster.

Effects of intestinal microbiota on physiological metabolism and pathogenicity of Vibrio

Vibrio species are disseminated broadly in the marine environment. Some of them can cause severe gastroenteritis by contaminating seafood and drinking water, such as Vibrio parahaemolyticus, Vibrio cholerae, and Vibrio vulnificus. However, their pathogenic mechanism still needs to be revealed to prevent and reduce morbidity. This review comprehensively introduces and discusses the common pathogenic process of Vibrio including adhesion, cell colonization and proliferation, and resistance to host immunity. Vibrio usually produces pathogenic factors including hemolysin, type-III secretion system, and adhesion proteins. Quorum sensing, a cell molecular communication system between the bacterial cells, plays an important role in Vibrio intestinal invasion and colonization. The human immune system can limit the virulence of Vibrio or even kill the bacteria through different responses. The intestinal microbiota is a key component of the immune system, but information on its effects on physiological metabolism and pathogenicity of Vibrio is seldom available. In this review, the effects of intestinal microorganisms and their metabolites on the invasion and colonization of common pathogenic Vibrio and VBNC status cells are discussed, which is conducive to finding the next-generation prebiotics. The strategy of dietary intervention is discussed for food safety control. Finally, future perspectives are proposed to prevent Vibrio infection in aquaculture.

The role of dctP gene in regulating colonization, adhesion and pathogenicity of Vibrio alginolyticus strain HY9901

Vibriosis caused by Vibrio alginolyticus has severely affected the development of mariculture industry in recent decades. DctP, a tripartite ATP-independent periplasmic transporter solute-binding subunit, is thought to be one of the virulence factors in Vibrio. In this study, the results displayed no difference in morphological characteristics and growth between ΔdctP (dctP mutant strain) and WT (wild-type strain). Nevertheless, the ability of swarming motility, biofilm formation, ECPase formation, cell adhesion and colonized ability of ΔdctP significantly decreased compared to those of WT. The LD₅₀ of ΔdctP significantly increased by 40-fold compared to that of WT. The transcriptome analysis demonstrated the deletion mutation of dctP could regulate the expression levels of 22 genes related to colonization, adhesion and pathogenicity in V. alginolyticus. The analysis of qRT-PCR showed the transcriptome data were reliable. These results reveal the effect of attenuated function of DctP on colonization, adherence and pathogenicity by controlling the expression of related gene.

Secretion Systems in Gram-Negative Bacterial Fish Pathogens

Bacterial fish pathogens are one of the key challenges in the aquaculture industry, one of the fast-growing industries worldwide. These pathogens rely on arsenal of virulence factors such as toxins, adhesins, effectors and enzymes to promote colonization and infection. Translocation of virulence factors across the membrane to either the extracellular environment or directly into the host cells is performed by single or multiple dedicated secretion systems. These secretion systems are often key to the infection process. They can range from simple single-protein systems to complex injection needles made from dozens of subunits. Here, we review the different types of secretion systems in Gram-negative bacterial fish pathogens and describe their putative roles in pathogenicity. We find that the available information is fragmented and often descriptive, and hope that our overview will help researchers to more systematically learn from the similarities and differences between the virulence factors and secretion systems of the fish-pathogenic species described here.

Effect of Food Matrix Type on Growth Characteristics of and Hemolysin Production by Vibrio alginolyticus

