Pathogen bacteria adhesion to skin mucus of fishes

Perfluorooctane Sulfonamide Induced Autotoxic Effects on the Zebrafish Immune System

Perfluorooctane sulfonamide (PFOSA) is an immediate perfluorooctanesulfonate (PFOS) precursor (PreFOS). Previous studies have shown PFOSA to induce stronger toxic responses compared to other perfluorinated compounds (PFCs). However, the specific nature of PFOSA-induced toxicity, whether autonomous or mediated by its metabolite PFOS, has not been fully elucidated. This study systematically investigates the immunomodulatory effects of PFOSA and PFOS in zebrafish (Danio rerio). Exposure to PFOSA compromised the zebrafish's ability to defend against pathogenic infections, as evidenced by increased bacterial adhesion to their skin and reduced levels of the biocidal protein lysozyme (LYSO). Moreover, PFOSA exposure was associated with disruptions in inflammatory markers and immune indicators, along with a decrease in immune cell counts. The findings from this study suggest that the immunotoxicity effects of PFOSA are primarily due to its own toxicity rather than its metabolite PFOS. This conclusion was supported by dose-dependent responses, the severity of observed effects, and multivariate analysis. In addition, our experiments using NF-κB-morpholino knock-down techniques further confirmed the role of the Nuclear factor-κappa B pathway in mediating PFOSA-induced immunotoxicity. In conclusion, this study reveals that PFOSA impairs the immune system in zebrafish through an autotoxic mechanism, providing valuable insights for assessing the ecological risks of PFOSA.

Evaluation of the Biological Activities of Peptides from Epidermal Mucus of Marine Fish Species from Chilean Aquaculture

The skin of fish is a physicochemical barrier that is characterized by being formed by cells that secrete molecules responsible for the first defense against pathogenic organisms. In this study, the biological activity of peptides from mucus of Seriola lalandi and Seriolella violacea were identified and characterized. To this purpose, peptide extraction was carried out from epidermal mucus samples of juveniles of both species, using chromatographic strategies for purification. Then, the peptide extracts were characterized to obtain the amino acid sequence by mass spectrometry. Using bioinformatics tools for predicting antimicrobial and antioxidant activity, 12 peptides were selected that were chemically produced by simultaneous synthesis using the Fmoc-Tbu strategy. The results revealed that the synthetic peptides presented a random coil or extended secondary structure. The analysis of antimicrobial activity allowed it to be discriminated that four peptides, named by their synthesis code 5065, 5069, 5070, and 5076, had the ability to inhibit the growth of Vibrio anguillarum and affected the copepodite stage of C. rogercresseyi. On the other hand, peptides 5066, 5067, 5070, and 5077 had the highest antioxidant capacity. Finally, peptides 5067, 5069, 5070, and 5076 were the most effective for inducing respiratory burst in fish leukocytes. The analysis of association between composition and biological function revealed that the antimicrobial activity depended on the presence of basic and aromatic amino acids, while the presence of cysteine residues increased the antioxidant activity of the peptides. Additionally, it was observed that those peptides that presented the highest antimicrobial capacity were those that also stimulated respiratory burst in leukocytes. This is the first work that demonstrates the presence of functional peptides in the epidermal mucus of Chilean marine fish, which provide different biological properties when the fish face opportunistic pathogens.

Potential Involvement of the South American Lungfish Intelectin-2 in Innate-Associated Immune Modulation

Intelectins belong to a family of lectins with specific and transitory carbohydrate interaction capabilities. These interactions are related to the activity of agglutinating pathogens, as intelectins play a significant role in immunity. Despite the prominent immune defense function of intelectins, limited information about its structural characteristics and carbohydrate interaction properties is available. This study investigated an intelectin transcript identified in RNA-seq data obtained from the South American lungfish (Lepidosiren paradoxa), namely LpITLN2-B. The structural analyses predicted LpITLN2-B to be a homo-trimeric globular protein with the fibrinogen-like functional domain (FReD), exhibiting a molecular mass of 57 kDa. The quaternary structure is subdivided into three monomers, A, B, and C, and each domain comprises 11 β-sheets: an anti-parallel β-sheet, a β-hairpin, and a disordered β-sheet structure. Molecular docking demonstrates a significant interaction with disaccharides rather than monosaccharides. The preferential interaction with disaccharides highlights the potential interaction with pathogen molecules, such as LPS and Poly(I:C). The hemagglutination assay inhibited lectins activity, especially maltose and sucrose, highlighting lectin activity in L. paradoxa samples. Overall, our results show the potential relevance of LpITLN2-B in L. paradoxa immune defense against pathogens.

