Role of mucosal immune response and histopathological study in European eel (Anguilla anguilla L.) intraperitoneal challenged by Vibrio anguillarum or Tenacibaculum soleae

Biological studies reveal the role of trpA gene in biofilm formation, motility, hemolysis and virulence in Vibrio anguillarum

Vibrio anguillarum is a pathogen responsible for vibriosis in aquaculture animals. The formation of bacterial biofilm contributes to infections and increases resistance to antibiotics. Tryptophanase and its substrate tryptophan have been recognized as signal molecules regulating bacterial biofilm formation. This study aimed to investigate the role of the trpA gene encoding tryptophan synthase in V. anguillarum through constructing a trpA mutant (ΔtrpA) and its complemented strain (CΔtrpA). The ΔtrpA produced less tryptophan compared to the WT and ΔtrpA strains, with almost no detectable indole synthesis in the ΔtrpA mutant. RNAseq analysis showed that 152 genes were differentially expressed in the ΔtrpA mutant, including 64 upregulated and 88 downregulated genes. KEGG enrichment analysis and qRT-PCR validation indicated that genes associated with bacterial chemotaxis, two component system, quorum sensing and biofilm formation were downregulated. Crystal violet staining confirmed that the ΔtrpA mutant decreased biofilm formation due to the reduced tryptophan and indole. Our studies also showed that TrpA plays an important role in the motility and hemolytic ability of V. anguillarum. Finally, the virulence of the ΔtrpA mutant was evaluated using a Tetrahymena infection model, which demonstrated that the virulence of ΔtrpA mutant was significantly attenuated. These findings provide insight into the role of trpA gene in biofilm formation, motility, hemolysis, and virulence in V. anguillarum.

Short-term induced swimming activity enhanced innate immune parameters and antioxidant status of European eel (Anguilla anguilla)

The swimming activity, although an essential trait in the life cycle of fish, is still poorly understood in farmed fish. The current study aimed to investigate the impact of short-term induced swimming on the immune and antioxidant defence systems in European eel (Anguilla anguilla). Sixteen male yellow European eels (total length: 39.9 ± 0.7 cm; body weight: 108.8 ± 6.1 g) were individually placed in swimming flumes and divided into two groups: i) no swimming (n = 8); and ii) induced-swimming (n = 8) at 0.3 body lengths (BL)·s-1 for 7 h. Swimming resulted in a 2-fold lower cortisol concentration in plasma, whereas plasma glucose, lactate, and several immune-related parameters did not present variations between groups. Interestingly, swimming led to higher lysozyme, peroxidase, and protease activities in skin mucus, whereas bactericidal activity did not show differences among groups. Additionally, the gene expression of interleukin 1 beta showed an up-regulation in the skin of fish with induced swimming, while no differences were observed in the head-kidney or gills. Furthermore, modulation of the antioxidant status was observed in the liver and posterior skeletal muscle after induced swimming. Fish subjected to swimming showed lower lipid peroxidation and higher reduced glutathione levels, increasing the reduced/oxidized glutathione ratio. However, no variations in the antioxidant status were observed between groups in the anterior skeletal muscle. This study showed modulation of immune and oxidative stress markers in European eels upon short-term induced swimming compared to non-swimming fish.

Molecular characterization and gene expression of pattern recognition receptors in brown-marbled grouper (Epinephelus fuscoguttatus) fingerlings responding to vibriosis infection

