Immunogenicity study of an expressed outer membrane protein U of Vibrio vulnificus in Japanese eel (Anguilla japonica)

RNA-seq analysis revealed the pathogenicity of Vibrio vulnificus to American eel (Anguilla rostrata) and the strategy of host anti-V. vulnificus infection

Vibrio vulnificus is a commonly pathogenic bacterium in cultivated eels, but its pathogenicity to American eel (Anguilla rostrata) and the molecular mechanism of host anti-V. vulnificus infection remains uncertain. In this study, American eels were infected with different dose of V. vulnificus to determine the LD50. Then, bacterial load in the liver and kidney histopathology were assessed post the LD50 of V. vulnificus infection. Additionally, gene expressions of 18 immune related genes in the liver, spleen and kidney were detected. Furthermore, transcriptome sequencing and enrichment of differentially expressed genes (DEGs) were analyzed in the eel spleens between pre-infection (Con_0), post-36 h (Vv_36), and post-60 h (Vv_60) infection. The results showed that LD50 of V. vulnificus to American eels was determined to be 5.0 × 105 cfu/g body weight, and the bacterial load peaked at 24 and 12 h post the infection (hpi) in the kidney and liver, respectively. The histopathology was highlighted by necrotic hepatocytes and splenic cells, congestion blood vessels in liver and spleen, atrophied glomeruli and vacuolization of renal tubular epithelial cells. The results of RT-PCR revealed that 18 host immune-related genes showed significantly up or downregulated expression post-infection compare to that of pre-infection. Finally, results of the RNA-seq revealed 16 DEGs play essential role to the immunosuppression in American eels, and the protein-protein interactions shed light on the widespread upregulation GEGs related to metabolism and immune response maintained the host cell homeostasis post the V. vulnificus infection, shedding new light on our understanding of the V. vulnificus pathogenesis towards understudied American eel and the host anti-V. vulnificus infection strategies in gene transcript.

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.

Mechanisms Underlying the Virulence Regulation of Vibrio alginolyticus ND-01 pstS and pstB with a Transcriptomic Analysis

Vibrio alginolyticus is a common opportunistic pathogen of fish, shrimp, and shellfish, and many diseases it causes can result in severe economic losses in the aquaculture industry. Causing host disease was confirmed by several virulence factors of V. alginolyticus. To date, there have been no reports on the effect of the pstS gene on its virulence regulation of V. alginolyticus. The virulence mechanism of target genes regulating V. alginolyticus is worthy of further study. Previous studies found that Fructus schisandrae (30 mg/mL) inhibited the growth of V. alginolyticus ND-01 (OD600 = 0.5) for 4 h, while the expressions of pstS and pstB were significantly affected by F. schisandrae stress. So, we speculated that pstS and pstB might be the virulence genes of V. alginolyticus, which were stably silenced by RNAi to construct the silencing strains pstS-RNAi and pstB-RNAi, respectively. After the expression of pstS or pstB gene was inhibited, the adhesion capacity and biofilm formation of V. alginolyticus were significantly down-regulated. The chemotaxis and biofilm formation ability of pstS-RNAi was reduced by 33.33% and 68.13% compared with the wild-type strain, respectively. Sequence alignment and homology analysis showed that pstS was highly conserved, which suggested that pstS played a vital role in the secretion system of V. alginolyticus. The pstS-RNAi with the highest silencing efficiency was selected for transcriptome sequencing. The Differentially Expressed Genes (DEGs) and GO terms were mapped to the reference genome of V. alginolyticus, including 1055 up-regulated genes and 1134 down-regulated genes. The functions of the DEGs were analyzed by GO and categorized into different enriched functional groups, such as ribosome synthesis, organelles, biosynthesis, pathogenesis, and secretion. These DEGs were then mapped to the reference KEGG pathways of V. alginolyticus and enriched in commonalities in the metabolic, ribosomal, and bacterial secretion pathways. Therefore, pstS and pstB could regulate the bacterial virulence of V. alginolyticus by affecting its adhesion, biofilm formation ability, and motility. Understanding the relationship between the expressions of pstS and pstB with bacterial virulence could provide new perspectives to prevent bacterial diseases.

