Six genes of ompA family shuffling for development of polyvalent vaccines against Vibrio alginolyticus and Edwardsiella tarda

Transfer of maternal immunity using a polyvalent vaccine and offspring protection in Nile tilapia, Oreochromis niloticus

Background

Vaccination is an effective and alternative means of disease prevention, however, it cannot be conducted on the offspring of fish. For this process to take place, the transfer of maternal immunity should be implemented. This study aims to determine the effectiveness of transferring immunity from the broodstock to the offspring using a polyvalent vaccine against Aeromonas hydrophila, Streptococcus agalactiae, and Pseudomonas fluorescens in Nile tilapia, Oreochromis niloticus.

Methods

Nile tilapia broodstock with an average weight of 203g (±SD 23) was reared in spawning ponds until mass spawning and harvested one week post-spawning for vaccination. After being vaccinated according to the treatment, each fish broodstock was reared in 3x3 m cages installed in an earthen pond with a density of 20 broodstock, consisting of 15 females and 5 males. The vaccine used was a formalin-killed whole-cell vaccine at a density of 10 10 cfu/mL injected intramuscularly ( i.m.) at a dose of 0.4 mL/kg fish. Nile tilapia was injected with a vaccine used as a treatment. Example include A. hydrophila monovalent (MA) , S. agalactiae monovalent (MS) , P. fluorescens monovalent (MP), A. hydrophila and S. agalactiae bivalent (BAS) , A. hydrophila and P. fluorescens bivalent (BAP), P. fluorescens and S. agalactiae bivalent (BPS), and A. hydrophila, S. agalactiae, and P. fluorescens polyvalent vaccines (PAPS). While the control was fish that were injected with a PBS solution. The broodstock's immune response was observed on the 7 th, 14 th, 21 st, and 28 th days, while the immune response and challenge test on the offspring was conducted on the 10 th, 20 th, 30 th, and 40 th day during the post-hatching period. The parameters observed consisted of total leukocytes, phagocytic activity, antibody titer, lysozyme, and relative survival percentage (RPS).

Result

The application of PAPS in broodstock could significantly induce the best immune response and immunity to multiple diseases compared to other treatments. The RPS of the PAPS was also higher than the other types of vaccines. This showed that the transfer of immunity from the broodstock to the Nile tilapia offspring could protect it against bacterial diseases such as A. hydrophila, S. agalactiae, and P. fluorescens.

Conclusion

The application of polyvalent vaccine A. hydrophila, S. agalactiae, P. fluorescens vaccines increased the broodstock's immune response and it was transferred to their offsprings. Polyvalent vaccines derived from maternal immunity can protect offspring from disease up to 30 days of age. They were able to produce tilapia seeds that are immune to diseases caused by A. hydrophila, S. agalactiae, and P. fluorescens.

Reverse Vaccinology and Immunoinformatic Approach for Designing a Bivalent Vaccine Candidate Against Hepatitis A and Hepatitis B Viruses

Hepatitis A and B are two crucial viral infections that still dramatically affect public health worldwide. Hepatitis A Virus (HAV) is the main cause of acute hepatitis, whereas Hepatitis B Virus (HBV) leads to the chronic form of the disease, possibly cirrhosis or liver failure. Therefore, vaccination has always been considered the most effective preventive method against pathogens. At this moment, we aimed at the immunoinformatic analysis of HAV-Viral Protein 1 (VP1) as the major capsid protein to come up with the most conserved immunogenic truncated protein to be fused by HBV surface antigen (HBs Ag) to achieve a bivalent vaccine against HAV and HBV using an AAY linker. Various computational approaches were employed to predict highly conserved regions and the most immunogenic B-cell and T-cell epitopes of HAV-VP1 capsid protein in both humans and BALB/c. Moreover, the predicted fusion protein was analyzed regarding primary and secondary structures and also homology validation. Afterward, the three-dimensional structure of vaccine constructs docked with various toll-like receptors (TLR) 2, 4 and 7. According to the bioinformatics tools, the region of 99-259 amino acids of VP1 was selected with high immunogenicity and conserved epitopes. T-cell epitope prediction showed that this region contains 32 antigenic peptides for Human leukocyte antigen (HLA) class I and 20 antigenic peptides in terms of HLA class II which are almost fully conserved in the Iranian population. The vaccine design includes 5 linear and 4 conformational B-cell lymphocyte (BCL) epitopes to induce humoral immune responses. The designed VP1-AAY-HBsAg fusion protein has the potency to be constructed and expressed to achieve a bivalent vaccine candidate, especially in the Iranian population. These findings led us to claim that the designed vaccine candidate provides potential pathways for creating an exploratory vaccine against Hepatitis A and Hepatitis B Viruses with high confidence for the identified strains.

