Pathogenicity of Edwardsiella tarda to olive flounder, Paralichthys olivaceus (Temminck & Schlegel)

Identification of the matrix metalloproteinase (MMP) gene family in Japanese flounder (Paralichthys olivaceus): Involved in immune response regulation to temperature stress and Edwardsiella tarda infection

The Matrix metalloproteinase (MMP) gene family is responsible for regulating the degradation of Extra Cellular Matrix (ECM) proteins, which are important for physiological processes such as wound healing, tissue remodeling, and stress response. Although MMPs have been studied in many species, their role in immune response in Japanese flounder (Paralichthys olivaceus) is still not fully understood. This study conducted a comprehensive analysis of MMPs in flounder, including gene structures, evolutionary relationships, conserved domains, molecular evolution, and expression patterns. Analysis revealed that MMP genes could be grouped into 17 subfamilies and were evolutionarily conserved and functionally-constrained. Meanwhile, MMP genes were found to express in different embryonic and larval stages and might play the role of sentinel in healthy tissues. Furthermore, expression profiling showed that MMPs had diverse functions in environmental stress, with 60% (9/15) and 73% (11/15) of MMPs showing differential expression patterns under temperature stress and Edwardsiella tarda (E. tarda) infection, respectively. These findings provide a useful resource for understanding the immune functions of MMP genes in Japanese flounder.

TNFR2 is a regulatory target of pol-miR-194a and promotes the antibacterial immunity of Japanese flounder Paralichthys olivaceus

MicroRNAs (miRNAs) are regulatory RNAs that modulate target gene expression after transcription. Pol-miR-194a had been reported to be a miRNA of Japanese flounder (Paralichthys olivaceus) involved in Edwardsiella tarda infection. Here, we identified tumor necrosis factor receptor 2 (TNFR2) as a target gene of pol-miR-194a. Pol-miR-194a markedly repressed the protein expression of flounder TNFR2 (PoTNFR2) via specific interaction with the 3'UTR of PoTNFR2. PoTNFR2 responded to E. tarda infection in a manner that was opposite to that of pol-miR-194a and inhibited E. tarda invasion by activating the NF-κB pathway. Consistently, dysregulation of PoTNFR2 had a significant impact on E. tarda dissemination in flounder tissues. Together, these results add new insights into the regulation mechanism and immune function of fish TNFR2 and pol-miR-194a.

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.

Vitellogenin-derived fragment in embryos of Japanese flounder Paralichthys olivaceus with binding and bactericidal activities against an infectious bacterium via an interaction with saccharides

Thirty- and 90-kDa proteins with binding ability to Edwardsiella tarda, a causative bacterium of Edwardsiellosis in fish, were purified from the embryo of Japanese flounder Paralichthys olivaceus. The proteins were isolated with affinity chromatography, in which the bacterium was used as a ligand and galactose, mannose, and ethylenediaminetetraacetic acid (EDTA) were used as elution agents, followed by gel filtration chromatography. N-terminal amino acid sequencing and liquid chromatography with quadrupole time-of-flight tandem mass spectrometry (LC/Q-TOF-MS) analysis revealed that the 90-kDa protein was lipovitellin heavy-chain (LvH), which is one of the proteolytically cleaved products of maternal vitellogenin (Vg) and represents the main precursor of the egg yolk in teleosts, and the 30-kDa protein was an N-terminal bit of LvH. On the other hand, Vg in the serum of the mother fish did not bind to E. tarda. While the 90-kDa protein did not show anti-bacterial activity, the 30-kDa protein strongly exhibited activity toward E. tarda, with a minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) below 0.06 μM, suggesting that the latter protein plays an important role during embryogenesis in the flounder. This is the first report showing that Vg-derived products have monosaccharides-binding activity and a fragment derived from LvH exhibits bactericidal activity.

Potential Efficacy of Chitosan-Poly (Lactide-Co-Glycolide)-Encapsulated Trivalent Immersion Vaccine in Olive Flounder (Paralichthys olivaceus) Against Viral Hemorrhagic Septicemia Virus, Streptococcus parauberis Serotype I, and Miamiensis avidus (Scuticociliate)

Olive flounder (Paralichthys olivaceus) is the most valuable aquaculture species in Korea, corresponding to ~60% of its total production. However, infectious diseases often break out among farmed flounders, causing high mortality and substantial economic losses. Although some deleterious pathogens, such as Vibrio spp. and Streptococcus iniae, have been eradicated or contained over the years through vaccination and proper health management, the current disease status of Korean flounder shows that the viral hemorrhagic septicemia virus (VHSV), Streptococcus parauberis, and Miamiensis avidus are causing serious disease problem in recent years. Furthermore, these three pathogens have differing optimal temperature and can attack young fingerlings and mature fish throughout the year-round culture cycle. In this context, we developed a chitosan-poly(lactide-co-glycolide) (PLGA)-encapsulated trivalent vaccine containing formalin-killed VHSV, S. parauberis serotype-I, and M. avidus and administered it to olive flounder fingerlings by immersion route using a prime-boost strategy. At 35 days post-initial vaccination, three separate challenge experiments were conducted via intraperitoneal injection with the three targeted pathogens at their respective optimal temperature. The relative percentages of survival were 66.63%, 53.3%, and 66.75% in the group immunized against VHSV, S. parauberis serotype-I, and M. avidus, respectively, compared to the non-vaccinated challenge (NVC) control group. The immunized fish also demonstrated significantly (p < 0.05) higher specific antibody titers in serum and higher transcript levels of Ig genes in the mucosal and systemic tissues than those of NVC control fish. Furthermore, the study showed significant (p < 0.05) upregulation of various immune genes in the vaccinated fish, suggesting induction of strong protective immune response, ultimately leading to improved survival against the three pathogens. Thus, the formulated mucosal vaccine can be an effective prophylactic measure against VHS, streptococcosis, and scuticociliatosis diseases in olive flounder.