Fulvic acid modulates mucosal immunity in fish skin: Sustainable aquaculture solution or environmental risk factor?

This is the first study determining the effects of bath exposure to fulvic acid, a humic substance, on the skin mucosal immunity of rainbow trout (Oncorhynchus mykiss). Humic substances have recently been gaining attention for their increasing concentrations in aquatic ecosystems and their use as supplements in sustainable aquaculture. This study demonstrated that water exposure to fulvic acid at concentrations of 5 mg C/L and 50 mg C/L increased lysozyme and alkaline phosphatase activities in the mucus by approximately 2-fold and 2.5 to 3.2-fold, respectively. Furthermore, exposure to 50 mg C/L resulted in a 77.0% increase in mucosal immunoglobulin concentrations compared to the other groups. Importantly, all mucus samples demonstrated significant antibacterial activity against Yersinia ruckeri, with control mucus reducing bacterial growth by 44.5% and exposure to fulvic acid increasing this effect to 26.3%. Although these modulations show promise for application in aquaculture, alterations of the beneficial microbiota from long-term exposure in natural waters can be expected. Monitoring the rising concentrations of humic substances in natural water bodies is therefore urgently needed. Overall, this study represents the first investigation revealing the ability of humic substances to modulate skin mucosal immunity and the capacity to combat microorganisms.

Effects of stress conditions on plasma parameters and gene expression in the skin mucus of farmed rainbow trout (Oncorhynchus mykiss)

The aim of this study was to investigate the physiological response of rainbow trout (Oncorhynchus mykiss) before slaughtering in the last phase of farming analyzing skin mucus and plasma. Two groups of rainbow trout were considered: Group UN ("unstressed"), represented by fish randomly captured from raceways, in the last phase of a standard fattening cycle; Group S ("stressed"), collected at the end of the pre-slaughtering tank, soon after slaughtering. The fish skin mucus was swabbed from head to tail using a sterile plastic spatula and the blood was collected through an endocardial puncture. qRT-PCR was used to study the gene expression in skin mucus. The mRNA expression levels of the IL-6 and IgD genes were higher in the S than in the Group UN. The plasma analysis showed an only a decrease in the glucose plasma levels in the Group S when compared to the Group UN. The present results indicated that the procedures adopted after slaughtering only affected changes in plasma glucose and skin mucus activity in rainbow trout suggesting that management protocol was compatible with non-stressful farming conditions.

Acellular fish skin for wound healing

Fish skin grafting as a new skin substitute is currently being used in clinical applications. Acceleration of the wound healing, lack of disease transmission, and low cost of the production process can introduce fish skin as a potential alternative to other grafts. An appropriate decellularization process allows the design of 3D acellular scaffolds for skin regeneration without damaging the morphology and extracellular matrix content. Therefore, the role of decellularization processes is very important to maintain the properties of fish skin. In this review article, recent studies on various decellularization processes as well as biological, physical, and mechanical properties of fish skin and its applications with therapeutic effects in wound healing were investigated.

Amoeba species colonizing the gills of rainbow trout (Oncorhynchus mykiss) in Swiss aquaculture

Nodular gill disease (NGD) is an infectious condition characterized by proliferative gill lesions leading to respiratory problems, oxygen deficiency and mortality in fish. Globally, NGD primarily impacts freshwater salmonids in intensive aquaculture systems. In recent years, numerous outbreaks of severe gill disease have affected more than half of the larger rainbow trout (Oncorhynchus mykiss) farms in Switzerland, mainly during spring and early summer. Mortality has reached up to 50% in cases where no treatment was administered. Freshwater amoeba are the presumed aetiologic agent of NGD. The gross gill score (GS) categorising severity of gill pathology is a valuable first-line diagnostic tool aiding fish farmers in identifying and quantifying amoebic gill disease (AGD) in farmed marine salmonids. In this study, the GS was adapted to the NGD outbreak in farmed trout in Switzerland. In addition to scoring disease severity, gill swabs from NGD-affected rainbow trout were sampled and amoeba were cultured from these swabs. Morphologic and molecular methods identified six amoeba strains: Cochliopodium sp., Naegleria sp., Vannella sp., Ripella sp., Saccamoeba sp. and Mycamoeba sp. However, the importance of the different amoeba species for the onset and progression of NGD still has to be evaluated. This paper presents the first description of NGD with associated amoeba infection in farmed rainbow trout in Switzerland.