ABSTRACT

The growth of and hemolysin production by two V. alginolyticus strains (HY9901 and ATCC 17749T) at 30°C were investigated in briny tilapia, shrimp, scallop, oyster, pork, chicken, freshwater fish, and egg fried rice. Bacteria were enumerated by plate counting. Hemolysin production was evaluated with blood agar and hemolytic titer tests. The two V. alginolyticus strains had similar growth and hemolysin production patterns in all tested foods. Based on the goodness-of-fit primary model statistics (coefficient of determination, mean square error, bias factor, and accuracy factor), the modified Gompertz model was a better fit than the logistic model to V. alginolyticus growth in foods. Growth kinetic parameters of V. alginolyticus had a higher μmax and shorter λ in the following order: briny tilapia > shrimp > freshwater fish > egg fried rice > scallop > oyster > chicken > pork. V. alginolyticus levels were similar at the stationary phase, with no significant growth difference between raw and cooked foods. Significantly higher thermostable direct hemolysin activity (P < 0.05) was found for V. alginolyticus in the following order: briny tilapia > freshwater fish > shrimp > chicken > egg fried rice > scallop > oyster > pork. However, the hemolytic titer was not consistent with the thermostable direct hemolysin activity and was significantly higher (P < 0.05) in the following order: briny tilapia > egg fried rice > shrimp > freshwater fish > chicken > scallop > oyster > pork. Contrary to current belief, V. alginolyticus produced more hemolysin in some nonseafoods (freshwater fish, egg fried rice, and chicken) than in scallops or oysters. This report is the first on the growth and toxicity of V. alginolyticus in different food matrices and confirms that some nonseafoods can be contaminated with pathogenic V. alginolyticus. These results should increase awareness of nonseafood safety issues and improve the accuracy of V. alginolyticus risk assessments.

HIGHLIGHTS

First Succinylome Profiling of Vibrio alginolyticus Reveals Key Role of Lysine Succinylation in Cellular Metabolism and Virulence

Recent studies have shown that a key strategy of many pathogens is to use post-translational modification (PTMs) to modulate host factors critical for infection. Lysine succinylation (Ksuc) is a major PTM widespread in prokaryotic and eukaryotic cells, and is associated with the regulation of numerous important cellular processes. Vibrio alginolyticus is a common pathogen that causes serious disease problems in aquaculture. Here we used the affinity enrichment method with LC-MS/MS to report the first identification of 2082 lysine succinylation sites on 671 proteins in V. alginolyticus, and compared this with the lysine acetylation of V. alginolyticus in our previous work. The Ksuc modification of SodB and PEPCK proteins were further validated by Co-immunoprecipitation combined with Western blotting. Bioinformatics analysis showed that the identified lysine succinylated proteins are involved in various biological processes and central metabolism pathways. Moreover, a total of 1,005 (25.4%) succinyl sites on 502 (37.3%) proteins were also found to be acetylated, which indicated that an extensive crosstalk between acetylation and succinylation in V. alginolyticus occurs, especially in three central metabolic pathways: glycolysis/gluconeogenesis, TCA cycle, and pyruvate metabolism. Furthermore, we found at least 50 (7.45%) succinylated virulence factors, including LuxS, Tdh, SodB, PEPCK, ClpP, and the Sec system to play an important role in bacterial virulence. Taken together, this systematic analysis provides a basis for further study on the pathophysiological role of lysine succinylation in V. alginolyticus and provides targets for the development of attenuated vaccines.

Identification of Vibrio alginolyticus virulent strain-specific DNA regions by suppression subtractive hybridization and PCR

AIMS

Vibrio alginolyticus was frequently isolated from diseased farmed fish in the coaster waters of Hainan Island over the past two decades. In this study, we attempted to identify candidates of virulent strain-specific DNA regions for this pathogen.

METHODS AND RESULTS

Suppression subtractive hybridization (SSH) and PCR were successively performed between the typical virulent strain and avirulent strain of V. alginolyticus, in which they shared 99·54% homology of 16S rDNAs. Out of 2873 subtracted clones, nine clones were finally indicated to harbour virulent strain-specific DNA fragments. The receivable functions of the major fragments in the nine clones were believed to encode methyl-accepting chemotaxis protein (n = 1), type VI secretion system-associated FHA domain protein TagH (n = 1), diguanylate cyclase (n = 1), AraC family transcriptional regulator (n = 1), ABC-type uncharacterized transport system permease component (n = 1) and hypothetical proteins (n = 4). Two hypothetical proteins contain several disordered regions.

CONCLUSIONS

Some specific DNA regions existed in the virulent strain of V. alginolyticus, and the SSH assay could be a highly sensitive method for identifying virulent regions in pathogens.