Japanese flounder pol-miR-3p-2 suppresses Edwardsiella tarda infection by regulation of autophagy via p53

MicroRNAs (miRNAs) are post-transcriptional regulators that play vital roles in diverse physiological processes including immunity. In this study, we investigated the regulatory mechanism and function of a novel Japanese flounder (Paralichthys olivaceus) miRNA, pol-miR-3p-2. pol-miR-3p-2 was responsive in expression to the infection of the bacterial pathogen Edwardsiella tarda. pol-miR-3p-2 negatively regulated the expression of p53 through interaction with the 3'UTR of p53. Overexpression of pol-miR-3p-2 promoted autophagy, resulting in augmented production of LC3-II, while knockdown of p53 increased the level of beclin, a key factor of autophagy. In vivo and in vitro studies showed that E. tarda infection induced autophagy in flounder, and pol-miR-3p-2 inhibited the infectivity of E. tarda. Together these results indicate that pol-miR-3p-2 regulates autophagy through the target gene p53, thus revealing a regulatory link between p53 and autophagy in teleost, and that pol-miR-3p-2 plays an important role in the immune defense against E. tarda.

Edwardsiella piscicida: a significant bacterial pathogen of cultured fish

Edwardsiella piscicida, a Gram-negative, facultative aerobic pathogen belonging to the Enterobacteriaceae family, is the etiological agent of edwardsiellosis in fish and a significant problem in global aquaculture. E. piscicida has been reported from a broad geographical range and has been isolated from more than 20 fish host species to date, but this is likely to be an underestimation, because misidentification of E. piscicida as other species within the genus remains to be resolved. Common clinical signs associated with edwardsiellosis include, but are not limited to, exophthalmia, haemorrhages of the skin and in several internal organs, mild to moderate dermal ulcerations, abdominal distension, discoloration in the fish surface, and erratic swimming. Many antibiotics are currently effective against E. piscicida, although legal restrictions and the cost of medicated feeds have encouraged significant research investment in vaccination for the management of edwardsiellosis in commercial aquaculture. Here we summarise the current understanding of E. piscicida and highlight the difficulties with species assignment and the need for further research on epidemiology and strain variability.

Intracellular Bacterial Infections: A Challenge for Developing Cellular Mediated Immunity Vaccines for Farmed Fish

Aquaculture is one of the most rapidly expanding farming systems in the world. Its rapid expansion has brought with it several pathogens infecting different fish species. As a result, there has been a corresponding expansion in vaccine development to cope with the increasing number of infectious diseases in aquaculture. The success of vaccine development for bacterial diseases in aquaculture is largely attributed to empirical vaccine designs based on inactivation of whole cell (WCI) bacteria vaccines. However, an upcoming challenge in vaccine design is the increase of intracellular bacterial pathogens that are not responsive to WCI vaccines. Intracellular bacterial vaccines evoke cellular mediated immune (CMI) responses that “kill” and eliminate infected cells, unlike WCI vaccines that induce humoral immune responses whose protective mechanism is neutralization of extracellular replicating pathogens by antibodies. In this synopsis, I provide an overview of the intracellular bacterial pathogens infecting different fish species in aquaculture, outlining their mechanisms of invasion, replication, and survival intracellularly based on existing data. I also bring into perspective the current state of CMI understanding in fish together with its potential application in vaccine development. Further, I highlight the immunological pitfalls that have derailed our ability to produce protective vaccines against intracellular pathogens for finfish. Overall, the synopsis put forth herein advocates for a shift in vaccine design to include CMI-based vaccines against intracellular pathogens currently adversely affecting the aquaculture industry.

