The Role of Flagellin B in Vibrio anguillarum-Induced Intestinal Immunity and Functional Domain Identification

Transcriptome research conducted on the liver and spleen of Jinhu grouper (Epinephelus fuscoguttatus♀ × Epinephelus tukula♂) reveals the mechanism in response to Vibrio anguillarum infection

Vibrio anguillarum serves as a pathogenic organism in aquaculture, leading to a lethal hemolytic septicemia in aquatic species. Whereas little study has evaluated the molecular mechanism of the infection caused by V. anguillarum in Jinhu grouper. In this study, analysis of the transcriptome was conducted on the liver and spleen tissues from Jinhu groupers infected with V. anguillarum infection. We identified 2978 DEGs in the liver group and 2506 DEGs in spleen group, including 1689 and 1502 up-regulated genes and 1289 and 1004 down-regulated genes, respectively. Gene set enrichment analysis revealed a significant reduction in genes associated with metabolism such as carbon metabolism and glycolysis/gluconeogenesis in the liver, while upregulation of genes linked to the above pathways as well as in the citrate cycle in the spleen. In addition, the upregulated genes in the liver and spleen are both enriched in the cell cycle. Subsequent investigation into the principal DEGs implicated in the TLR pathways showed that V. anguillarum infection may activate the TLR pathway by overexpression of the tlr5 and promote the synthesis of proinflammatory cytokines il1β and il-8. Among these, 11 genes related to metabolism, cell cycle and immunity were selected and characterized. Overall, our research indicates that V. anguillarum can affect the metabolism and cell cycle while also triggering immune defense reactions in Jinhu grouper.

Influence of Vibrio anguillarum culture conditions on the efficacy of bacterin-based vaccines in lumpfish (Cyclopterus lumpus)

Lumpfish (Cyclopterus lumpus) is used as cleaner fish to control sea lice infestations in Atlantic salmon (Salmo salar) farms across the North Atlantic. Vibrio anguillarum, the causative agent of vibriosis, is a recurrent bacterial pathogen affecting lumpfish. Bacterin-based vaccines are frequently used to control vibriosis in finfish, but their efficacy is not always consistent. Culture conditions significantly influence synthesis of bacterial outer membrane and secreted proteins, which are critical antigens, and thus impact the immunogenicity of bacterin-based vaccines. In this study, we assessed the effect of V. anguillarum culture conditions on vaccine efficacy in lumpfish. V. anguillarum was cultured under iron-limited at 15 °C, and iron-rich or iron-limited conditions at 28 °C with 2 % NaCl, and these cultures were used to prepare bacterins. A commercial vaccine was used as positive control, while PBS and PBS adjuvant were negative controls. Lumpfish were intraperitoneally immunized and challenged 12 weeks post-immunization with 10-100 times the LD50 dose of V. anguillarum. Bacterins prepared from V. anguillarum grown under iron-limited conditions at 28 °C with 2 % NaCl and mixed with adjuvant conferred the highest protection compared to other preparations and commercial vaccines. In contrast, bacterins derived from V. anguillarum cultured under iron-limited conditions at 15 °C conferred the lowest protection. Reverse vaccinology and transcriptomic analyses of V. anguillarum grown under optimal immunogenic conditions revealed 323 upregulated genes, of which 211 were high-antigenicity proteins suitable for subunit vaccines. This study provides critical knowledge for effective vaccine formulation against V. anguillarum and identifies potential antigens for subunit vaccine development.

Genomic and Transcriptomic Diversification of Flagellin Genes Provides Insight into Environmental Adaptation and Phylogeographic Characteristics in Aeromonas hydrophila

Aeromonas hydrophila is an opportunistic motile pathogen with a broad host range, infecting both terrestrial and aquatic animals. Environmental and geographical conditions exert selective pressure on both geno- and phenotypes of pathogens. Flagellin, directly exposed to external environments and containing important immunogenic epitopes, may display significant variability in response to external conditions. In this study, we conducted a comparative analysis of ~  150 A. hydrophila genomes, leading to the identification of six subunits of the flagellin gene (fla-1 to fla-4, flaA, and flaB). Individual strains harbored different composition of flagellin subunits and copies. The composition of subunits showed distinct patterns depending on environmental sources. Strains from aquatic environments were mainly comprised of fla-1 to fla-4 subunits, while terrestrial strains predominated in groups harboring flaA and flaB subunits. Each flagellin showed varying levels of expression, with flaA and flaB demonstrating significantly higher expression compared to others. One of the chemotaxis pathways that control flagellin movement through a two-component system was significantly upregulated in flaA(+ 1)/flaB(+ 1) group, whereas flaA and flaB showed different transcriptomic expressions. The genes positively correlated with flaA expression were relevant to biofilm formation and bacterial chemotaxis, but flaB showed a negative correlation with the genes in ABC transporters and quorum sensing pathway. However, the expression patterns of fla-2 to fla-4 were identical. This suggests various types of flagellin subunits may have different biological functions. The composition and expression levels of flagellin subunits could provide valuable insights into the adaptation of A. hydrophila and the differences among strains in response to various external environments.