SIGNIFICANCE AND IMPACT OF THE STUDY

This report is the first to describe the identification of virulent strain-specific DNA regions in the V. alginolyticus genome, which is helpful in developing virulent strain-specific rapid detection methods and is a pivotal precondition for clarifying the molecular virulence mechanism of V. alginolyticus.

A T3SS Regulator Mutant of Vibrio alginolyticus Affects Antibiotic Susceptibilities and Provides Significant Protection to Danio rerio as a Live Attenuated Vaccine

Vibrio alginolyticus is a major cause of Vibriosis in farmed marine aquatic animals and has caused large economic losses to the Asian aquaculture industry in recent years. Therefore, it is necessary to control V. alginolyticus effectively. The virulence mechanism of V. alginolyticus, the Type III secretion system (T3SS), is closely related to its pathogenicity. In this study, the T3SS gene tyeA was cloned from V. alginolyticus wild-type strain HY9901 and the results showed that the deduced amino acid sequence of V. alginolyticus tyeA shared 75–83% homology with other Vibrio spp. The mutant strain HY9901ΔtyeA was constructed by Overlap-PCR and homologous recombination techniques. The HY9901ΔtyeA mutant exhibited an attenuated swarming phenotype and an ~40-fold reduction in virulence to zebrafish. However, the HY9901ΔtyeA mutant showed no difference in growth, biofilm formation and ECPase activity. Antibiotic susceptibility test was observed that wild and mutant strains were extremely susceptible to Amikacin, Minocycline, Gentamicin, Cefperazone; and resistant to oxacillin, clindamycin, ceftazidime. In contrast wild strains are sensitive to tetracycline, chloramphenicol, kanamycin, doxycycline, while mutant strains are resistant to them. qRT-PCR was employed to analyze the transcription levels of T3SS-related genes, the results showed that compared with HY9901 wild type, ΔtyeA had increased expression of vscL, vscK, vscO, vopS, vopN, vscN, and hop. Following vaccination with the mutant strain, zebrafish had significantly higher survival than controls following infection with the wild-type HY9901 (71.2% relative percent survival; RPS). Analysis of immune gene expression by qPCR showed that vaccination with HY9901ΔtyeA increased the expression of IgM, IL-1β, IL-6, and TNF-α in zebrafish. This study provides evidence of protective efficacy of a live attenuated vaccine targeting the T3SS of V. alginolyticus which may be facilitated by up-regulated pro-inflammatory and immunoglobulin-related genes.

Fast, simple and highly specific molecular detection of Vibrio alginolyticus pathogenic strains using a visualized isothermal amplification method

Background

Vibrio alginolyticus is an important pathogen that has to be closely monitored and controlled in the mariculture industry because of its strong pathogenicity, quick onset after infection and high mortality rate in aquatic animals. Fast, simple and specific methods are needed for on-site detection to effectively control outbreaks and prevent economic losses. The detection specificity towards the pathogenic strains has to be emphasized to facilitate pointed treatment and prevention. Polymerase chain reaction (PCR)-based molecular approaches have been developed, but their application is limited due to the requirement of complicated thermal cycling machines and trained personnel.

Results

A fast, simple and highly specific detection method for V. alginolyticus pathogenic strains was established based on isothermal recombinase polymerase amplification (RPA) and lateral flow dipsticks (LFD). The method targeted the virulence gene toxR, which is reported to have good coverage for V. alginolyticus pathogenic strains. To ensure the specificity of the method, the primer-probe set of the RPA system was carefully designed to recognize regions in the toxR gene that diverge in different Vibrio species but are conserved in V. alginolyticus pathogenic strains. The primer-probe set was determined after a systematic screening of amplification performance, primer-dimer formation and false positive signals. The RPA-LFD method was confirmed to have high specificity for V. alginolyticus pathogenic strains without any cross reaction with other Vibrio species or other pathogenic bacteria and was able to detect as little as 1 colony forming unit (CFU) per reaction without DNA purification, or 170 fg of genomic DNA, or 6.25 × 10³ CFU/25 g in spiked shrimp without any enrichment. The method finishes detection within 30 min at temperatures between 35 °C and 45 °C, and the visual signal on the dipstick can be directly read by the naked eye. In an application simulation, randomly spiked shrimp homogenate samples were 100% accurately detected.