Genomic Analysis of miR-21-3p and Expression Pattern with Target Gene in Olive Flounder

MicroRNAs (miRNAs) act as regulators of gene expression by binding to the 3’ untranslated region (UTR) of target genes. They perform important biological functions in the various species. Among many miRNAs, miR-21-3p is known to serve vital functions in development and apoptosis in olive flounder. Using genomic and bioinformatic tools, evolutionary conservation of miR-21-3p was examined in various species, and expression pattern was analyzed in olive flounder. Conserved sequences (5’-CAGUCG-3’) in numerous species were detected through the stem-loop structure of miR-21-3p. Thus, we analyzed target genes of miR-21-3p. Among them, 3’ UTR region of PPIL2 gene indicated the highest binding affinity with miR-21-3p based on the minimum free energy value. The PPIL2 gene showed high expression levels in testis tissue of the olive flounder, whereas miR-21-3p showed rather ubiquitous expression patterns except in testis tissue, indicating that miR-21-3p seems to control the PPIL2 gene expression in a complementary repression manner in various tissues of olive flounder. Taken together, this current study contributes to infer the target gene candidates for the miR-21-3p using bioinformatics tools. Furthermore, our data offers important information on the relationship between miR-21-3p and target gene for further functional study.

Combined effects of Chinese medicine feed and ginger extract bath on co-infection of Ichthyophthirius multifiliis and Dactylogyrus ctenopharyngodonid in grass carp

Dactylogyrus ctenopharyngodonid and Ichthyophthirius multifiliis are two important ectoparasites of freshwater fish. Co-infection by the two parasites leads to high fish mortality and results in heavy economic losses. This study aimed to evaluate the efficacy of medicated feed and a ginger extract bath against D. ctenopharyngodonid and I. multifiliis on grass carp and investigate the hematological response of grass carp co-infected by the two parasites. These results demonstrated that red blood cell (RBC) and thrombocyte percentage among leucocytes significantly decreased after grass carp were co-infected by D. ctenopharyngodonid and I. multifiliis. The monocyte and neutrophil percentages significantly increased with the increment of parasite mean intensities, while the lymphocyte percentage decreased. The activities of serum acid phosphatase (ACP), alkaline phosphatase (AKP), lysozyme (LZM), and superoxide dismutase (SOD) significantly increased after co-infection. When grass carp treated with medicated feed containing 4% of Astragalus membranaceus, Allium sativum, Morus alba, and Glycyrrhiza uralensis, the activities of ACP, AKP, LZM, and SOD were significantly enhanced, and the mean intensities of D. ctenopharyngodonid and I. multifiliis were significantly decreased. When grass carp was treated with medicated feed and a 4-mg/L ginger extract bath, all parasites were eliminated during 28 days. The bath of ginger extract at a concentration of 4 mg/L kept a low mean intensity of I. multifiliis and D. ctenopharyngodonid, then the two parasites were eliminated by oral administration of the medicated feed with an immunostimulant (Chinese medicine compound).

Three-dimensional visualization of green fluorescence protein-labelled Edwardsiella tarda in whole Medaka larvae

The invasive fish pathogen Edwardsiella tarda is common in aquatic environments and causes the environmentally and economically destructive emphysematous putrefactive disease called edwardsiellosis. In order to understand the organism's infection pathway, medaka larvae (Oryzias latipes) were immersion-infected with E. tarda labelled with green fluorescence protein (GFP) and then visualized in three dimensions under confocal laser microscopy and light-sheet fluorescence microscopy. Confocal microscopy revealed GFP-labelled E. tarda in the mouth, head, gill bridges, gill cover, skin, membrane fin, gastrointestinal tract and air bladder, and in the caudal vein, somite veins, caudal artery and caudal capillaries. Light-sheet microscopy additionally showed GFP-labelled E. tarda in the pharyngeal cavity, muscle of the pectoral fin and cardiac atrium and ventricle. These findings suggest that during its infection of fish, E. tarda initially adheres to, and invades, the epithelial cells of the skin, gills and gastrointestinal tract (through the pharyngeal cavity); E. tarda then enters the blood vessels to access organs, including the air bladder and heart.

Proteomic characterization of the interactions between fish serum proteins and waterborne bacteria reveals the suppression of anti-oxidative defense as a serum-mediated antimicrobial mechanism

Fish blood is one of the crucial tissues of innate immune system, but the full repertoire of fish serum components involved in antibacterial defense is not fully identified. In this study, we demonstrated that turbot serum, but not the heat-inactivated control, significantly reduced the number of Edwardsiella tarda (E. tarda). By conjugating serum proteins with fluorescent dyes, we showed that E. tarda were coated with multiple fish proteins. In order to identify these proteins, we used E. tarda to capture turbot serum proteins and subjected the samples to shotgun proteomic analysis. A total of 76 fish proteins were identified in high confidence, including known antimicrobial proteins such as immunoglobins and complement components. 34 proteins with no previously known immunological functions were also identified. The expression of one of these proteins, IQ motif containing H (IQCH), was exclusively in fish brain and gonads and was induced during bacterial infection. This approach also allowed the study of the corresponding proteomic changes in E. tarda exposed to turbot serum, which is a general decrease of bacterial protein expression except for an upregulation of membrane components after serum treatment. Interestingly, while most other known stresses stimulate bacterial antioxidant enzymes, fish serum induced a rapid suppression of antioxidant proteins and led to an accumulation of reactive oxygen species. Heat treatment of fish serum eliminated this effect, suggesting that heat labile factors in the fish serum overrode bacterial antioxidant defenses. Taken together, this work offers a comprehensive view of the interactions between fish serum proteins and bacteria, and reveals previously unknown factors and mechanisms in fish innate immunity.