Efficacy of polyvalent vaccine on immune response and disease resistance against streptococcosis/lactococcosis and yersiniosis in rainbow trout (Oncorhynchus mykiss)

Diseases are the most significant challenge in the development and stability of aquaculture. In this study, the immunogenic efficiency of polyvalent streptococcosis/lactococcosis and yersiniosis vaccines was evaluated by injection and immersion methods in rainbow trout.. The 450 fish with an average weight of 50 ± 5 g were divided into three treatments and three replications as follows: injection vaccine treatment, immersion vaccine treatment and control group without vaccine administration. Fish were kept for 74 days and sampling was done on days 20, 40 and 60. Then, from the 60th to the 74th day, the immunized groups were challenged with three bacteria Streptococcus iniae (S. iniae), Lactococcus garvieae (L. garvieae) and Yersinia ruckeri (Y. ruckeri) separately. A significant difference was observed in the weight gained (WG) in the immunized groups compared to the control group (P < 0.05). The relative survival percentage (RPS) after 14 days of challenge with S. iniae, L. garvieae and Y. ruckeri in the injection group compared to the control group increased respectively (60%, 60% and 70%), (P < 0.05). Also, RPS in the immersion group had an increase respectively (30%, 40% and 50%) after the challenge with S. iniae, L.garvieae and Y. ruckeri compared to the control group. Immune indicators such as antibody titer, complement and lysozyme activity significantly increased in comparison to the control group (P < 0.05). In general, it can be concluded that applying three vaccines by injection and immersion method has significant effects on immune protection and survival rate. However, the injection method is more effective and more suitable than the immersion method.

Strategies for Prevention and Control of Vibriosis in Asian Fish Culture

It is estimated that vibriosis account for about half of the economic losses in Asian fish culture. Consequently, the prevention and control of vibriosis is one of the priority research topics in the field of Asian fish culture disease. Relevant measures have been proposed to control some Vibrios that pose a threat to Asian fish culture, but there are currently only a few effective vaccines available to combat these Vibrios. The purpose of our review is to sum up the main prevention methods and the latest control strategies of seven Vibrio species that cause great harm to Asian aquaculture, including Vibrio harveyi, Vibrio vulnificus, Vibrio parahaemolyticus, Vibrio mimicus, Vibrio anguillarum, Vibrio alginolyticus and Vibrio cholerae. Strategies such as antibiotics, probiotics, bacteriophages, antimicrobials from plants and other natural sources, as well as vaccines, are compared and discussed here. We expect this review will provide some new views and recommendations for the future better prevention and control of vibriosis in Asian fish culture.

Taurine promotes Oreochromis niloticus survival against Edwardsiella tarda infection

Edwardsiella tarda represents one of the most important pathogens that infects a variety of hosts including aquatic animals and humans. The outbreak of E. tarda infection is frequently reported in aquaculture that causes huge economic loss. Due to the widespread of antibiotic resistance, available antibiotics to treat bacterial infection are limited. Therefore, enhancing aquatic animals to survive upon E. tarda infection become an urgent issue. In this study, we profiled the metabolomic change of tilapia in-between the dying and survival fish upon E. tarda infection. The dying and survival fish mounts differential metabolic response, from which we identify a key metabolite, taurine, whose abundance is increased in both the survival group and the dying group but is more significant in the survival group. Exogenous taurine increases tilapia survival rate by 37.5% upon E. tarda infection. Further quantitative PCR analysis demonstrate taurine increases the expression of immune genes in liver, spleen and head kidney. Therefore, our study shows a new strategy to enhance fish immune response against bacterial infection.