Conclusions

The RPA-LFD method developed in this study is fast, simple, highly specific and does not require complicated equipment. This method is applicable for on-site detection of V. alginolyticus pathogenic strains for the mariculture industry.

Vibrio Proteases for Biomedical Applications: Modulating the Proteolytic Secretome of V. alginolyticus and V. parahaemolyticus for Improved Enzymes Production

Proteolytic enzymes are of great interest for biotechnological purposes, and their large-scale production, as well as the discovery of strains producing new molecules, is a relevant issue. Collagenases are employed for biomedical and pharmaceutical purposes. The high specificity of collagenase-based preparations toward the substrate strongly relies on the enzyme purity. However, the overall activity may depend on the cooperation with other proteases, the presence of which may be essential for the overall enzymatic activity, but potentially harmful for cells and tissues. Vibrios produce some of the most promising bacterial proteases (including collagenases), and their exo-proteome includes several enzymes with different substrate specificities, the production and relative abundances of which strongly depend on growth conditions. We evaluated the effects of different media compositions on the proteolytic exo-proteome of Vibrio alginolyticus and its closely relative Vibrio parahaemolyticus, in order to improve the overall proteases production, as well as the yield of the desired enzymes subset. Substantial biological responses were achieved with all media, which allowed defining culture conditions for targeted improvement of selected enzyme classes, besides giving insights in possible regulatory mechanisms. In particular, we focused our efforts on collagenases production, because of the growing biotechnological interest due to their pharmaceutical/biomedical applications.

Protection against Vibrio alginolyticus in pearl gentian grouper (♀Epinephelus fuscoguttatus × ♂Epinephelus lanceolatu) immunized with an acfA-deletion live attenuated vaccine

Vibrio alginolyticus is well-known as an opportunistic Gram-negative pathogen, which endangers the development of global aquaculture as well as human health. In this study, a ΔacfA mutant strain and complementation of the ΔacfA mutant (C-acfA) were constructed. The ΔacfA mutant was tested in pearl gentian grouper (♀Epinephelus fuscoguttatus × ♂Epinephelus lanceolatu) to observe the changes in virulence and evaluate its potential as an attenuated live vaccine. The results showed that the ΔacfA mutant caused a high antibody titer and a significant reduction in the ability to colonize the intestine of pearl gentian grouper. Grouper vaccinated with ΔacfA mutant were more tolerant of the infection by virulent V. alginolyticus HY9901 without inducing clinical symptoms and obvious pathological changes. The relative percent survival value of pearl gentian grouper vaccinated with ΔacfA mutant intraperitoneal injection reached 81.1% after challenging with V. alginolyticus HY9901. The specific antibody titers immunized with ΔacfA was significantly higher than that in the PBS group. The antibody titer of ΔacfA group displayed the tendency of rising up from the first to fourth week and declining from fifth to eighth week and reached the peak at the fourth week. In the meanwhile, the expression level of genes associated with immunity, including IL-1β, TNF-α, IL-16, IgM, CD8α and MHC-Iα, was up-regulated after vaccination, indicating that the ΔacfA can induce effective and durable immune response in pearl gentian grouper and it may be an effective attenuated live vaccine candidate for the prevention of infections by V. alginolyticus.

Isolation and characterization of a virulence related Vibrio alginolyticus strain Wz11 pathogenic to cuttlefish, Sepia pharaonis