Comparative proteomic analysis unravels a role for EsrB in the regulation of reactive oxygen species stress responses in Edwardsiella piscicida

As a leading pathogen, Edwardsiella piscicida can cause hemorrhagic septicemia in fish and gastro-intestinal infections in humans. The two-component regulatory system EsrA-EsrB plays essential roles in pathogenesis through the type III and type VI secretion systems, and hemolysin production in E. piscicida It is unclear whether other virulence- or stress response-associated genes are regulated by EsrA-EsrB. In this study, the proteomes of wild-type E. piscicida EIB202 and esrB mutant strains were compared to reveal EsrB regulon components after growth in Luria-Bertani broth (LB). Overall, the expression levels of nine genes exhibited significant changes, and five of them required the presence of EsrB, while others exhibited higher levels in the esrB mutant. The diverse functions of these proteins were identified, including amino acid metabolism, oxidative stress defense and energy production. Interestingly, superoxidase dismutase and thiol peroxidase were the most significantly down-regulated by EsrB. Furthermore, other reported reactive oxygen species (ROS) resistance-related genes were also down-regulated by EsrB as revealed by quantitative real-time. Compared with the wild-type and the complement strain esrB⁺, ΔesrB displayed a significantly enhanced ROS resistance. These results demonstrated that EsrB plays important roles in the ROS resistance pathway in E. piscicida grown in LB conditions.

Eha, a regulator of Edwardsiella tarda, required for resistance to oxidative stress in macrophages

Edwardsiella tarda is distributed widely in a variety of hosts. Eha has recently been found to be its virulence regulator. In order to explore the mechanism of its regulation, we investigated the survival rates of wild type strain ET13, and its eha mutant and complemented strains in RAW264.7 macrophages under light microscopic observation as well as by counting bacterial CFUs on the plates. All of the different strains could live within the macrophages; however, the intracellular numbers of the wild type were significantly higher than the mutant when the incubation time extended 4 h or 6 h (P < 0.05). Furthermore, more ROS were produced by the mutant-infected cells, indicating that Eha may enhance ET13's capacity to detoxify ROS. In agreement with this, we found that the mutant exhibited more sensitivity by H₂O₂ disk inhibitory assay and less survival ability with H₂O₂ treatment. We further demonstrated that the bacterial antioxidant enzymes SodC and KatG were regulated by Eha with qRT-PCR and β-galactosidase assay. Collectively, our data show Eha is required for E. tarda to resist the oxidative stress from the macrophages.

Enhanced immune response and resistance to edwardsiellosis following dietary chitooligosaccharide supplementation in the olive flounder (Paralichthys olivaceus)

This study was conducted to evaluate the effects of dietary chitooligosaccharide (COS) supplementation on peripheral leukocyte count, head kidney leukocyte phagocytic rate, phagocytic index, respiratory burst activity, serum lysozyme activity, and immune protection in Paralichthys olivaceus. A total of 300 flounder with an average body weight of 80-100 g were randomly assigned into four dietary groups: (I) basic diet (control), basic diet containing (II) 0.5% COS, and (III) 1% COS, fed continuously for 28 d, and (IV) basic diet containing 1% COS fed in 14 d intervals. Continuous feeding of 0.5% and 1% COS diets for 28 d significantly increased the number of peripheral leukocytes, head kidney leukocyte phagocytic rate, phagocytic index, respiratory burst activity, and serum lysozyme activity (P < 0.05 or P < 0.01). After a 10 d Edwardsiella tarda challenge, the immune protection rates in the 0.5% and 1% COS groups were 30% and 60%, respectively. No control fish survived the E. tarda challenge treatment. Most immune indices were slightly lower after removal of COS from the diet for 14 d, but all immune indices were observed to recover after another 14 d of COS supplementation. This study demonstrates that supplementation of a basic diet with COS enhances the non-specific immune response and improves survival rates following infection with E. tarda in P. olivaceus. An optimized interval feeding strategy with diets containing 1% COS may have potential applications in the prevention of disease in aquacultured P. olivaceus.

Immunostimulatory effects of artificial feed supplemented with a Chinese herbal mixture on Oreochromis niloticus against Aeromonas hydrophila

The effects of a Chinese herbal mixture (CHM) composed of astragalus, angelica, hawthorn, Licorice root and honeysuckle on immune responses and disease resistant of Nile tilapia (Oreochromis niloticus GIFT strain) were investigated in present study. Fish were fed diets containing 0 (control), 0.5%, 1.0%, 1.5% or 2.0% CHM (w/w) for 4 weeks. And series of immune parameters including lysozyme, cytokine genes TNF-α and IL-1β, superoxide dismutase (SOD), peroxidase (POD), malondialdehyde (MDA) were measured during test period. After four weeks of feeding, fish were infected with Aeromonas hydrophila and mortalities were recorded. Results of this study showed that feeding Nile tilapia with CHM-supplementation diet stimulated lysozyme activity, SOD activity and POD activity in serum, induced TNF-α and IL-1β mRNA expression in head kidney and spleen, but decreased serum MDA content. All CHM-supplemental groups showed reduced mortalities following A. hydrophila infection compared with the group fed the control diet. These results suggested that this CHM can be applied as a tilapia feed supplement to elevate fish immunity and disease resistance against A. hydrophila.