Anguilla sp. diseases diagnoses and treatments: The ideal methods at the crossroads of conservation and aquaculture purposes

Anguilla anguilla, A. japonica and A. rostrata are the most fished and consumed eel species. However, these species are Critically Endangered, Endangered and Endangered, respectively. A combination of factors is thought to be responsible for their decline including fisheries, climate change, habitat destruction, barriers to migration, pollution and pathogens. Among them, viruses, bacteria and parasites are causing weakening of wild eels and serious economic losses for fishermen and eel farmers. Early detection of pathogens is essential to provide appropriate responses both for conservation reasons and to limit economic losses. Classic diagnosis approaches are time consuming and invasive and usual treatments, for example, antipathogenic substances are becoming obsolete because of pathogen resistance and environmental impact problems. The need for early and non-invasive diagnostic methods as well as effective and environmentally friendly treatments has increased. Vaccine development and diet supplementation have known a growing interest since their use could allow prevention of diseases. In this review, we summarize the main pathogens-viruses, bacteria and parasites-of the three northern temperate eel species, the methods used to detect these pathogens and the different treatments used. We discussed and highlighted the need for non-invasive, rapid and efficient detection methods, as well as effective and environmentally friendly treatments for both conservation and aquaculture purposes.

Recombinant expression and immunogenicity evaluation of a TonB-dependent receptor of Vibrio parahaemolyticus

This study constructed the recombinant plasmid of a TonB-dependent receptor from V. parahaemolyticus and evaluated the immunogenicity of the recombinant protein in mice. The TonB-dependent receptor gene (GI: 28901321) was obtained by PCR amplification and cloned into plasmid pET-32a (+). The recombinant plasmids were transformed into Escherichia coli BL21, and the protein expression was induced by isopropyl-β-d-thiogalactopyranoside (IPTG). The 6 × His-tagged TonB-dependent receptor inclusion bodies were purified by Ni-NTA Agarose column and renatured by gradient urea dialysis. The soluble and inclusion bodies of the TonB-dependent receptor were emulsified with Freund's adjuvant and subcutaneously injected into BALB/c mice. The serum titers with seven V. parahaemolyticus strains, eight Vibrio species, and nine other bacteria were studied by enzyme-linked immunosorbent assay and immunoblotting. The results showed that the serum homogenously bound the target protein in the V. parahaemolyticus cell lysates. The titers against the immunized protein were above 89K, while the titer against whole cells of seven V. parahaemolyticus strains ranged from 4.12K to 12.5K. However, the titers were higher for the soluble TonB-dependent receptor. The serums reacted with E. coli strains but did not cross-react with eight Vibrio species and Photobacterium damselae. These results showed that the TonB-dependent receptor proteins in this study were immunogenic, and the serums showed adequate specificity for V. parahaemolyticus. However, the availability of the TonB-dependent receptor on V. parahaemolyticus cells is probably limited.

Structure, regulation, and host interaction of outer membrane protein U (OmpU) of Vibrio species

OmpU is a multimeric, cation selective outer membrane protein of Vibrio and related species that non-covalently interact with peptidoglycan layer. Interaction of OmpU with human host cells triggers signaling pathways to promote cytokine secretion, reactive oxygen species production, and caspase independent death in immune and epithelial cells. Non-choleric OmpU imparts resistance to antimicrobial peptides and induces actin cytoskeletal reorganization in the host cells. Further, OmpU isolated from Vibrio species elicits an immune response in several aquaculture hosts. Importantly, in-vivo studies using recombinant OmpU or OmpU derived mimotopes reveal a short-lasting immunity, and protection against Vibrio in the aquaculture sector. In conclusion, OmpU is a key adhesion protein and an important virulence factor for successful colonization of Vibrio species into hosts. This review article provides a broad overview of structural, regulatory, and functional mechanisms of OmpU in normal and disease states.