Vibrio alginolyticus is a ubiquitous marine opportunistic pathogen that can infect various hosts in marine environment. In the present study, V. alginolyticus strain Wz11 was isolated from diseased cuttlefish, Sepia pharaonis, with 20% of promoted death and high survival capacity in skin mucus and tissue liquid. Its growth, siderophore production, and expressions of haemolysin and swarming related genes were characterized under iron limited conditions. The minimal inhibitory concentration (MIC) of 2,2'-dipyridyl (DP) to V. alginolyticus strain Wz11 was 640 μM. While growth of V. alginolyticus strain Wz11 was inhibited by DP, production of iron-seizing substances, haemolytic activity and swarming motility were increased. Moreover, expressions of haemolysin related genes tlh, tdh and vah and flagellar related genes flgH, fliC, fliD and fliS were also characterized using real-time reverse transcriptase PCR. Expression of tdh was up-regulated to 7.7-fold, while expressions of tlh and vah were down-regulated to 0.016-fold and 0.03-fold, respectively. The expression of fliC, flgH, fliD and fliS was up-regulated to 4.9-, 3.8-, 8.6- and 4.5-fold, respectively. Concluded from our results suggested that V. alginolyticus strain Wz11 was considered as a potential pathogen of S. pharaonis, and iron level played an important role in the production of iron-seizing substances, and activities of haemolysin and bacterial swarming as well as their related gene expressions.

Expression of Collagenase is Regulated by the VarS/VarA Two-Component Regulatory System in Vibrio alginolyticus

Vibrio alginolyticus is an opportunistic pathogen in both humans and marine animals. Collagenase encoded by colA is considered to be one of the virulence factors. Expression of colA is regulated by multiple environmental factors, e.g., temperature, growth phase, and substrate. To elucidate the mechanism of regulation of colA expression, transposon mutagenesis was performed. VarS, a sensor histidine kinase of the two-component regulatory system, was demonstrated to regulate the expression of colA. VarA, a cognate response regulator of VarS, was also identified and shown to be involved in the regulation of colA expression. In vitro phosphorylation assays showed that phosphorylated VarS acted as a phosphoryl group donor to VarA. A site-directed mutagenesis study showed that the His300, Asp718 and His874 residues in VarS were essential for the phosphorylation of VarS, and the Asp54 residue in VarA was likely to receive the phosphoryl group from VarS. The results demonstrate that the VarS/VarA two-component regulatory system regulates the expression of collagenase in V. alginolyticus.

Potential antitumor therapeutic application of Grimontia hollisae thermostable direct hemolysin mutants

We report on the preparation of a new type of immunotoxin by conjugation of an epidermal growth factor receptor (EGFR)-binding peptide and an R46E mutation of thermostable direct hemolysin from Grimontia hollisae, (Gh-TDH^R46E/EB). The hybrid immunotoxin was purified to homogeneity and showed a single band with slight slower mobility than that of Gh-TDH^R46E. Cytotoxicity assay of Gh-TDH^R46E/EB on EGFR highly, moderately, low, and non-expressed cells, A431, MDA-MB-231, HeLa, and HEK293 cells, respectively, showed apparent cytotoxicity on A431 and MDA-MB-231 cells but not on HeLa or HEK293 cells. In contrast, no cytotoxicity was observed for these cells treated with either Gh-TDH^R46E or EB alone, indicating enhanced cytotoxic efficacy of Gh-TDH^R46E by the EGFR binding moiety. Further antitumor activity assay of Gh-TDH^R46E/EB in a xenograft model of athymic nude mice showed obvious shrinkage of tumor size and degeneration, necrosis, and lesions of tumor tissues compared to the normal tissues. Therefore, the combination of Gh-TDH^R46E with target affinity agents opens new possibilities for pharmacological treatment of cancers and potentiates the anticancer drug's effect.

Genetic characterization of trh positive Vibrio spp. isolated from Norway

The thermostable direct hemolysin (TDH) and/or TDH-related hemolysin (TRH) genes are carried by most virulent Vibrio parahaemolyticus serovars. In Norway, trh+ V. parahaemolyticus constitute 4.4 and 4.5% of the total number of V. parahaemolyticus isolated from blue mussel (Mytilus edulis) and water, respectively. The trh gene is located in a region close to the gene cluster for urease production (ure). This region was characterized in V. parahaemolyticus strain TH3996 and it was found that a nickel transport operon (nik) was located between the first gene (ureR) and the rest of the ure cluster genes. The organization of the trh-ureR-nik-ure gene cluster in the Norwegian trh+ isolates was unknown. In this study, we explore the gene organization within the trh-ureR-nik-ure cluster for these isolates. PCR analyses revealed that the genes within the trh-ureR-nik-ure gene cluster of Norwegian trh+ isolates were organized in a similar fashion as reported previously for TH33996. Additionally, the phylogenetic relationship among these trh+ isolates was investigated using Multilocus Sequence Typing (MLST). Analysis by MLST or ureR-trh sequences generated two different phylogenetic trees for the same strains analyzed, suggesting that ureR-trh genes have been acquired at different times in Norwegian V. parahaemolyticus isolates. MLST results revealed that some pathogenic and non-pathogenic V. parahaemolyticus isolates in Norway appear to be highly genetically related.