Characterization of ready-to-eat fish surface as a potential source of contamination of Vibrio parahaemolyticus biofilms

The worldwide consumption of ready-to-eat seafood products has steadily increased due to a range of health benefits. However, depending on the handling or cutting process of raw fish, ready-to-eat sashimi can be exposed to microbiological risks that can lead to foodborne infection by marine pathogens. Since surface characteristics are key factors for microbial adhesion and biofilm formation, the present study aims to determine the correlation between raw fish skin properties and Vibrio parahaemolyticus biofilm formation. We analyzed V. parahaemolyticus biofilms (ATCC 17802; initially inoculated ca. 2 or 4 log CFU/cm²) on fish skin (gizzard shad, pomfret, red snapper, and mackerel; fish species served as sashimi without peeling the skin) formed under simulated marine environments (incubating in artificial seawater with rocking motion at 30 °C, the maximum temperature of seasonal seawater) for 24 h. The characteristics of fish skin were determined using confocal laser scanning microscopy/scanning electron microscopy. V. parahaemolyticus showed higher biofilm counts on fish skins than on stainless steel, which was used as a control (P < 0.05). In particular, V. parahaemolyticus formed biofilms with significantly higher levels of bacterial populations on gizzard shad and pomfret (ca. 5 log CFU/cm²; P < 0.05), highlighting the relationship between the biofilm formation level and the characteristics of gizzard shad and pomfret skins. The surface roughness of fish skins, including the main roughness parameters (Ra, Rq, and Rz), influenced the attachment of V. parahaemolyticus (P < 0.05). Additionally, images of V. parahaemolyticus biofilms suggested that different topographical profiles of fish species (e.g., mucus, unique structural features, etc.) could cause V. parahaemolyticus to exhibit different biofilm phenotypes, such as sticking to or entangling on the fish skin surface. The major findings of this study provide various phenotypic adhesions of V. parahaemolyticus to fish skin in considerations of the potential hazard for the consumption of ready-to-eat sashimi served with its skin.

Toxicity assessment of lead, nickel and cadmium on zebra fish augmented with Bacillus xiamenensis VITMSJ3: An insight on the defense mechanism against oxidative stress due to heavy metals

Increase urbanization in recent years has let to discharge of heavy metals into the environment which has caused severe impacts on soil as well as water. Therefore the current study was aimed to assess the toxicity of lead (Pb), nickel (Ni), and cadmium (Cd) from the contaminated water using zebra fish Danio rerio and detoxification of metals upon augmentation with Bacillus xiamenensis. Exposure doses till 150 mg L^-1 of Pb, Ni and Cd in water showed lethal effects on fish. Similarly the histopathological analysis showed severe tissue disruption in the gills and liver which were less upon supplementation with bacterial strain VITMSJ3. On the 20th day, the uptake concentration of Pb, Ni and Cd in zebra fish was found to be 87 mg L^-1, 89 mg L^-1 and 91 mg L^-1 respectively with VITMSJ3, from the water. Antioxidant enzymatic activities showed an increase upon bacterial supplementation, which reduced the oxidative stress. Further SEM-EDAX analysis confirmed the presence of Pb, Ni and Cd ions adsorbed on the gills. The results clearly showed less oxidative damages in fish with increased head and reduced tail %. Overall, the results showed a significant difference (p < 0.05) among the treatments compared with the control.

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.

Mediterranean Sea heatwaves jeopardize greater amberjack's (Seriola dumerili) aquaculture productivity through impacts on the fish microbiota

Climate change is dramatically increasing the frequency and severity of marine heatwaves (MHWs) in the Mediterranean basin, strongly affecting marine food production systems. However, how it will shape the ecology of aquaculture systems, and the cascading effects on productivity, is still a major knowledge gap. The present work aims to increase our understanding of future impacts, caused by raising water temperatures, on the interaction between water and fish microbiotas, and consequential effects upon fish growth. Thus, the bacterial communities present in the water tanks, and mucosal tissues (skin, gills and gut), of greater amberjack farmed in recirculatory aquaculture systems (RAS), at three different temperatures (24, 29 and 33 °C), were characterized in a longitudinal study. The greater amberjack (Seriola dumerili) is a teleost species with high potential for EU aquaculture diversification due to its fast growth, excellent flesh quality and global market. We show that higher water temperatures disrupt the greater amberjack's microbiota. Our results demonstrate the causal mediation exerted by this bacterial community shifts on the reduction of fish growth. The abundance of members of the Pseudoalteromonas is positively correlated with fish performance, whereas members of the Psychrobacter, Chryseomicrobium, Paracoccus and Enterovibrio are suggested as biomarkers for dysbiosis, at higher water temperatures. Hence, opening new evidence-based avenues for the development of targeted microbiota-based biotechnological tools, designed to increase the resilience and adaptation to climate change of the Mediterranean aquaculture industry.