The pathogen recognition system involves receptors and genes that play a crucial role in activating innate immune response in brown-marbled grouper (Epinephelus fuscoguttatus) as a control agent against various infections including vibriosis. Here, we report the molecular cloning of partial open reading frames, sequences characterization, and expression profiles of Pattern Recognition Receptors (PRRs) in brown-marbled grouper. The PRRs, namely pglyrp5, tlr5, ctlD, and ctlE in brown-marbled grouper, possess conserved domains and showed shared evolutionary relationships with other fishes, humans, mammals, birds, reptilians, amphibians, and insects. In infection experiments, up to 50% mortality was found in brown-marbled grouper fingerlings infected with Vibrio alginolyticus compared to 27% mortality infected Vibrio parahaemolyticus and 100% survival of control groups. It is also demonstrated that all four PRRs had higher expression in samples infected with V. alginolyticus compared to V. parahaemolyticus. This PRRs gene expression analysis revealed that all four PRRs expressed rapidly at 4-h post-inoculation even though the Vibrio count was only detected earliest at 12-h post-inoculation in samples. The highest expression recorded was from V. alginolyticus inoculated fish spleen with up to 73-fold change for pglyrp5 gene, followed by 14 to 38-fold expression for the same treatment in spleen, head kidney, and blood samples for other PRRs, namely tlr5, ctlD, and ctlE genes. Meanwhile less than a 10% increase in expression of all four genes was detected in spleen, head kidney, and blood samples inoculated with V. parahaemolyticus. These findings indicated that pglyrp5, tlr5, ctlD, and ctlE play important roles in the early immune response to vibriosis infected, brown-marbled grouper fingerlings.

The effects of two dietary synbiotics on growth performance, hematological parameters, and nonspecific immune responses in Japanese Eel

OBJECTIVE

Feed additives have attracted increased attention in aquaculture due to their ability to modulate fish gut microbiota, resulting in improved fish growth and immunity. This study assessed the effects of two synbiotics in Japanese Eel Anguilla japonica: Bacillus subtilis with mannooligosaccharides (MOS) and Enterococcus faecium with fructooligosaccharides (FOS).

METHODS

Six diets, including a control (CON) diet, oxytetracycline (OTC) diet, and four synbiotic diets (B. subtilis at 1 × 106 or 1 × 107 colony-forming units [CFU]/g with MOS at 5 g/kg [BS6MO and BS7MO; collectively, BSMOS diets] and E. faecium at 1 × 106 or 1 × 107 CFU/g with FOS at 5 g/kg [EF6FO and EF7FO; collectively, EFFOS diets]), were fed to triplicate groups of 20 fish (average weight ± SD = 6.00 ± 0.07 g) for 8 weeks.

RESULT

Fish fed the BSMOS diets showed significantly higher weight gain, specific growth rate (SGR), and feed efficiency compared to fish fed the CON and OTC diets, but the values were not significantly different from those of fish fed the EFFOS diets. Weight gain and SGR of fish that were given EFFOS diets were not significantly different from those of fish fed all other diets. Fish fed the OTC diet showed a higher mean aspartate aminotransferase level, although the difference was not statistically significant. The myeloperoxidase activity of fish fed the BS7MO diet was significantly higher than those of fish receiving all other diets, and the superoxide dismutase activity of fish fed the BS7MO diet was also significantly higher than that of fish fed the EF7FO diet. Overall, the BSMOS synbiotic diets were significantly more effective than the CON diet in enhancing fish survival against a Vibrio anguillarum challenge.

CONCLUSION

Our findings suggest that synbiotics can be a preferable alternative to antibiotics in aquaculture.

Effects of Shrimp Shell-Derived Chitosan on Growth, Immunity, Intestinal Morphology, and Gene Expression of Nile Tilapia (Oreochromis niloticus) Reared in a Biofloc System

Chitosan (CH) shows great potential as an immunostimulatory feed additive in aquaculture. This study evaluates the effects of varying dietary CH levels on the growth, immunity, intestinal morphology, and antioxidant status of Nile tilapia (Oreochromis niloticus) reared in a biofloc system. Tilapia fingerlings (mean weight 13.54 ± 0.05 g) were fed diets supplemented with 0 (CH0), 5 (CH5), 10 (CH10), 20 (CH20), and 40 (CH40) mL·kg-1 of CH for 8 weeks. Parameters were assessed after 4 and 8 weeks. Their final weight was not affected by CH supplementation, but CH at 10 mL·kg-1 significantly improved weight gain (WG) and specific growth rate (SGR) compared to the control (p < 0.05) at 8 weeks. Skin mucus lysozyme and peroxidase activities were lower in the chitosan-treated groups at weeks 4 and 8. Intestinal villi length and width were enhanced by 10 and 20 mL·kg-1 CH compared to the control. However, 40 mL·kg-1 CH caused detrimental impacts on the villi and muscular layer. CH supplementation, especially 5-10 mL·kg-1, increased liver and intestinal expressions of interleukin 1 (IL-1), interleukin 8 (IL-8), LPS-binding protein (LBP), glutathione reductase (GSR), glutathione peroxidase (GPX), and glutathione S-transferase (GST-α) compared to the control group. Overall, dietary CH at 10 mL·kg-1 can effectively promote growth, intestinal morphology, innate immunity, and antioxidant capacity in Nile tilapia fingerlings reared in biofloc systems.