VscO, a putative T3SS chaperone escort of Vibrio alginolyticus, contributes to virulence in fish and is a target for vaccine development

Type III secretion system (T3SS) in Vibrio alginolyticus is essential for its pathogenesis. VscO's homologous proteins FliJ, InvI and YscO have been suggested to be putative chaperone escorts although its function in V. alginolyticus is unclear. To investigate the physiological role of VscO, a mutant strain of V. alginolyticus with an in-frame deletion of the vscO gene was constructed in the present study. One finding was that the mRNA expression levels of SycD, VopB and VopD proteins decreased in the ΔvscO mutant. In addition, the ΔvscO mutant showed an attenuated swarming ability and a ten-fold decrease in the virulence to fish. However, the ΔvscO mutant showed no difference in the biofilm formation and ECPase activity. Complementation of the mutant strain with the vscO gene could restore the phenotypes of the wild-type strain. Finally, the recombinant VscO protein caused a high antibody titer and an effective protection against lethal challenge with the wild-type strain V. alginolyticus. These results indicated that VscO protein has a specific role in the pathogenesis of V. alginolyticus and it may be a candidate antigen for development of a subunit vaccine against vibriosis.

The thermostable direct hemolysin from Grimontia hollisae causes acute hepatotoxicity in vitro and in vivo

Background

G. hollisae thermostable direct hemolysin (Gh-TDH) is produced by most strains of G. hollisae. This toxin has been reported to be absorbed in the intestines in humans. Secondary liver injury might be caused by venous return of the toxin through the portal system. We aimed to firstly analyze the in vitro and in vivo hepatotoxicity of Gh-TDH.

Methods

Liver cells (primary human non-cancer cell and FL83B mouse cells) were treated and mice (BALB/c) were fed with this toxin to investigate its hepatotoxicity. Morphological examination and cytotoxicity assays using liver cells were also performed. Fluorescein isothiocyanate-conjugated toxin was used to analyze the localization of this protein in liver cells. Mice were subjected to liver function measurements and liver biopsies following toxin treatment and wild-type bacterial infection. PET (positron emission tomography)/CT (computed tomography) images were taken to assess liver metabolism during acute injury and recovery.

Results

The effect of hepatotoxicity was dose and time dependent. Cellular localization showed that the toxin was initially located around the cellular margins and subsequently entered the nucleus. Liver function measurements and liver biopsies of the mice following treatment with toxin or infection with wild-type Grimontia hollisae showed elevated levels of transaminases and damage to the periportal area, respectively. The PET/CT images revealed that the reconstruction of the liver continued for at least one week after exposure to a single dose of the toxin or bacterial infection.

Conclusions

The hepatotoxicity of Gh-TDH was firstly demonstrated. The damage was located in the periportal area of the liver, and the liver became functionally insufficient.

Induction of apoptosis in sea bream fibroblasts by Vibrio harveyi haemolysin and evidence for an anti-apoptotic role of heat shock protein 70

In this study, we exposed black sea bream, Mylio macrocephalus (Basilewsky), fibroblast (BSF) and silver sea bream, Sparus sarba Forsskål, fibroblast (SSF) cell lines to a recombinant Vibrio harveyi haemolysin (VHH) and investigated mechanisms involved in apoptosis. A decrease in mitochondrial membrane potential, followed by an increase in caspase 3 activity, occurred within 2-8 h of VHH exposure, in both cell lines; however, VHH did not alter cellular levels of reactive oxygen species. As heat shock protein 70 (HSP70) is known to prevent the onset of apoptosis in certain mammalian cells, we aimed to test whether such a protective effect is operative in VHH-exposed fibroblasts. The amounts of HSP70 were elevated in SSF and BSF via an acute heat shock or an acute heat shock followed by a 6 h recovery. It was found that the VHH-mediated reduction in mitochondrial membrane potential was suppressed in cells that had a 6 h post-heat shock recovery, and the protective effect of heat shock-induced HSP70 was attenuated following treatment of cells with the HSP70 inhibitor, quercetin. This study demonstrates how haemolysin causes cell death via induction of apoptosis and provides evidence as to the role of HSP70 as an anti-apoptotic factor.