An overview of the structure and function of the flagellar hook FlgE protein

The flagellum is an important organelle for the survival of bacteria and consists of a basal body, hook, and filament. The FlgE protein is the subunit of the hook that connects the basal body and the filament and determines the motility of bacteria. Also, flgE gene plays an essential role in flagellar biosynthesis, swimming ability and biofilm formation. Although the intact flagella and the major component filament have been extensively studied, so far, little is known about the comprehensive understanding of flagellar hook and FlgE. Here in this review, we summarize the structures of flagellar hook and its subunit FlgE in various species and physiological functions of FlgE, including the hook assembly, the structural characteristics of flagellar hook, the mechanical properties of hook, and the similarities and differences between FlgE (hook) and FlgG (distal rod), with special attention on the interaction of FlgE with other molecules, the antigenicity and pro-inflammatory effect of FlgE, and cross-linking of FlgE in spirochetes. We hope our summary of this review could provide a better understanding of the FlgE protein and provide some useful information for developing new effective antibacterial drugs in the future work.

Dietary powder and molecular imprinted polymer nanoencapsulated sodium propionate to enhance growth performance, digestive enzymes activity, antioxidant defense, and mucosal immune response in African cichlid (Labidochromis lividus) fingerlings

This study was conducted to examine the effects of powder sodium propionate (P-SP) and SP-loaded molecular imprinted polymer (MIP) nanoparticles (MIP-SP NPs) on the growth, skin mucosal immune parameters, and digestive and liver enzymes activities of African cichlid (Labidochromis lividus) fingerlings. Fish with an average weight of 500±2 mg were stocked into 12 experimental units and fed on experimental diets prepared by supplementing the basal diet (control) with MIP NPs, P-SP (5 g SP Kg-1 of dry diet), and MIP-SP NPs for 8 weeks. The findings demonstrated that growth indices improved in the MIP-SP NPs followed by the P-SP dietary group compared to the control groups (P<0.05). The activity of digestive enzymes of lipase, trypsin, protease, and alkaline phosphatase was higher in the fish fed SP-supplemented diets than in the controls (P<0.05). The protease and lipase activities in the MIP-SP NPs dietary group increased by 29.33% and 48.81% compared to the control, respectively (P<0.05). In addition, the alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels of liver tissue decreased in the SP dietary groups, while the catalase (CAT), superoxide dismutase (SOD), and alkaline phosphatase (ALP) levels increased compared to the control groups (P<0.05). The highest SOD and ALP levels were observed in the fish fed on the MIP-SP NPs-supplemented diet (P<0.05). Furthermore, the skin mucosal immune indices including, alternative haemolytic complement activity (ACH50), lysozyme, and total immunoglobulin (Ig) levels increased in the MIP-SP NPs and P-SP dietary groups compared to the controls (P<0.05). The findings indicated that sodium propionate encapsulated in molecularly imprinted polymer nanoparticles could enhance the efficiency of dietary SP in African cichlid fish.

A novel gnotobiotic experimental system for Atlantic salmon (Salmo salar L.) reveals a microbial influence on mucosal barrier function and adipose tissue accumulation during the yolk sac stage

Gnotobiotic models have had a crucial role in studying the effect that commensal microbiota has on the health of their animal hosts. Despite their physiological and ecological diversity, teleost fishes are still underrepresented in gnotobiotic research. Moreover, a better understanding of host-microbe interactions in farmed fish has the potential to contribute to sustainable global food supply. We have developed a novel gnotobiotic experimental system that includes the derivation of fertilized eggs of farmed and wild Atlantic salmon, and gnotobiotic husbandry of fry during the yolk sac stage. We used a microscopy-based approach to estimate the barrier function of the skin mucus layer and used this measurement to select the derivation procedure that minimized adverse effects on the skin mucosa. We also used this method to demonstrate that the mucus barrier was reduced in germ-free fry when compared to fry colonized with two different bacterial communities. This alteration in the mucus barrier was preceded by an increase in the number of cells containing neutral mucosubstances in the anterior segment of the body, but without changes in the number of cells containing acidic substances in any of the other segments studied along the body axis. In addition, we showed how the microbial status of the fry temporarily affected body size and the utilization of internal yolk stores during the yolk sac stage. Finally, we showed that the presence of bacterial communities associated with the fry, as well as their composition, affected the size of adipose tissue. Fry colonized with water from a lake had a larger visceral adipose tissue depot than both conventionally raised and germ-free fry. Together, our results show that this novel gnotobiotic experimental system is a useful tool for the study of host-microbe interactions in this species of aquacultural importance.