Construction of the flagellin F mutant of Vibrio parahaemolyticus and its toxic effects on silver pomfret (Pampus argenteus) cells

Vibrio parahaemolyticus causes diseases in aquatic organisms, leading to substantial financial losses to the aquaculture industry; its flagellin F (flaF) protein triggers severe inflammation in host cells. To enhance the understanding of the function of flaF in V. parahaemolyticus infection, in this study, a flaF-deficient mutant was constructed by employing two-step homologous recombination. The flaF-deficient mutant induced a significantly lower toll-like receptor 5 (TLR5) expression and apoptosis in fish intestinal epithelial cells than the wild-type V. parahaemolyticus. Furthermore, fluorescence labelling and microscopy analysis of TLR5 showed that V. parahaemolyticus and its mutant strain significantly enhanced TLR5 expression. Additionally, the findings suggest that flaF deletion did not significantly affect the expression of myeloid differentiation factor 88 (MyD88) and interleukin-8 (IL-8) induced by V.parahaemolyticus. In summary, V. parahaemolyticus induced a TLR5-dependent inflammatory response and apoptosis through MyD88, which was observed to be influenced by flaF deletion. In this study, we obtained stable mutants of V. parahaemolyticus via target gene deletion-which is a rapid and effective approach-and compared the induction of inflammatory response and apoptosis by V. parahaemolyticus and its mutant strain, providing novel perspectives for functional gene research in V. parahaemolyticus.

The role of flagellin F in Vibrio Parahaemolyticus-induced intestinal immunity and functional domain identification

The marine pathogen Vibrio parahaemolyticus has caused huge economic losses to aquaculture. Flagellin is a key bacterial virulence factor that induces an inflammatory response via activation of Toll-like receptor 5 (TLR5) signaling. Herein, to explore the inflammatory activity of V. parahaemolyticus flagellins (flaA, flaB, flaC, flaD, flaE, and flaF), we investigated their ability to induce apoptosis in a fish cell line. All six flagellins induced severe apoptosis. Moreover, treatment with V. parahaemolyticus flagellins increased TLR5 and myeloid differentiation factor 88 (MyD88) expression and the production of TNF-α and IL-8 significantly. This indicated that flagellins might induce a TLR5-meditated immune response via an MyD88-dependent pathway. FlaF exhibited the strongest immunostimulatory effect; therefore, the interaction between TLR5 and flaF was screened using the yeast two-hybrid system. A significant interaction between the two proteins was observed, indicating that flaF binds directly to TLR5. Finally, the amino acids that participate in the TLR5-flaF interaction were identified using molecular simulation, which indicated three binding sites. These results deepen our understanding of the immunogenic properties of flagellins from V. parahaemolyticus, which could be used for vaccine development in the future.

Comparative proteomics study of exosomes in Vibrio harveyi and Vibrio anguillarum

Exosomes are a class of extracellular vesicles released by bacteria and contain diverse biomolecules. In this study, we isolated exosomes from Vibrio harveyi and Vibrio anguillarum, which are both serious pathogens in mariculture, using a supercentrifugation method, and the proteins in the exosomes of these two vibrios were analyzed by LC-MS/MS proteomics. Exosome proteins released by V. harveyi and V. anguillarum were different; they not only contained virulence factors (such as lipase and phospholipase in V. harveyi, metalloprotease and hemolysin in V. anguillarum), but also participated in the important life activities of bacteria (such as fatty acid biosynthesis, biosynthesis of antibiotics, carbon metabolism). Subsequently, to verify whether the exosomes participated in bacterial toxicity, after Ruditapes philippinarum was challenged with V. harveyi and V. anguillarum, the corresponding genes of virulence factors from exosomes screened by proteomics were tested by quantitative real-time PCR. All the genes detected were upregulated which suggested that exosomes were involved in vibrio toxicity. The results could provide an effective proteome database for decoding the pathogenic mechanism of vibrios from the exosome perspective.