Immunostimulatory effect of artificial feed supplemented with indigenous plants on Clarias gariepinus against Aeromonas hydrophila

The antibacterial activity of methanol extracts of Ficus benghalensis (prop-root) and Leucaena leucocephala (pod seed) was evaluated by measurement of zone of inhibition against pathogenic bacteria, Escherichia coli and Aeromonas hydrophila. Control artificial feed and artificial feed supplemented with 5% powder of F. benghalensis and L. leucocephala were prepared. Juvenile Clarias gariepinus were divided into four groups, acclimatized to laboratory conditions and fed with respective feeds for 20 days prior to the experiment. Immunomodulatory response of supplementary feed was studied by challenging the fish intraperitoneally at weekly intervals, with A. hydrophila. One set of fish, not challenged with A. hydrophila was used as a negative control, to analyze any detrimental effect of supplementary feed, while positive control, comprised of challenged fish fed with non-supplemented feed. Other two groups of fish were challenged with A. hydrophila and fed with respective supplementary feeds. Blood was collected on weekly intervals for four weeks and serum samples were analyzed to evaluate the damage of fish by A. hydrophila through liver function tests. The increase in the levels of Serum glutamic oxaloacetic transaminase (SGOT) and Serum Glutamic pyruvate transaminase (SGPT) in positive control group indicated the damage of liver & kidney. However the levels did not change significantly in fish fed with supplementary feeds when compared to negative control group. Nitric oxide, SOD, ALP and lipid peroxidase indicated lower stress levels in these fish compared to positive control. Fish fed with supplementary feed showed increased lysozyme activity and phagocytic index indicating an increase in non-specific immune response. The immunoglobulin levels of in serum were analyzed by homologous sandwich ELISA, which showed higher antibody production in fish fed with supplementary feed. The current study suggests conclusively, immunostimulatory role of F. benghalensis (prop-roots) and L. leucocephala (pod seed) in C. gariepinus when supplemented in artificial feed.

Mutations of flagellar genes fliC12, fliA and flhDC of Edwardsiella tarda attenuated bacterial motility, biofilm formation and virulence to fish

AIMS

The aim of this study was to investigate functions of flagellar genes fliC2, fliC12, fliA and flhDC in a bacterial fish pathogen Edwardsiella tarda.

METHODS AND RESULTS

In this study, functions of flagellar genes, fliC2, fliC12 (fliC1 + fliC2), fliA and flhDC (flhD + flhC) of Edw. tarda H1 were analysed by constructing in-frame deletion mutants respectively and complementary strains fliC2(+) and fliA(+) . Electron microscopy revealed that in-frame deletion of fliC12, fliA and flhDC significantly impaired the number and length of flagellar filaments, resulting in loss of both swimming and swarming motilities of the bacteria. In addition, compared to the wild-type strain and complementary strains, the flagellum-impaired mutants exhibited reduced biofilm formation ability, showed decreased ability in adherence and internalization to Epithelioma papulosum cyprini (EPC) cells and reduced pathogenicity to zebrafish.

CONCLUSIONS

These results indicated that fliC12, fliA and flhDC of Edw. tarda played essential roles in flagellar filaments structure, bacteria motility, biofilm formation, adherence, internalization and pathogenicity of this bacterium.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study revealed that flagella function in facilitating virulence and it may provide a new target for vaccines against Edw. tarda infection.

Effects of dietary supplementation of citrus by-products fermented with a probiotic microbe on growth performance, innate immunity and disease resistance against Edwardsiella tarda in juvenile olive flounder, Paralichthys olivaceus (Temminck & Schlegel)

Two consecutive studies were conducted to evaluate the dietary supplementation of citrus by-products (CB) fermented with probiotic bacteria on growth performance, feed utilization, innate immune responses and disease resistance of juvenile olive flounder. In Experiment I, five diets were formulated to contain 0% (control) or 3% four different CB fermented with Bacillus subtilis (BS), Enterococcus faecium (EF), Lactobacillus rhamnosus (LR) and L. plantarum (LP) (designated as CON, CBF-BS, CBF-EF, CBF-LR and CBF-LP, respectively). During 10 weeks of a feeding trial, growth performance and feed efficiency were not significantly different among all the fish groups. However, fish fed CBF containing diets had significantly higher survivals than the CON group. Disease resistance of fish against Edwardsiella tarda was increased by the fermentation of CB. In Experiment II, we chose the BS as a promising probiotic and formulated five diets to contain 0%, 2%, 4%, 6% and 8% CBF-BS. Growth performance was not significantly affected by the CBF-BS supplementation during 6 weeks of a feeding trial. Innate immunity of fish was significantly enhanced by CBF-BS supplementation. Myeloperoxidase and lysozyme activities were increased in a dose-dependent manner by dietary CBF-BS inclusions. In a consecutive challenge test against E. tarda, an increased disease resistance was found by CBF-BS supplementation. These studies indicate that the fermentation process of CB with probiotic has beneficial effects on innate immunity and thereby increases disease resistance of olive flounder against E. tarda. Bacillus subtilis can be used as a promising probiotic microbe for by-product fermentation in fish feeds.