Evaluation of the β-barrel outer membrane protein VP1243 as a candidate antigen for a cross-protective vaccine against Vibrio infections

Vibrio parahaemolyticus is a Gram-negative halophilic bacterium that causes acute gastroenteritis after the consumption of contaminated food, wound infection, and seizures. Antibiotic therapy is the main method for controlling Vibrio infections, which inevitably leads to drug resistance. Therefore, a vaccine is urgently needed to avoid this problem. Outer membrane proteins (OMPs) play a pivotal role in the interaction between the host immune system and bacteria. VP1243 is an OMP of V. parahaemolyticus, and it possessed immunogenicity in our previous study. The present study found that VP1243 was widely distributed, highly conserved and possessed similar surface epitopes among the major Vibrio species. The protein stimulated a strong antibody response and induced cross-reactive immune responses in V. parahaemolyticus, V. alginolyticus and V. vulnificus. Notably, it provided 100% immune protection against lethal challenges by the three Vibrio species in mice immunized with VP1243. Efficient clearance of cells of the three Vibrio bacterial species was observed in immunized mice. These findings provide solid evidence for VP1243 as a promising candidate for the development of a versatile vaccine to protect against Vibrio infections.

Application of Outer Membrane Protein-Based Vaccines Against Major Bacterial Fish Pathogens in India

Aquaculture is one of the fastest-growing food-producing sectors in the world. However, its growth is hampered by various disease problems due to infectious microorganisms, including Gram-negative bacteria in finfish aquaculture. Disease control in aquaculture by use of antibiotics is not recommended as it leads to antibiotic residues in the final product, selection, and spread of antibiotic resistance in the environment. Therefore, focus is on disease prevention by vaccination. All Gram-negative bacteria possess surface-associated outer membrane proteins (OMPs), some of which have long been recognized as potential vaccine candidates. OMPs are essential for maintaining the integrity and selective permeability of the bacterial membrane and play a key role in adaptive responses of bacteria such as solute and ion uptake, iron acquisition, antimicrobial resistance, serum resistance, and bile salt resistance and some adhesins have virulence attributes. Antigenic diversity among bacterial strains even within the same bacterial species has constrained vaccine developments, but OMPs that are conserved across serotypes could be used as potential candidates in vaccine development, and several studies have demonstrated their efficacy and potential as vaccine candidates. In this review, we will look into the application of OMPs for the design of vaccines based on recombinant proteins, subunit vaccines, chimeric proteins, and DNA vaccines as new-generation vaccine candidates for major bacterial pathogens of fish for sustainable aquaculture.

Construction expression and immunogenicity of a novel trivalent outer membrane protein (OmpU-A-II) from three bacterial pathogens in Japanese eels (Anguilla japonica)

Vibrio vulnificus, Edwardsiella anguillarum and Aeromonas hydrophila are three common bacterial pathogens in cultivated eels. To protect farming eels from infection by these pathogens, a trivalent outer membrane protein (OMP) containing partial sequences of OmpU from V. vulnificus, OmpA from E. anguillarum and OmpII from A. hydrophila was expressed and purified; then, the OMP was used as a vaccine to immunize Japanese eels (Anguilla japonica). Whole-blood cell proliferation, antibody titres and complement and lysozyme activities were detected at different days post-immunization (dpi), and the relative per cent survival (RPS) was determined after eels were infected with V. vulnificus, E. anguillarum or A. hydrophila at 28 dpi. The results showed that the OMP significantly stimulates the antibody titres. At 14 days after the challenge (i.e. at 28 dpi), the RPS of OMP against V. vulnificus, E. anguillarum and A. hydrophila was 20%, 70% and 11.1%, respectively. The construction, expression and immunogenicity of a trivalent Omp were reported for the first time, and this study will provide a valuable reference for the development of fish multiplex vaccines.