The feasibility of skin mucus replacing exosome as a pool for bacteria-infected markers development via comparative proteomic screening in teleost

In fish, skin mucus forms a protective barrier between the body surface and the external water environment, thus providing the most direct and intuitive clues to monitor the subject's health condition. To explore the impact of the Vibrio harveyi pathogen on teleost, the proteome of epidermal mucus from control and sick Cynoglossus semilaevis were screened through iTRAQ followed with LC-MS/MS. 1531 credible proteins were obtained relating to structural, metabolic and immunological functions. 335 different expressed proteins (DEPs) were identified, with 166 up-regulated and 169 down-regulated in MS. 62 proteins were characterized, including 22 up-regulated proteins and 40 down-regulated proteins. Integrated analysis of DE-miRNAs and DEPs from miRomics and proteomics were conducted to show the indirect regulatory relationship. Comparative analysis of DEPs between mucus and exosomes demonstrated that exosomes contributed the most DEPs of all mucus DEPs. 125 proteins are DEPs only in exosomes, which presented minor difference in total mucus. Expression of Aminopeptidase (anpep), Calcium-transporting ATPase, Histone H2B and H2A confirmed implied fine discriminative power with infected C. semilaevis, among which Calcium-transporting ATPase and H2B also appeared in list of exosomal markers. This study might shed the light on effective biomarker digging at other extended screening scenarios.

Sex and reproductive development impact skin mucosal epithelium immunity, antimicrobial capacity, and up-regulation of immune-related gene of goldfish (Carassius auratus)

This study evaluated the epidermis mucosal capacity of goldfish (Carassius auratus) during different stages of reproductive development in both females and males. In this regard, the activity of mucolytic immune enzymes, i.e., lysozyme, complement and peroxidase, as well as the activity of alkaline phosphatase (ALP) were evaluated. There were five stages for females i.e., immature (f1), cortical alveoli (f2), early and late-vitellogenesis (vtg) (f3 and f4) and ripe (f5); as well as two stages for males spermatogenesis (m1) and spermiation (m2). Some stages were also examined for the mucosal antimicrobial activity against specific pathogens. The results showed that the mucosal lysozyme activity increased significantly during vitellogenesis (P < 0.05), but no lysozyme activity was detected in plasma. On the contrary, the complement activity was only observed in female plasma, and it was significantly higher at f3 compared to the other developmental stages. Both the plasma and mucosal ALP and peroxidase activities showed a significant increase by female reproductive development with the highest amounts at f4. Contrary to the female, no significant changes were observed in plasma and mucosal immune agents and biochemistry of the male. The f5-staged goldfish showed the highest antimicrobial activities against Gram-positive bacteria, i.e., Streptococcus faecium, Staphylococcus aureus and Micrococcus luteus (P < 0.05). Our results also represented the up-regulation of lysozyme (c-lys) gene expression by effects of female maturational development in ovary, liver and skin, while male goldfish showed no significant changes in c-lys expression. Moreover, there were positive correlations between c-lys expression, mucosal lysozyme activity and calcium levels in females (P < 0.01). Overall, our findings revealed that vtg process improves mucosal innate immunity that leads to activate antimicrobial components at spawning season.

Supplementing artemisinin positively influences growth, antioxidant capacity, immune response, gut health and disease resistance against Vibrio parahaemolyticus in Litopenaeus vannamei fed cottonseed protein concentrate meal diets

Artemisinin (ART) is a kind of Chinese herbal medicine worth exploring, which obtains various physiological activities. In order to study the prebiotic effect of ART on Litopenaeus vannamei fed cottonseed protein concentrate meal diets, six groups of isonitrogenous and isolipid diets were prepared (including the fish meal control group, FM; cottonseed protein concentrate replacing 30% fishmeal protein and supplementing ART groups: ART0, ART0.3, ART0.6, ART0.9, and ART1.2). The feeding trials was lasted for 56 days. The results showed that the final body weight, weight gain and specific growth rate of the ART0.6 group were the highest, yet the feed coefficient rate of the ART0.6 group was the lowest significantly (P < 0.05). There was no significant difference in survival rate among treatments (P > 0.05). In serum, the content of malondialdehyde in ART0 group was the highest (P < 0.05); the activities of superoxide dismutase, catalase, phenol oxidase and lysozyme increased firstly and then decreased among the ARTs groups (P < 0.05). The activities of intestinal digestive enzymes (including the trypsin, lipase and amylase) showed an upward trend among the ARTs groups (P < 0.05). The histological sections showed that the intestinal muscle thickness, fold height and fold width in the FM group were significantly better than those in the ART0 group; while the mentioned above morphological indexes in the ART0 group were significantly lowest among the ARTs groups (P < 0.05). Sequencing of intestinal microbiota suggested that the microbial richness indexes firstly increased and then decreased (P < 0.05); the bacterial community structure of each treatment group was almost close; the relative abundance of pathogenic bacteria decreased significantly (P < 0.05), such as the Proteobacteria and Cyanobacteria at phylum level, besides the Vibrio and Candidatus Bacilloplasma at genus level. In intestinal tissue, the relative expression levels of TOLL1, TRAF6 and Pehaeidih3 showed up-regulated trends, while the expression of Crustin and LZM firstly up-regulated and then down-regulated (P < 0.05). The challenge experiment suggested that the cumulative mortality of FM group was significantly lower than that of ART0 group; besides the cumulative mortality firstly increased and then decreased between the ARTs groups (P < 0.05). In conclusion, the dietary supplementation of ART can improve the growth, antioxidant capacity, immune response, gut health and disease resistance of the shrimp. To be considered as a dietary immune enhancer, the recommended supplementation level of ART in shrimp's cottonseed protein concentrate meal diets is 0.43%.