Selection of normalization factors for quantitative real time RT-PCR studies in Japanese flounder (Paralichthys olivaceus) and turbot (Scophthalmus maximus) under conditions of viral infection

Disease outbreaks caused by iridoviruses are known to affect farmed flounder (Paralichthys olivaceus) and turbot (Scophthalmus maximus). To facilitate quantitative real time RT-PCR (qRT-PCR) analysis of gene expression in flounder and turbot during viral infection, we in this study examined the potentials of 9 housekeeping genes of flounder and turbot as internal references for qRT-PCR under conditions of experimental infection with megalocytivirus, a member of the Iridoviridae family. The mRNA levels of the 9 housekeeping genes in the brain, gill, heart, intestine, kidney, liver, muscle, and spleen of flounder and turbot were determined by qRT-PCR at 24h and 72h post-viral infection, and the expression stabilities of the genes were determined with geNorm and NormFinder algorithms. The results showed that (i) viral infection induced significant changes in the mRNA levels of the all the examined genes in a manner that was dependent on both tissue type and infection stage; (ii) for a given time point of infection, stability predictions made by the two algorisms were highly consistent for most tissues; (iii) the optimum reference genes differed at different infection time points at least in some tissues; (iv) at both examined time points, no common reference genes were identified across all tissue types. These results indicate that when studying gene expression in flounder and turbot in relation to viral infection, different internal references may have to be used not only for different tissues but also for different infection stages.

PoCRIP1, Paralichthys olivaceus cysteine-rich intestinal protein 1: molecular characterization, expression analysis upon Edwardsiella tarda challenge and a possible role in the immune regulation

Cysteine-rich intestinal protein (CRIP) is a LIM domain protein containing a zinc-finger motif and plays a role in the regulation of the inflammatory immune response. In the present study, we isolated a CRIP1 cDNA, designated PoCRIP1, from an olive flounder Paralichthys olivaceus intestine cDNA library by EST analysis. The PoCRIP cDNA consists of 421 bp with a polyadenylation signal sequence, AATAAA, and a poly(A) tail; it encodes a polypeptide of 76 amino acids containing a double zinc-finger motif (Cys(2)HisCys and Cys(4) sequences). The deduced amino acid sequence of PoCRIP1 showed 75.3-94.7% homology with CRIP1s of other species, including mammals. The PoCRIP1 transcript was highly expressed in the intestine and pyloric ceca and moderately expressed in the gill, heart, kidney, liver, muscle, spleen, skin, and stomach of normal conditioned flounder. Inducible expression of the PoCRIP1 transcript was observed in flounder challenged with Edwardsiella tarda, an economically important pathogen for aquaculture of flounder. Over-expression of PoCRIP1 augmented p65-driven flounder IL-6 promoter activity in HINAE cells. These results suggest that PoCRIP1 may function in the immune response of the flounder through the regulation of cytokine expression.

Evaluation of housekeeping genes as references for quantitative real time RT-PCR analysis of gene expression in Japanese flounder (Paralichthys olivaceus)

Japanese flounder (Paralichthys olivaceus) is an important economic fish species cultured worldwide. In this report, we compared the potentials of ten housekeeping genes as quantitative real time RT-PCR (qRT-PCR) references for the study of gene expression in Japanese flounder under normal physiological conditions and during bacterial infection. For this purpose, the expression of the ten genes in eight flounder tissues (liver, spleen, kidney, heart, muscle, brain, gill, and intestine) was determined by qRT-PCR before and after bacterial infection. The expression levels of the housekeeping genes were then compared and evaluated with geNorm and NormFinder algorithms. The results showed that before bacterial infection, the tested genes exhibited tissue-specific expressions to various degrees, with β-actin and ubiquitin-conjugating enzyme being ranked as the most stable genes across tissue types. Following bacterial challenge, all the tested genes varied in expression levels in tissue-dependent manners and no cross-all-tissue type reference gene was identified among the examined panel of housekeeping genes; however, α-tubulin was recognized as the most stable gene in four (spleen, heart, muscle, and gill) of the eight examined tissues. These results indicate that for qRT-PCR analysis of gene expression in Japanese flounder as a function of bacterial infection, the choice of reference genes should be made according to tissue type.

Detection of type III secretion gene as an indicator for pathogenic Edwardsiella tarda

AIMS

To differentiate pathogenic and nonpathogenic Edwardsiella tarda strains based on the detection of type III secretion system (T3SS) gene using polymerase chain reaction (PCR).