Identification of Polyvalent Vaccine Candidates From Extracellular Secretory Proteins in Vibrio alginolyticus

Bacterial infections cause huge losses in aquaculture and a wide range of health issues in humans. A vaccine is the most economical, efficient, and environment-friendly agent for protecting hosts against bacterial infections. This study aimed to identify broad, cross-protective antigens from the extracellular secretory proteome of the marine bacterium Vibrio alginolyticus. Of the 69 predicted extracellular secretory proteins in its genome, 16 were randomly selected for gene cloning to construct DNA vaccines, which were used to immunize zebrafish (Danio rerio). The innate immune response genes were also investigated. Among the 16 DNA vaccines, 3 (AT730_21605, AT730_22220, and AT730_22910) were protective against V. alginolyticus infection with 47–66.7% increased survival compared to the control, while other vaccines had lower or no protective effects. Furthermore, AT730_22220, AT730_22910, and AT730_21605 also exhibited cross-immune protective effects against Pseudomonas fluorescens and/or Aeromonas hydrophila infection. Mechanisms for cross-protective ability was explored based on conserved epitopes, innate immune responses, and antibody neutralizing ability. These results indicate that AT730_21605, AT730_22220, and AT730_22910 are potential polyvalent vaccine candidates against bacterial infections. Additionally, our results suggest that the extracellular secretory proteome is an antigen pool that can be used for the identification of cross-protective immunogens.

The two-component system RstAB regulates production of a polysaccharide capsule with a role in virulence in the marine pathogen Photobacterium damselae subsp. damselae

The marine bacterium Photobacterium damselae subsp. damselae (Pdd) causes disease in marine animals and humans. Previous studies demonstrated that mutation of the two-component system RstAB strongly impacts virulence of this pathogen, but the RstAB regulon has not been thoroughly elucidated. We here compared the transcriptomes of Pdd RM-71 and ΔrstA and ΔrstB derivatives using RNA-seq. In accordance with previous studies, RstAB positively regulated cytotoxins Dly, PhlyP and PhlyC. This analysis also demonstrated a positive regulation of outer membrane proteins, resistance against antimicrobials and potential virulence factors by this system. Remarkably, RstAB positively regulated two hitherto uncharacterised gene clusters involved in the synthesis of a polysaccharide capsule. Presence of a capsular layer in wild-type cells was confirmed by transmission electron microscopy, whereas rstA and rstB mutants were non-capsulated. Mutants for capsule synthesis genes, wza and wzc exhibited acapsular phenotypes, were impaired in resistance against the bactericidal action of fish serum and mucus, and were strongly impaired in virulence for fish, indicating a major role of capsule in virulence. Collectively, this study demonstrates that RstAB is a major positive regulator of key virulence factors including a polysaccharide capsule essential for full virulence in a pathogenic Photobacterium.

Succinate Promotes Phagocytosis of Monocytes/Macrophages in Teleost Fish

Development of immunity-based strategy to manage bacterial infection is urgently needed in aquaculture due to the widespread of antibiotic-resistant bacteria. Phagocytosis serves as the first line defense in innate immunity that engulfs bacteria and restricts their proliferations and invasions. However, the mechanism underlying the regulation of phagocytosis is not fully elucidated and the way to boost phagocytosis is not yet explored. In this manuscript, we profiled the metabolomes of monocytes/macrophages isolated from Nile tilapia, prior and after phagocytosis on Vibrio alginolyticus. Monocytes/macrophages showed a metabolic shift following phagocytosis. Interestingly, succinate was accumulated after phagocytosis and was identified as a crucial biomarker to distinguish before and after phagocytosis. Exogenous succinate increased the phagocytotic rate of monocytes/macrophages in a dose-dependent manner. This effect was dependent on the TCA cycle as the inhibitor of malonate that targets succinate dehydrogenase abrogated the effect. Meanwhile, exogenous succinate regulated the expression of genes associated with innate immune and phagocytosis. In addition, succinate-potentiated phagocytosis was applicable to both gram-negative and -positive cells, including V. alginolyticus, Edwardsiella tarda, Streptococcus agalactiae, and Streptococcus iniae. Our study shed light on the understanding of how modulation on host's metabolism regulates immune response, and this can be a potent therapeutic approach to control bacterial infections in aquaculture.