Alterations in certain immunological parameters in the skin mucus of the carp, Cirrhinus mrigala, infected with the bacteria, Edwardsiella tarda

The bacterial fish pathogen Edwardsiella tarda causes heavy stock mortality, severely hampering fish production, resulting in great economic loss to the farming industry. The first biological barriers that confer immune protection against pathogen entry are the fish mucosal surfaces. The present study was undertaken to investigate the influence of E. tarda on certain enzymatic and non-enzymatic parameters in the skin mucous secretions of the fish Cirrhinus mrigala using spectrophotometry and zymography. Fish were randomly divided into three groups: control, vehicle control, and infected. A sublethal dose of E. tarda (2.2 × 106 CFU/fish) suspended in 50 μL of PBS was injected intra-peritoneally at 0 day (d). Subsequently, mucus samples were collected at 2 d, 4 d, 6 d and 8 d post-infection. The activities of lysozyme (LYZ), protease (PROT), alkaline phosphatase (ALP), acid phosphatase (ACP), catalase (CAT), peroxidase (PER), superoxide dismutase (SOD), and glutathione S-transferase (GST) decreased significantly in the skin mucus of the challenged fish, indicating the suppressed immune system and decreased antioxidant capacity of C. mrigala to E. tarda infection. Lipid peroxidation (LPO) and total nitrate-nitrite were significantly higher at several time points post-infection, suggesting that physiological functions have been impaired following pathogen challenge. The present findings could be relevant for fish aquaculture and underline the importance of skin mucus not only for assessing fish immune status but also for identifying early warning signals of disease caused by pathogens.

Effects of Dietary Limonene Supplementation on Growth Performance and Immunological Parameters of Common Carp, Cyprinus carpio, Challenged by Aeromonas hydrophila

This study examined the impact of dietary limonene treatment on the growth performance, immune response, and disease resistance of common carp, Cyprinus carpio. The fish were fed with either a control diet (CTL; no limonene supplementation) or four experimental diets containing 50 (50 L), 100 (100 L), 200 (200 L), and 400 (400 L) mg/kg limonene over a 70-day period, followed by Aeromonas hydrophila challenge. The 200 L treatment resulted in a significant decrease in FCR compared to the CTL treatment. The highest post-challenge mortality was associated with the CTL treatment (62.7%), while the 200 L treatment had the lowest mortality (30.7%). Before the challenge, dietary limonene significantly increased humoral and skin mucosal immune parameters compared to the CTL treatment. The highest leukocyte, lymphocyte counts, skin mucosal protease activity, and intestinal lactic acid bacteria were observed in the 200 L treatment before the challenge. The highest plasma lysozyme activity was observed in the 400 L treatment, whereas the highest skin mucosal lysozyme and peroxidase activities were observed in the 100 L and 200 L treatments before the challenge. There were no significant differences in the blood neutrophil, monocyte, and eosinophil counts, humoral alternative complement activity, skin mucosal alkaline phosphatase activity, and the intestinal total viable bacteria among the treatments before the challenge. After the challenge, the 200 L treatment exhibited the highest leukocyte, neutrophil, and monocyte count, skin mucosal immune parameters, and intestinal lactic acid bacteria, whereas the highest blood eosinophil count was observed in the 100 L, 200 L, and 400 L treatments. At this time, the lowest blood lymphocyte counts were observed in the 100 L and 200 L, but the lowest intestinal total viable bacteria were observed in the 100 L, 200 L, and 400 L treatments. Based on these findings, dietary limonene at 200 mg/kg is ideal for common carp to promote feed efficiency, innate immunity boosting, and resistance against A. hydrophila.