METHODS AND RESULTS

Primers were designed to amplify Edw. tarda T3SS component gene esaV, catalase gene katB, haemolysin gene hlyA and 16S rRNA gene as an internal positive control. Genomic DNAs were extracted using a commercial isolation kit from 36 Edw. tarda strains consisting of 18 pathogenic and 18 nonpathogenic strains, and 50 ng of each DNA was used as the template for PCR amplification. PCR was performed with a thermocycler (TaKaRa TP600) in a 25-μl volume. Products of esaV were detected in all pathogenic strains, but not in nonpathogenic strains; katB was detected in all pathogenic strains and one of nonpathogenic strains; hlyA was not detected in any strains.

CONCLUSIONS

The detection of esaV gene can be used for the assessment of pathogenic Edw. tarda strains.

SIGNIFICANCE AND IMPACT OF THE STUDY

The strategy using T3SS gene as the virulence indicator provides a useful tool for the clinical assessment of pathogenic Edw. tarda strains and prediction of edwardsiellosis risk in fish culture environments.

Atypical Aeromonas salmonicida infection in the black rockfish, Sebastes schlegeli Hilgendorf, in Korea

Cultured black rockfish, Sebastes schlegeli, suffered mass mortalities during winter 2008 and spring 2009 in Korea, showing clinical signs of ulcer lesions and haemorrhages over their body surface. The aetiological agent was identified as Aeromonas salmonicida (strains RFAS-1, -2 and -3), which is a non-pigmented, slow-growing bacterium. Phenotypes of RFAS strains showed variation, while 16S rRNA, gyrB, rpoD, dnaJ and recA gene sequences of all the strains were affiliated to A. salmonicida. In particular, vapA gene sequences of the strains were most closely related to one of the five subspecies of A. salmonicida subsp. masoucida (=KCCM 40239(T) ). LD(50) values of RFAS-1 for intraperitoneal and intramuscular injection were 1.5 × 10(5.25) and 1.5 × 10(6.4) cfu/rockfish, respectively. However, A. salmonicida strains KCCM 40239(T) and SAS-1, which originate from masou and chum salmon, respectively, were not pathogenic to black rockfish. RFAS strains, possessing A-layer protein on their surface, exhibited β-haemolytic activity against rockfish erythrocytes and capability to survive in rockfish serum, which seem to be associated with virulence.

Characterization of Edwardsiella tarda waaL: roles in lipopolysaccharide biosynthesis, stress adaptation, and virulence toward fish

Edwardsiella tarda is the causative agent of edwardsiellosis in fish. The genome sequence of a virulent strain EIB202 has been determined. According to the genome sequence, the lipopolysaccharide (LPS) synthesis cluster containing a putative O-antigen ligase gene waaL was identified. Here, the in-frame deletion mutant ΔwaaL was constructed to analyze the function of WaaL in E. tarda EIB202. The ΔwaaL mutant displayed absence in O-antigen side chains in the LPS production. The ΔwaaL mutant exhibited an increased sensitivity to hydrogen peroxide indicating that the LPS was involved in the endurance to the oxidative stress in hosts during infection. In addition, the resistance of ΔwaaL to serum and polymyxin B decreased remarkably. The ΔwaaL mutant was also attenuated in virulence, showed an impaired ability in internalization of epithelioma papulosum cyprinid (EPC) cells and a comparatively poor ability of proliferation in vivo, which was in line with the increased LD(50) value. These results indicated that waaL gene was a functional member of the gene cluster involved in LPS synthesis and highlighted the importance of the O-antigen side chains to stress adaption and virulence in E. tarda, signifying the gene as a potential target for live attenuated vaccine against this bacterium.

Immune responses and expression profiles of some immune-related genes in Indian major carp, Labeo rohita to Edwardsiella tarda infection

Edwardsiella tarda is an important Gram-negative bacterium that causes systemic infections in a wide range of hosts including fish. The pathogenic mechanisms in this disease are still poorly understood in fish. Indian major carp, Labeo rohita were intraperitoneally challenged with a pathogenic isolate of E. tarda to measure sequential changes in immunity level. A significant decrease in the superoxide production, myeloperoxidase, alternative complement activity, total protein levels and antiprotease activity of serum was marked in the infected fish. However, the serum lysozyme activity and haemagglutination titre were raised in the infected fish. Similarly, a significant rise in specific antibody titre was noticed on and after 10 days post-challenge. This study also elucidates the changes in the relative expression of some immune-related genes viz., interleukin 1-beta (IL-1beta), inducible nitric oxide synthase (iNOS), complement component C3, beta(2)-microglobulin, CXCa, tumor necrosis factor-alpha (TNFalpha), and C-type and G-type lysozymes during the infection. Significant up-regulation of IL-1beta, iNOS, C3, CXCa and expression of both types of lysozyme genes was noticed at 6-12 h post-challenge (h.p.c.) whereas down-regulation of beta(2)-microglobulin and TNFalpha genes was observed after 48 h p.c. The results obtained here strengthen the understanding on molecular pathogenesis of edwardsiellosis in L. rohita.