Vaccine Efficacy of a Newly Developed Feed-Based Whole-Cell Polyvalent Vaccine against Vibriosis, Streptococcosis and Motile Aeromonad Septicemia in Asian Seabass, Lates calcarifer

Multiple infections of several bacterial species are often observed under natural farm conditions. The infections would cause a much more significant loss compared to a single infectious agent. Vaccination is an essential strategy to prevent diseases in aquaculture, and oral vaccination has been proposed as a promising technique since it requires no handling of the fish and is easy to perform. This research attempts to develop and evaluate a potential feed-based polyvalent vaccine that can be used to treat multiple infections by Vibrios spp., Streptococcus agalactiae, and Aeromonas hydrophila, simultaneously. The oral polyvalent vaccine was prepared by mixing formalin-killed vaccine of V. harveyi, S. agalactiae, and A. hydrophila strains with commercial feed pellet, and palm oil as an adjuvant was added to improve their antigenicity. Thereafter, a vaccinated feed pellet was tested for feed quality analysis in terms of feed stability in water, proximate nutrient analysis, and palatability, safety, and growth performance using Asian seabass, Lates calcarifer as a fish host model. For immune response analysis, a total of 300 Asian seabass juveniles (15.8 ± 2.6 g) were divided into two groups in triplicate. Fish of group 1 were not vaccinated, while group 2 was vaccinated with the feed-based polyvalent vaccine. Vaccinations were carried out on days 0 and 14 with oral administration of the feed containing the bacterin at 5% body weight. Samples of serum for antibody and lysozyme study and the spleen and gut for gene expression analysis were collected at 7-day intervals for 6 weeks. Its efficacy in protecting fish was evaluated in aquarium challenge. Following vaccination by the polyvalent feed-based vaccine, IgM antibody levels showed a significant (p < 0.05) increase in serum against Vibrio harveyi, Aeromonas hydrophila, and Streptococcus agalactiae and reached the peak at week 3, 5, and 6, respectively. The high-stimulated antibody in the serum remained significantly higher than the control (p < 0.05) at the end of the 6 weeks vaccination trial. Not only that, but the serum lysozyme level was also increased significantly at week 4 (p < 0.05) as compared to the control treatment. The immune-related gene, dendritic cells, C3, Chemokine ligand 4 (CCL4), and major histocompatibility complex class I (MHC I) showed significantly higher expression (p < 0.05) after the fish were vaccinated with the oral vaccine. In the aquarium challenge, the vaccine provided a relative percentage survival of 75 ± 7.1%, 80 ± 0.0%, and 80 ± 0.0% after challenge with V. harveyi, A. hydrophila, and S. agalactiae, respectively. Combining our results demonstrate that the feed-based polyvalent vaccine could elicit significant innate and adaptive immunological responses, and this offers an opportunity for a comprehensive immunization against vibriosis, streptococcosis, and motile aeromonad septicemia in Asian seabass, Lates calcarifer. Nevertheless, this newly developed feed-based polyvalent vaccination can be a promising technique for effective and large-scale fish immunization in the aquaculture industry shortly.

Malate enhances survival of zebrafish against Vibrio alginolyticus infection in the same manner as taurine

Development of low-cost and eco-friendly approaches to fight bacterial pathogens is especially needed in aquaculture. We previously showed that exogenous malate reprograms zebrafish’s metabolome to potentiate zebrafish survival against Vibrio alginolyticus infection. However, the underlying mechanism is unknown. Here, we use GC-MS based metabolomics to identify the malate-triggered metabolic shift. An activated TCA cycle and elevated taurine are identified as the key metabolic pathways and the most crucial biomarker of the reprogrammed metabolome, respectively. Taurine elevation is attributed to the activated TCA cycle, which is further supported by the increased expression of genes in the metabolic pathway of taurine biosynthesis from the isocitrate of the TCA cycle to taurine. Exogenous taurine increases the survival of zebrafish against V. alginolyticus infection as malate did. Moreover, exogenous taurine and malate regulate the expression of innate immunity genes and promote the generation of reactive oxygen species and nitrogen oxide in a similar way. The two metabolites can alleviate the excessive immune response to bacterial challenge, which protects fish from bacterial infection. These results indicate that malate enhances the survival of zebrafish to V. alginolyticus infection via taurine. Thus, our study highlights a metabolic approach to enhance a host’s ability to fight bacterial infection.