Immunization of a novel bivalent outer membrane protein simultaneously resisting Aeromonas hydrophila, Edwardsiella anguillarum and Vibrio vulnificus infection in European eels (Angullia angullia)

In cultivated European eels, Aeromonas hydrophila, Edwardsiella anguillarum and Vibrio vulnificus are three important bacterial pathogens. In this study, European eels (Anguilla anguilla) were immunized by the bivalent expression products of the outer membrane protein (Omp) gene from A. hydrophila (OmpⅡ) and E. anguillarum (OmpA), and the effects of the bivalent protein (rOmpⅡ-A) on the immune function of the European eel were detected. Three hundred eels were divided average into three groups of PBS, adjuvant and rOmp. Eels of three goups were injected intraperitoneal with 0.2 mL of PBS (0.01 mol/L, pH7.4), PBS + F (PBS mixed equal volume of freund's uncomplete adjuvant) or rOmpⅡ-A (1 mg mL^-1 rOmpⅡ-A mixed equal volume of freund's uncomplete adjuvant). Four immune-related genes expression, proliferation of whole blood cells, serum and skin mucus antibody titer, superoxide dismutase (SOD) activity and the relative percent of survival (RPS) were studied at different days (or hours) post the immunization. The results showed that the igm, lysC, mhc2 and sod gene in the liver, spleen, kidney and intestine tract were significant increased in the Omp group; On the 28 day post the immunization (dpi), blood cell proliferation was increased in the Omp group, and on the 14, 21, 28 and 42 dpi, antibody titers in serum and mucus of the Omp group were significantly higher than that of the PBS and adjuvant group, regardless of coating with bacteria or Omp antigen. The SOD activity of Omp group increased significantly in liver, kidney, skin mucus and serum from 14 to 42 dpi, especially in serum. Eels chanllenged by A. hydrophila, E. anguillarum and V. vulnificus in the bivalent Omp group showed the RPS were 83.33%, 55.56% and 44.44%, respectively. The results of this study showed that immunization of the bivalent Omp could effectively improve the immune function of European eels, and produced effectively protection to A. hydrophila and E. anguillarum infection. Simultaneously, the bivalent Omp also produced distinct cross-protection to the eels challenged by V. vulnificus.

Immunization of a novel outer membrane protein from Aeromonas hydrophila simultaneously resisting A. hydrophila and Edwardsiella anguillarum infection in European eels (Angullia angullia)

In cultivated European eels, Aeromonas hydrophila, Edwardsiella anguillarum and Vibrio vulnificus are three important bacterial pathogens. In this study, an expressed recombinant Outer membrane proteinⅡ (rOmpⅡ) from A. hydrophila was intraperitoneally injected into European eels (Angullia angullia). All examined eels were equally divided into three groups. One group was injected with PBS only (PBS group), one group was injected with 1:1 mixture of PBS and Freund's incomplete adjuvant (PBS + F, adjuvant group), and the third group was injected with 1:1 mixture of 1 mg mL^-1 rOmpⅡ and Freund's incomplete adjuvant (rOmpⅡ+F, OmpⅡ group). The immunogenicity of OmpⅡ was studied by detecting the expression of 4 immune-related genes, stimulation index (SI) of the whole blood cell, serum antibody titer, lysozyme and Superoxide Dismutase (SOD) activity, and relative percent of survival (RPS) rate. The results showed that gene expression of MHC-Ⅱ, LysC, SOD and IgM in the OmpⅡ group significantly increased in liver, spleen, kidney and intestine. At 28 days post the immunization (dpi), the SI of whole blood cells in the OmpⅡ group increased significantly; at 14, 21, 28 and 42 dpi, the serum antibody titers against A. hydrophila and E. anguillarum in the OmpⅡ group were significantly higher than that of the PBS and the adjuvant group; the SOD in the OmpⅡ group was found increased significantly in liver, kidney, mucus and serum. On the 28 dpi, eels were challenged by A. hydrophila, E. anguillarum and V. vulnificus for cross protection study. The results showed that the RPS of the OmpⅡ group were 83.33%, 55.56% and 33.33% respectively. These results showed that the expressed OmpⅡ from A. hydrophila significantly improve the immune function of Europena eels and their resistance to the infection of A. hydrophila and E. anguillarum simultaneously.

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.