Healthy and infected Atlantic salmon (Salmo salar) skin-mucus response to Tenacibaculum dicentrarchi under in vitro conditions

Tenacibaculosis caused by Tenacibaculum dicentrarchi is the second most important bacterial disease that affects the Chilean salmon industry. The impacted fish show severe external gross skin lesions on different areas of the body. The external mucus layer that covers fish skin contains numerous immune substances that act as one of the main defense barriers against microbial colonization and invasions by potential pathogens. The present in vitro study aimed to evaluate and elucidate the role of the external mucus layer in the susceptibility of Atlantic salmon (Salmo salar) to three Chilean T. dicentrarchi strains and the type strain. For this, mucus collected from healthy and diseased (i.e., with T. dicentrarchi) Atlantic salmon were used, and various antibacterial and inflammatory parameters were analysed. The T. dicentrarchi strains were attracted to the mucus of Atlantic salmon regardless of health status. All four strains adhered to the skin mucus and very quickly grew using the mucus nutrients. Once infection was established, different mucosal defense components were activated in the fish, but the levels of bactericidal activity and of other enzymes were insufficient to eliminate T. dicentrarchi. Alternatively, this pathogen may be able to neutralize or evade these mechanisms. Therefore, the survival of T. dicentrarchi in fish skin mucus could be relevant to facilitate the colonization and subsequent invasion of hosts. The given in vitro results suggest that greater attention should be given to fish skin mucus as a primary defense against T. dicentrarchi.

Immune response of European sea bass (Dicentrarchus labrax L.) against combination of antigens from three different pathogens

Three of the most important diseases of Mediterranean intensive European sea bass farming are, viral nervous necrosis (VNN) caused by the red grouper nervous necrosis virus (RGNNV) genotype of b-nodavirus, photobacteriosis caused by Photobacterium damselae subsp. piscicida (Phdp) and vibriosis caused mainly by the O1 serotype of Vibrio anguillarum (VaO1). Prevention against these diseases is performed through vaccination with a monovalent vaccine against the viral disease and, usually, with bivalent vaccines against the bacterial diseases. However, it is very difficult to program two vaccinations during the same season for the same fish stock and producers are forced to either vaccinate for the viral or the bacterial diseases or to perform double vaccination with both vaccines, without any prior knowledge on any interactions that may occur due to the plethora of antigens (Ag) injected. Ideally, therefore, a trivalent vaccine should be developed against all three diseases. The objective of this work was to analyse the immune response of sea bass against combinations of Ags from all three pathogens, namely viral particles, Phdp whole cells (WC), lipopolysaccharide (LPS), capsular polysaccharide (CPS) and extracellular products (ECPs) and VaO1 WC and ECPs in respect to the identification of any phenomena of immunodominance/immunosuppression between Ags with a view to select candidate Ags for inclusion in a trivalent vaccine formulation. Eight triplicate groups of fish were immunized with different combinations of the aforementioned Ags and another triplicate group served as negative control. Blood serum was isolated at various time-points post-immunization for the measurement of specific antibodies against each Ag and, in addition, leucocytes were isolated at day 29 post-immunization for analysis of various cellular activities. Results indicated that best levels of specific a-NNV virus antibodies (Abs) were produced when VaO1 ECPs were not included in the Ag combinations, in contrast to the leucocytes proliferation assay where best stimulation against NNV Ags was measured when VaO1 ECPs were present in Ag combinations. VaO1 ECPs apparently is a strong immunogen for both humoral and cellular responses but suppresses immunological reactions against the other Ags.VaO1 WC, Phdp LPS and ECPs raised good humoral immune responses in the groups with best responses against VNN Ags, but only VaO1 WC and Phdp ECPs provided good stimulation of leucocytes, with Phdp WC and CPS effecting either similar stimulation with untrained leucocytes (control groups) or down-stimulation. Results are discussed with a view to select Ags from all three pathogens for inclusion in trivalent vaccine against all three pathogens.