Development of loop-mediated isothermal amplification method for rapid detection of Streptococcus iniae, the causative agent of streptococcicosis in fish

Streptococcus iniae is a major pathogen that causes sever economic losses in tilapia aquaculture. A set of four specific primers was designed by targeting lctO gene. With Bst DNA polymerase, the target DNA can be clearly amplified for 60 min at 64 °C in a simple water bath. The sensitivity of the LAMP assay for the detection of S. iniae is about 12.4 cells per reaction in both of pure cultures and added fish tissues cultures. LAMP products could be judged with agar gel or naked eye after addition of SYBR Green I. There were no cross-reactions with other bacterial strains indicating high specificity of the LAMP. The LAMP method was also applied to detect S. iniae-infected tilapia tissues effectively. The LAMP assay reported here indicates the potential usefulness of the technique as a valuable simple, rapid alternative procedure for the detection of S. iniae during streptococcicosis monitoring of cultured fish.

Identification of a luxO-regulated extracellular protein Pep and its roles in motility in Vibrio alginolyticus

The fish pathogen Vibrio alginolyticus virulence factors and regulation mechanism are barely known except the quorum sensing regulated extracellular products. In the present study, the cleavable isotope-coded affinity tags (cICATs) were used to analyze the differentially expressed extracellular proteins of V. alginolyticus wild-type and a ΔluxO mutant, and a putative peptidase Pep was identified. Quantitative real-time reverse transcription PCR confirmed that the transcription of pep was down-regulated by LuxO. Furthermore, western blotting demonstrated that the expression of Pep was cell-density dependent and quorum sensing regulated in V. alginolyticus. The pep in-frame mutant abolished the swimming and swarming motility, suggesting that it was involved in motility regulation in V. alginolyticus. In liquid medium, the polar flagellum was observed intact in ∆pep mutant while the chemotaxis related gene cheY, which is a response regulator causing change in direction of flagellar rotation, was down-regulated in ∆pep mutant. When ∆pep mutant was grown on hard agar plate, the lateral flagella disappeared and the transcriptions of flagellar biogenesis genes flaK, fliS, lafK, and lafA were down-regulated in ∆pep mutant. These results suggested that Pep was a luxO-regulated extracellular protein and involved in the motility of V. alginolyticus.

Loop-mediated isothermal amplification method for rapid detection of Vibrio alginolyticus, the causative agent of vibriosis in mariculture fish

AIMS

The purpose of this study was to develop a loop-mediated isothermal amplification (LAMP) method for the rapid, sensitive and simple detection of Vibrio alginolyticus in mariculture fish.

METHODS AND RESULTS

LAMP primers were designed by targeting the gyrB gene. With Bst DNA polymerase, the target DNA can be clearly amplified for 60 min at 64 degrees C in a simple water bath. The detection sensitivity of the LAMP assay for the detection of V. alginolyticus is about 3.7 x 10(2) CFU ml(-1) (3.7 CFU per reaction). LAMP products could be judged with agar gel or naked eye after the addition of SYBR Green I. There were no cross-reactions with other bacterial strains indicating a high specificity of the LAMP. The LAMP method was applied to detect V. alginolyticus-infected fish tissues effectively.

CONCLUSIONS

The LAMP established in this study is a simple, sensitive, specific, inexpensive and rapid protocol for the detection of V. alginolyticus.

SIGNIFICANCE AND IMPACT OF THE STUDY

This LAMP method provides an important diagnostic tool for the detection of V. alginolyticus infection both in the laboratory and field.