Identification of Vibrio parahaemolyticus and Vibrio spp. Specific Outer Membrane Proteins by Reverse Vaccinology and Surface Proteome

The discovery of outer membrane proteins (OMPs) with desirable specificity and surface availability is a fundamental challenge to develop accurate immunodiagnostic assay and multivalent vaccine of pathogenic Vibrio species in food and aquaculture. Herein 101 OMPs were systemically screened from 4,831 non-redundant proteins of Vibrio parahaemolyticus by bioinformatical predication of signaling peptides, transmembrane (TM) α-helix, and subcellular location. The sequence homology analysis with 32 species of Vibrio spp. and all the non-Vibrio strains revealed that 15 OMPs were conserved in at least 23 Vibrio species, including BamA (VP2310), GspD (VP0133), Tolc (VP0425), OmpK (VP2362), OmpW (VPA0096), LptD (VP0339), Pal (VP1061), flagellar L-ring protein (VP0782), flagellar protein MotY (VP2111), hypothetical protein (VP1713), fimbrial assembly protein (VP2746), VacJ lipoprotein (VP2214), agglutination protein (VP1634), and lipoprotein (VP1267), Chitobiase (VP0755); high adhesion probability of flgH, LptD, OmpK, and OmpW indicated they were potential multivalent Vibrio vaccine candidates. V. parahaemolyticus OMPs were found to share high homology with at least one or two Vibrio species, 19 OMPs including OmpA like protein (VPA073), CsuD (VPA1504), and MtrC (VP1220) were found relatively specific to V. parahaemolyticus. The surface proteomic study by enzymatical shaving the cells showed the capsular polysaccharides most likely limited the protease action, while the glycosidases improved the availability of OMPs to trypsin. The OmpA (VPA1186, VPA0248, VP0764), Omp (VPA0166), OmpU (VP2467), BamA (VP2310), TolC (VP0425), GspD (VP0133), OmpK (VP2362), lpp (VPA1469), Pal (VP1061), agglutination protein (VP1634), and putative iron (III) compound receptor (VPA1435) have better availability on the cell surface.

Metabolic mechanism of colistin resistance and its reverting in Vibrio alginolyticus

Colistin is a last-line antibiotic against Gram-negative multidrug-resistant bacteria, but the increased resistance poses a huge challenge to this drug. However, the mechanisms underlying such resistance are largely unexplored. The present study first identified the mutations of two genes encoding AceF subunit of pyruvate dehydrogenase (PDH) and TetR family transcriptional regulator in colistin-resistant Vibrio alginolyticus (VA-R_CT ) through genome sequencing. Then, gas chromatography-mass spectroscopy-based metabolomics was adopted to investigate metabolic responses since PDH plays a role in central carbon metabolism. Colistin resistance was associated with the reduction of the central carbon metabolism and energy metabolism, featuring the alteration of the pyruvate cycle, a recently characterized energy-producing cycle. Metabolites in the pyruvate cycle reprogramed colistin-resistant metabolome to colistin-sensitive metabolome, resulting in increased gene expression, enzyme activity or protein abundance of the cycle and sodium-translocating nicotinamide adenine dinucleotide-ubiquinone oxidoreductase. This reprogramming promoted the production of the proton motive force that enhances the binding between colistin and lipid A in lipopolysaccharide. Moreover, this metabolic approach was effective against VA-R_CT in vitro and in vivo as well as other clinical isolates. These findings reveal a previously unknown mechanism of colistin resistance and develop a metabolome-reprogramming approach to promote colistin efficiency to combat with colistin-resistant bacteria.