Evaluating the Functional Properties of Spray-Dried Porcine Plasma in Gilthead Seabream (Sparus aurata) Fed Low Fish Meal Diets

Blood by-products are an untapped source of high-quality ingredients for aquafeeds, containing a broad variety of cytokines, hormones, growth factors, proteins, bioactive peptides, and amino acids. The effects of the spray-dried porcine plasma (SDPP), a type of processed animal protein on several immune parameters, were evaluated in sea bream using ex vivo and in vitro assays. In this study, fish were fed with two isoproteic, isolipidic, and isoenergetic diets: control diet (7% fish meal, FM) and SDPP diet (2% FM and 5% SDPP). At the end of the 92-days trial, those fed the SDPP diet were larger in body weight (p < 0.05) without differences in feed conversion ratio (p > 0.05). The ex vivo immune stimulation of splenocytes indicated that SDPP had a beneficial effect in promoting systemic immunity, since the surface cell marker (cd4), pro- (il-1β), and anti-inflammatory (tgf-β1) cytokines, and genes involved in humoral immunity (IgM) were up-regulated. The co-culture assays of skin mucus corroborated that SDPP enhanced the antibacterial capacity of mucus against V. anguillarum. In addition, main mucus biomarkers did not show significant differences, except for cortisol levels which were lower in the SDPP diet. The present study indicated that SDPP may be considered a functional ingredient in aquafeeds formulated with low FM levels.

Serum-derived exosomes induce proinflammatory cytokines production in Cynoglossus semilaevis via miR-133-3p

Exosomes are small membrane-enclosed vesicles secreted by various types of cells. In mammals, a wide range of physiological and pathological functions have been confirmed and attributed to EVs carrying a variety of molecular cargoes, including miRNAs. However, studies on the biological functions and related molecular mechanisms of serum exosomes isolated from teleost fish are limited. Indeed, the molecular mechanisms underlying the effects of serum exosomes on immune responses and inflammatory processes are unknown. Chinese tongue sole (Cynoglossus semilaevis) is an economically important species used widely in industrial aquaculture. Vibrio harveyi, a common bacterial pathogen that infects C. semilaevis and some other fish, causes excessive inflammatory reactions, which are characterized by skin ulceration. Here, we isolated serum-derived exosomes from C. semilaevis and investigated their effects on inflammatory processes following V. harveyi infection. We found that compared with uninfected fish, exosome abundance in infected fish blood increased with bacterial infection time, while expression of TNF-α increased, and that of IL-10 decreased, significantly. Moreover, artificial infection studies demonstrated that injection of serum exosomes isolated from infected fish increased expression of TNF-α, IL-6, and IL-8, which is consistent with the increase in proinflammatory cytokines induced by V. harveyi infection. To further investigate the mechanisms by which exosomes increase proinflammatory cytokine production, we performed miRNA expression profiling and found that 26 differentially expressed miRNAs were associated with bacterial infection and immune responses; of these, miR-133-3p was considerably more abundant in serum exosomes from infected fish. Bioinformatics analysis suggested that miR-133-3p inhibits NF-κB signaling pathways by targeting PP2A and affecting cytokine release. We also found that miR-133-3p increased expression of TNF-α, IL-6, and IL-8 in fish blood and kidney, whereas an miR-133-3p inhibitor showed the opposite results. Thus, the data suggest that serum exosomes participate in innate immunity in teleost fish by promoting inflammatory responses to bacterial infection. Exosome-mediated transfer of miR-133-3p increases expression of proinflammatory cytokines in C. semilaevis, resulting in excessive inflammatory responses during V. harveyi infection. These data may lead to development of methods and strategies that control skin ulceration in Chinese tongue sole.