The Francisella tularensis pathogenicity island encodes a secretion system that is required for phagosome escape and virulence

Francisella tularensis causes the human disease tularemia. F. tularensis is able to survive and replicate within macrophages, a trait that has been correlated with its high virulence, but it is unclear the exact mechanism(s) this organism uses to escape killing within this hostile environment. F. tularensis virulence is dependent upon the Francisella pathogenicity island (FPI), a cluster of genes that we show here shares homology with type VI secretion gene clusters in Vibrio cholerae and Pseudomonas aeruginosa. We demonstrate that two FPI proteins, VgrG and IglI, are secreted into the cytosol of infected macrophages. VgrG and IglI are required for F. tularensis phagosomal escape, intramacrophage growth, inflammasome activation and virulence in mice. Interestingly, VgrG secretion does not require the other FPI genes. However, VgrG and other FPI genes, including PdpB (an IcmF homologue), are required for the secretion of IglI into the macrophage cytosol, suggesting that VgrG and other FPI factors are components of a secretion system. This is the first report of F. tularensis FPI virulence proteins required for intramacrophage growth that are translocated into the macrophage.

Edwardsiella tarda T6SS component evpP is regulated by esrB and iron, and plays essential roles in the invasion of fish

Edwardsiella tarda is a gram-negative pathogen for hemorrhagic septicemia in a broad range of hosts. The type VI secretion system (T6SS) has recently been dissected in E. tarda to secrete EvpC, EvpI and a novel effector protein EvpP. In this study, sequencing and genetic alignments showed that evpP genes from different E. tarda isolates were highly similar and an evpP homolog was also found in Aeromonas hydrophila 0865 isolated from a diseased eel, suggesting the possible lateral gene transfer of evpP or the whole T6SS gene island. With reporter strains carrying gfp gene fused to the evpP promoter region, flow cytometric analysis revealed that transcription of evpP was positively regulated by either the two-component system EsrA-EsrB in E. tarda or the iron concentration in media. Compared with the parental strain, in-frame deletion of evpP in E. tarda EIB202 led to the significantly increased 50% lethal doses in zebrafish (Danio rerio) and Japanese flounder (Paralichthys olivaceus), decreased hemolytic activities, failure to adhere to mucus and reduced serum resistance, and complementation of an intact evpP gene restored these phenotypes in the evpP mutant. Investigation of infection kinetics indicated that the evpP deletion mutant was unable to proliferate in vivo, particularly in immune organs of fish. Moreover, the evpP deletion mutant exhibited incapacity to internalize in EPC cell model in vitro, demonstrating that EvpP in T6SS plays critical roles for invasion mechanism of E. tarda and merits as potential target for attenuated live vaccine construction.

Edwardsiellosis in fish: a brief review

Edwardsiellosis is one of the most important bacterial diseases in fish. Scientific work on this disease started more than forty years ago and numerous workers around the world are continually adding to the knowledge of the disease. In spite of this, not a single article that reviews the enormous scientific data thus generated is available in the English language. This article briefly discusses some of the recent research on edwardsiellosis, describing the pathogen's interaction with the host and environment, its pathogenesis and pathology as well as diagnostic, preventive and control measures.

Comparison of the responses of peritoneal macrophages from Japanese flounder (Paralichthys olivaceus) against high virulent and low virulent strains of Edwardsiella tarda

In vivo infection studies in Japanese flounder (Paralichthys olivaceus) demonstrated that the number of viable cells of the virulent strain (NUF251) of Edwardsiella tarda increased gradually in kidney and hepato-pancreas after intraperitoneal injection, but the low virulent strain (NUF194) did not. To gain insight into the virulence factors of E. tarda, in vitro responses of Japanese flounder (P. olivaceus) peritoneal macrophages to these strains were compared in terms of phagocytosis, bactericidal activity, and reactive oxygen species (ROS) generation as measured by chemiluminescence (CL) responses. Microscopic observation revealed that these two strains of E. tarda were phagocytosed by the peritoneal macrophages, and there was no significant difference in the mean numbers of ingested bacteria per macrophage between these strains. A gradual increase in the number of viable cells of the highly virulent strain within macrophages was observed during 9h post-phagocytosis, whereas no significant replication of the low virulent strain within macrophages was detected. These results suggest that the virulent strain of E. tarda has an ability to survive and replicate within macrophages, while the low virulent strain has no such ability. When the peritoneal macrophages were exposed to the opsonized low virulent E. tarda strain, a rapid increase in CL response was induced. However, the highly virulent strain caused only background level of CL response. By the subsequent stimulation with phorbol myristate acetate, the macrophages exposed to the virulent E. tarda strain showed extremely higher CL response than that of the one exposed to the low virulent E. tarda strain. These results suggest that the virulent E. tarda prevents the activation of ROS generation system during phagocytosis, though the system is still capable of responding to other stimulation. The virulent strain significantly reduced the CL response induced by xanthine/xanthine oxidase system, while the low virulent strain had almost no effect. Furthermore, the virulent strain showed greater resistance to H(2)O(2) than the low virulent strain. Our results suggest that the virulent strain of E. tarda is highly resistant to ROS, and such ability might allow the organism to survive and multiply within phagocytes, and may serve to disseminate E. tarda throughout the host during in vivo infection.