Comparative Proteomics and Secretomics Revealed Virulence and Antibiotic Resistance-Associated Factors in Vibrio parahaemolyticus Recovered From Commonly Consumed Aquatic Products

Vibrio parahaemolyticus is a seafoodborne pathogen that can cause severe gastroenteritis and septicemia diseases in humans and even death. The emergence of multidrug-resistant V. parahaemolyticus leads to difficulties and rising costs of medical treatment. The bacterium of environmental origins containing no major virulence genes (tdh and trh) has been reported to be associated with infectious diarrhea disease as well. Identification of risk factors in V. parahaemolyticus is imperative for assuming food safety. In this study, we obtained secretomic and proteomic profiles of V. parahaemolyticus isolated from 12 species of commonly consumed aquatic products and identified candidate protein spots by using two-dimensional gel electrophoresis and liquid chromatography tandem mass spectrometry techniques. A total of 11 common and 28 differential extracellular proteins were found from distinct secretomic profiles, including eight virulence-associated proteins: outer membrane channel TolC, maltoporin, elongation factor Tu, enolase, transaldolase, flagellin C, polar flagellin B/D, and superoxide dismutase, as well as five antimicrobial and/or heavy metal resistance-associated ABC transporter proteins. Comparison of proteomic profiles derived from the 12 V. parahaemolyticus isolates also revealed five intracellular virulence-related proteins, including aldehyde-alcohol dehydrogenase, outer membrane protein A, alkyl hydroperoxide reductase C, phosphoenolpyruvate-protein phosphotransferase, and phosphoglycerate kinase. Additionally, our data indicated that aquatic product matrices significantly altered proteomic profiles of the V. parahaemolyticus isolates with a number of differentially expressed proteins identified. The results in this study meet the increasing need for novel diagnosis candidates of the leading seafoodborne pathogen worldwide.

Metabolic modulation of redox state confounds fish survival against Vibrio alginolyticus infection

Vibrio alginolyticus threatens both humans and marine animals, but hosts respond to V. alginolyticus infection is not fully understood. Here, functional metabolomics was adopted to investigate the metabolic differences between the dying and surviving zebrafish upon V. alginolyticus infection. Tryptophan was identified as the most crucial metabolite, whose abundance was decreased in the dying group but increased in the survival group as compared to control group without infection. Concurrently, the dying zebrafish displayed excessive immune response and produced higher level of reactive oxygen species (ROS). Interestingly, exogenous tryptophan reverted dying rate through metabolome re‐programming, thereby enhancing the survival from V. alginolyticus infection. It is preceded by the following mechanism: tryptophan fluxed into the glycolysis and tricarboxylic acid cycle (TCA cycle), promoted adenosine triphosphate (ATP) production and further increased the generation of NADPH. Meanwhile, tryptophan decreased NADPH oxidation. These together ameliorate ROS, key molecules in excessive immune response. This is further supported by the event that the inhibition of pyruvate metabolism and TCA cycle by inhibitors decreased D. reiro survival. Thus, our data indicate that tryptophan is a key metabolite for the host to fight against V. alginolyticus infection, representing an alternative strategy to treat bacterial infection in an antibiotic‐independent way.

Identification and innate immunity mechanism of protective immunogens from extracellular proteins of Edwardsiella tarda

One of the most important emerging pathogens in the aquaculture industry is Edwardsiella tarda, and it causes extensive losses in farmed fish globally. The identification of protective immunogens against E. tarda is increasingly valued. We previously investigated 20 recombinant proteins of 38 E. tarda extracellular secretory proteins and identified 10 as protective immunogens in a zebrafish model. Here, we clone 10 of the remaining 18 genes, and the resulting recombinant proteins are used for evaluation of immune protection. ETAE_2147 (FliK), ETAE_0654 (PpdD), and ETAE_3259 (DamX) are identified as protective immunogens. Furthermore, their protection mechanism is explored by the detection of innate immunity genes encoding IL-1b, IL-6, IL-8, C3b, and NF-κB. The three protective immunogens stimulate zebrafish to produce higher and more lasting expression of the five immunity genes than non-protective immunogens during the first 48 h of infection. In addition, these protective immunogens are prone to be regulated by host products, which is helpful for cross-talk between host and pathogen, and thus they become vaccine candidates. These results highlight the way to understand the working mechanisms of protective immunogens.