Host–Pathogen Interactions of Marine Gram-Positive Bacteria

Simple Summary

Complex interactions between marine Gram-positive pathogens and fish hosts in the marine environment can result in diseases of economically important finfish, which cause economic losses in the aquaculture industry. Understanding how these pathogens interact with the fish host and generate disease will contribute to efficient prophylactic measures and treatments. To our knowledge, there are no systematic reviews on marine Gram-positive pathogens. Therefore, here we reviewed the host–pathogen interactions of marine Gram-positive pathogens from the pathogen-centric and host-centric points of view.

Abstract

Marine Gram-positive bacterial pathogens, including Renibacterium salmoninarum, Mycobacterium marinum, Nocardia seriolae, Lactococcus garvieae, and Streptococcus spp. cause economic losses in marine fish aquaculture worldwide. Comprehensive information on these pathogens and their dynamic interactions with their respective fish–host systems are critical to developing effective prophylactic measures and treatments. While much is known about bacterial virulence and fish immune response, it is necessary to synthesize the knowledge in terms of host–pathogen interactions as a centerpiece to establish a crucial connection between the intricate details of marine Gram-positive pathogens and their fish hosts. Therefore, this review provides a holistic view and discusses the different stages of the host–pathogen interactions of marine Gram-positive pathogens. Gram-positive pathogens can invade fish tissues, evade the fish defenses, proliferate in the host system, and modulate the fish immune response. Marine Gram-positive pathogens have a unique set of virulence factors that facilitate adhesion (e.g., adhesins, hemagglutination activity, sortase, and capsules), invasion (e.g., toxins, hemolysins/cytolysins, the type VII secretion system, and immune-suppressive proteins), evasion (e.g., free radical quenching, actin-based motility, and the inhibition of phagolysosomal fusion), and proliferation and survival (e.g., heme utilization and siderophore-mediated iron acquisition systems) in the fish host. After infection, the fish host initiates specific innate and adaptive immune responses according to the extracellular or intracellular mechanism of infection. Although efforts have continued to be made in understanding the complex interplay at the host–pathogen interface, integrated omics-based investigations targeting host–pathogen–marine environment interactions hold promise for future research.

Influence of black rice (Oryza sativa L.) bran derived anthocyanin-extract on growth rate, immunological response, and immune-antioxidant gene expression in Nile tilapia (Oreochromis niloticus) cultivated in a biofloc system

This study investigated the effects of dietary supplementation with anthocyanin extracted from black rice bran (AR) on the growth rate, immunological response, and expression of immune and antioxidant genes in Nile tilapia raised in an indoor biofloc system. A total of 300 Nile tilapia fingerlings (15.14 ± 0.032 g) were maintained in 150 L tanks and acclimatized for two weeks. Five experimental AR diets (0, 1, 2, 4, and 8 g kg^-1) with various anthocyanin doses were used to feed the fish. We observed that the growth and feed utilization of fish fed with different dietary AR levels increased significantly after eight weeks (p < 0.05). In addition, the serum immunity of fish fed AR diets was much greater than that of those fed non-AR diets (p < 0.05). However, there were little or no difference in between fish fed AR enriched diets and the control AR-free diet (p > 0.05). After eight weeks, fish fed AR-supplemented diets had significantly higher mRNA transcript levels in immune (interleukin [IL]-1, IL-8, and liposaccharide-binding protein [LBP]) and antioxidant (glutathione transferase-alpha [GST-α] and glutathione reductase [GSR]) genes compared to control fish fed the AR-free diet, with the greatest enhancement of mRNA transcript levels (in the case of IL-8 by up to about 5.8-fold) in the 4 g kg^-1 AR diet. These findings suggest that dietary inclusion of AR extract from black rice bran at 4-8 g kg^-1 could function as a herbal immunostimulant to enhance growth performance, feed consumption, and immunity in Nile tilapia.

Tollip suppresses MyD88-mediated NF-κB activation by enhancing MyD88 ubiquitination levels in large yellow croaker (Larimichthys crocea)