Modulation of growth, immune response, and immune-antioxidant related gene expression of Nile tilapia (Oreochromis niloticus) reared under biofloc system using mango peel powder

This study aimed to investigate the effects of mango peel powder (MGPP) on growth, innate immunity, and immune-antioxidant related gene expression of Nile tilapia reared under biofloc system. Three hundred Nile tilapia (average weight 14.78 ± 0.05 g) were distributed into 15 fiber tanks (300 L per tank) assigned to five treatments in triplication. Fish were fed basal diet containing different levels MGPP as follows: 0 (MGPP0: control), 6.25 (MGPP 6.25), 12.5 (MGPP 12.25), 25 (MGPP 25), and 50 (MGPP 50) g kg^-1 diet for 8 weeks. Specific growth rate (SGR), weight gain (WG), final weight (FW), feed conversion ratio (FCR), skin mucus of lysozyme (SMLA), and peroxidase activities (SMPA), serum of lysozyme (SL) and peroxidase (SP) were measured every for weeks; while immune-antioxidant-related gene expressions were determined after 8 weeks post-feeding. The results indicated that MGPP 25 diet resulted in higher SGR, WG, FW, and FCR but no significant differences among treatments were noticed. In terms of immune responses, lysozyme and peroxidase activities in mucus and serum were significantly higher in MGPP 12.5 and MGPP 25 diets against the control. Similarly, significant up-regulation of IL-1 and IL-8 gene expressions was observed in fish fed MGPP 25 against the control. However, no significant differences in LBP, GSTa, GPX, and GSR among treatments were observed. Overall, dietary inclusion of MGPP 25 significantly enhanced immune response and immune related gene expressions but not growth performance and antioxidant gene expressions. The results implied that MGPP can be potentially used as an immunostimulants in Nile tilapia culture.

Immune correlates of NF-κB and TNFα promoter DNA methylation in Japanese flounder (Paralichthys olivaceus) muscle and immune parameters change response to vibrio anguillarum infection

Vibrio anguillarum infection can activate NF-κB/TNFα pathway in the immune organs of fish. Fish muscle is also an important immune organ, but the research on its immune function is few. Our aim was to study regulating mechanism of NF-κB and TNFα gene expressions in the muscle of Japanese flounder (Paralichthys olivaceus) which was under Vibrio anguillarum infection (0, 24, 48, 72 and 96 h). The results showed that the expressions of NF-κB and TNFα increased significantly at 48 h, and there was a significant positive correlation between them. In situ hybridization confirmed the co-existence of NF-κB and TNFα genes in Japanese flounder muscle. Interestingly, the expression of the TNFα gene was regulated by the DNA methylation and its methylation level was negatively correlated with the expression. The lowest methylation level of TNFα occurred at 48 h under Vibrio anguillarum infection (P < 0.05). And more, when the fragment (-2122 ∼ -730) was deleted on TNFα gene promoter, double luciferase activity was the highest, indicating that fragment (-730-0) was the transcription factor binding region. The site (-78 ~ -69) on the fragment (-730-0) binding NF-κB was mutated, and double luciferase activity decreased significantly. The results confirmed that the site (-78 ~ -69) was indeed an important binding site for NF-κB. In addition, the activity of TNFα in the serum of Japanese flounder changed with the prolongation of vibrio anguillarum infection, and the concentration of other immune factors such as ALP, ALT, AST and LDH also changed in the muscle under vibrio anguillarum infection. They all showed a trend of first increasing and then decreasing. Above studies implied that Japanese flounder responded to Vibrio anguillarum infection at the immune level with the change of its methylation status and the activation of transcription factor. By studying the mechanism of immune pathways, understanding the response to immune stress is great significant to the research of fish breeding for disease resistance.