Comparison of Extracellular Proteins from Virulent and Avirulent Vibrio parahaemolyticus Strains To Identify Potential Virulence Factors

Vibrio parahaemolyticus is a leading seafood-borne pathogen that causes gastroenteritis, septicemia, and serious wound infections due to the actions of virulence-associated proteins. We compared the extracellular proteins of nonvirulent JHY20 and virulent ATCC 33847 V. parahaemolyticus reference strains. Eighteen extracellular proteins were identified from secretory profiles, and 11 (68.75%) of the 16 proteins in ATCC 33847 are associated with virulence and/or protection against adverse conditions: trigger factor, chaperone SurA, aspartate-semialdehyde dehydrogenase, 4-hydroxy-3-methylbut-2-en-1-yl diphosphate synthase, glutamate 5-kinase, alanine dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, outer membrane protein OmpV, ribosome-associated inhibitor A, chaperone protein Skp, and universal stress protein. Two nontoxic-related proteins, amino acid ABC transporter substrate-binding protein and an uncharacterized protein, were identified in JHY20. The results provide a theoretical basis for supporting safety risk assessment of aquatic foods, illuminate the pathogenic mechanisms of V. parahaemolyticus, and assist the identification of novel vaccine candidates for foodborne pathogens.

Metabolites-Enabled Survival of Crucian Carps Infected by Edwardsiella tarda in High Water Temperature

Temperature is one of the major factors that affect the outbreak of infectious disease. Lines of evidences have shown that virulence factors can be controlled by thermo-sensors in bacterial pathogens. However, how temperature influences host's responses to the pathogen is still largely unexplored, and the study of this might pave the way to develop strategies to manage pathogenic bacterial infection. In the present study, we show that finfish Carassius carassius, the crucian carp that is tolerant to a wide range of temperatures, is less susceptible to bacterial infection when grown in 20°C than in 30°C. The different responses of C. carassius to bacterial infection could be partially explained by the distinct metabolisms under the specific temperatures: C. carassius shows elevated tricarboxylic acid cycle (TCA cycle) but decreased taurine and hypotaurine metabolism as well as lower biosynthesis of unsaturated fatty acids at 30°C. The decreased abundance of palmitate, threonine, and taurine represents the most characteristic metabolic feature. Consistently, exogenous palmitate, threonine, or taurine enhances the survival of C. carassius to bacterial infection at 30°C in a dose-dependent manner. This effect could be attributed to the inhibition on the TCA cycle by the three metabolites. This notion is further supported by the fact that low concentration of malonate, a succinate dehydrogenase inhibitor, increases the survival of C. carassius at 30°C as well. On the other hand, addition of the three metabolites rescued the decreased expression of pro-inflammatory cytokines including TNF-α1, TNF-α2, IL-1β1, IL-1β2, and lysozyme at 30°C. Taken together, our results revealed an unexpected relationship between temperature and metabolism that orchestrates the immune regulation against infection by bacterial pathogens. Thus, this study shed light on the modulation of finfish physiology to fight against bacterial infection through metabolism.

Phenylalanine enhances innate immune response to clear ceftazidime-resistant Vibrio alginolyticus in Danio rerio

Antibiotic-resistant bacteria becomes a major threat to the economy and food safety in aquaculture. Although the antibiotic-dependent strategy is still the mostly adopted option, the development of antibiotic-free approach is urgently needed to ameliorate the severe situation of the global antibiotic resistance. In the present study, we showed that modulating the metabolism of zebrafish, Danio reiro, would enhance D. rerio to clear ceftazidime-resistant Vibrio alginoyticus (Caz-R) in vivo. By generating Caz-R in vitro, we found Caz-R stays longer than ceftazidime-sensitive V. alginoyticus (Caz-S) in D. rerio, where Caz-R induced less potent immune response than that of Caz-S. The differential immune response was associated with different metabolism of the host. Through functional metabolomics, we identified a crucial biomarker, phenylalanine. The abundance of phenylalanine was increased in both of Caz-S and Caz-R infected hosts but the abundance was higher in Caz-S infected group. This specific difference indicated phenylalanine could be a metabolite required to clear Caz-R by the host. Exogenous phenylalanine would enhance the host's ability to remove Caz-R, which was through upregulated production of lysozyme and C3b. Thus, our study demonstrates a novel strategy to boost host's immune response to combat against antibiotic-resistant bacteria.

Immunogenicity of a bivalent protein as a vaccine against Edwardsiella anguillarum and Vibrio vulnificus in Japanese eel (Anguilla japonica)

The OMPs A (OmpA)—of Edwardsiella anguillarum and OmpU of V. vulnificus have been proven to be good antigens. In this study, after construction of a vector, a new recombinant Omp (rOMP) containing both OmpA and OmpU was expressed and purified. Then, the Japanese eels (Anguilla japonica) were intraperitoneally (i.p.) injected with the phosphate‐buffered saline (PBS group), formalin‐killed‐cell (FKC group) or the recombinant Omp (rOMP group). The stimulation index of the whole blood cells in eels from FKC group was significantly higher than the eels from PBS and rOMP groups at 28 dpi; serum titers of anti‐E. anguillarum and anti‐V. vulnificus antibody of eels from FKC and rOMP group increased significantly at 21 and 28 dpi; in the rOMP group, eels serum titer stayed at a high level on 42 dpi. The activities of lysozyme in skin mucus, liver, kidney, and serum in three groups exhibited considerable changes. The relative percent survival (RPS) rate of eels from rOMP group were 100% and 83% when challenged with V. vulnificus or E. anguillarum. These results indicated that inoculation of rOMP would protect Japanese eels against the infection by E. anguillarum and V. vulnificus.

NaCl promotes antibiotic resistance by reducing redox states in Vibrio alginolyticus

The development of antibiotic resistance in Vibrio alginolyticus represents a threat to human health and fish farming. Environmental NaCl regulation of bacterial physiology is well documented, but whether the regulation contributes to antibiotic resistance remains unknown. To explore this, we compared minimum inhibitory concentration (MIC) of V. alginolyticus cultured in different media with 0.5%-10% NaCl, and found that the MIC increased as the NaCl concentration increased, especially for aminoglycoside antibiotics. Consistent with this finding, internal NaCl also increased, while intracellular gentamicin level decreased. GC-MS-based metabolomics showed different distributions of pyruvate cycle intermediates among 0.5%, 4% and 10% NaCl. Differential activity of enzymes in the pyruvate cycle and altered expression of Na(+)-NQR led to a reducing redox state, characterized by decreased levels of NADH, proton motive force (PMF) and ATP. Meanwhile, NaCl negatively regulated PMF as a consequence of the reducing redox state. These together are responsible for the decreased intracellular gentamicin level with the increased external level of NaCl. Our study reveals a previously unknown redox state-dependent mechanism regulated by NaCl in V. alginolyticus that impacts antibiotic resistance.

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.

Differential Antibody Responses to Outer Membrane Proteins Contribute to Differential Immune Protections between Live and Inactivated Vibrio parahemolyticus

It is widely accepted that live vaccines elicit higher immune protection than inactivated vaccines. However, the mechanisms are largely unknown. Here, an array with 64 recombinant outer membrane proteins of Vibrio parahemolyticus was developed to explore antibody responses of live and inactivated V. parahemolyticus post immunization of the 8th, 12th, 16th and 20th day. Among the 64 outer membrane proteins, 28 elicited antibody generation. They were all detected in live vaccine-induced immunity but only 15 antibodies were found in inactivated vaccine-induced immunity. Passive immunization showed that higher percent survival was detected in live than inactivated vaccine-induced immunities. Active immunization indicated that out of 19 randomly selected outer membrane proteins, 5 stimulated immune protection against V. parahemolyticus infection. Among them, antibodies to VP2309 and VPA0526 were shared in mice immunized by live or inactivated vaccines, whereas antibodies to VPA0548, VPA1745, and VP1667 were only found in mice immunized by live vaccine. In addition, live V. parahemolyticus stimulated earlier antibody response than inactivated bacteria. These results indicate that not all of the outer membrane proteins elicited antibody responses when they work together in the form of live or inactivated bacteria; live vaccine elicits more protective antibodies, which contribute to higher immune protection in live vaccine than inactivated vaccine. Notably, the recombinant proteins might be different from those separated from live bacteria, and they might be different in their immunogenic potencies.