Toll-interacting protein (Tollip) plays an important role in the innate immune response by negative regulation of the TLR-IL-1R signaling pathway. MyD88 serves as a universal adaptor in TLR-mediated NF-κB activation. However, the regulation mechanisms of Tollip in piscine MyD88-mediated NF-κB activation is largely unknown. In the present study, the cDNA sequence of LcTollip was identified from the large yellow croaker (Larimichthys crocea). The putative LcTollip protein encoded 275 amino acid residues, containing a N-terminal TBD domain, a central C2 domain, and a C-terminal CUE domain. Quantitative PCR showed that the most predominant constitutive expression of LcTollip was detected in spleen. In addition, LcTollip transcripts enhanced significantly after LPS and poly I:C challenge (P < 0.05). Cellular localization revealed that LcTollip existed in the cytoplasm and nucleus. Furthermore, the overexpression plasmids of wild type LcTollip as well as its six domain truncated mutants of LcTollip were constructed by overlap PCR. Dual luciferase analysis showed that NF-κB activation could not be induced by overexpression of LcTollip or its domain truncated mutants alone. However, the LcMyD88-induced-NF-κB activation was significantly suppressed by overexpression with LcTollip, and the truncated mutants LcTollip-ΔTBD, LcTollip-ΔC2, LcTollip-ΔCUE and LcTollip-ΔTBDΔCUE while not by LcTollip-ΔLR and LcTollip-ΔTBDΔC2. Moreover, co-immunoprecipitation (Co-IP) assay revealed that the interaction between LcTollip and LcMyD88 was through CUE domain. More interesting, IP and immunoblotting examination of HEK293T cells co-transfected with LcMyD88, LcTollip and HA-ubiquitin showed that LcMyD88 induced a dose-dependent de-ubiquitination of LcTollip while LcTollip enhanced a dose-dependent ubiquitination of LcMyD88. However, protein degradation investigation displayed that the proteolysis and ubiquitination of LcMyD88 were not connected. Our findings suggested that the LcTollip might involve in negative regulation TLR pathway by suppressing LcMyD88-mediated immune activation and improving the ubiquitination level of LcMyD88.

Histology and morphometry of the skin of the trident goby Tridentiger brevispinis (Perciformes, Gobiidae)

The Korean trident goby, Tridentiger brevispinis, lives in adverse habitats that can easily become hypoxic due to low precipitation, regional dry periods, and high amounts of solar radiation. Histological and morphometric studies revealed the goby’s specialized skin (35.4–150.0 μm in thickness), consisting of an epidermis and dermis. The thicker epidermis comprises an outermost surface layer (having taste buds, stratified flattened cells, mucous cells, pigment cells, and stratified polygonal cells), middle layer (having stratified polygonal cells), and stratum germinativum (stratified columnar cells). In particular, the dermis has scales, well-developed vascularization, and a few blood capillaries just above the basement membrane, and a reduced diffusion distance was present in the lateral body. Consequently, adaptations such as thicker epidermis, well-developed vascularization, few blood capillaries, and a reduced diffusion distance may provide cutaneous respiration for survival in poorly oxygenated water during the periodic dry season.

Seasonal variations in the skin epidermal structure and mucosal immune parameters of rainbow trout skin (Oncorhynchus mykiss) at different stages of farming

The aim of this study was to investigate the seasonal changes in the epidermal structure and the innate immunity parameters of skin mucus in rainbow trout. The skin epidermis and mucus samples were collected over three consecutive seasons including winter, spring and late summer from three different weight groups i.e., 2-20 g (W1), 100-200 g (W2) and 400-600 g (W3) fish. The skin mucosal immunity analysis of rainbow trout showed that the haemagglutination activity increased significantly with increasing fish size from W1 to W3 in all three seasons, while no significant seasonal changes occurred in haemagglutination activity. Moreover, the bactericidal activity against fish pathogens increased significantly with increasing water bacterial load in late summer. The SDS-PAGE analysis of mucus showed a high amount of low molecular weight proteins (<35 kDa) in the late summer that was correlated with the increase in bactericidal activity. Histological analysis of the epidermis structure of rainbow trout skin showed that the density and size of goblet cells and consequently the mucus secretion significantly increased in W3 group in all seasons. In all three weight groups of fish, the density of goblet cells significantly increased from winter to spring and late summer along with increasing water temperature. Moreover, the goblet cell density showed a significant positive relationship with the soluble protein concentration and haemagglutination activity (p < 0.01). The results of this study demonstrated the more active immune role of the skin epidermal cells and mucus in rainbow trout during summer to protect fish against the pathogenic microorganisms. Given its potent bactericidal properties and the lack of haemolytic activity, the rainbow trout mucus might be used as a safe and inexpensive source for developing antimicrobial agents to prevent and treat some bacterial diseases in human and fish.

Aeromonas hydrophila Induces Skin Disturbance through Mucosal Microbiota Dysbiosis in Striped Catfish (Pangasianodon hypophthalmus)

Bacterial pathogens are well equipped to adhere to and initiate infection in teleost fish. Fish skin mucus serves as the first barrier against environmental pathogens. The mucus harbors commensal microbes that impact host physiological and immunological responses. However, how the skin mucosal microbiota responds to the presence of pathogens remains largely unexplored. Thus, little is known about the status of skin mucus prior to infection with noticeable symptoms. In this study, we investigated the interactions between pathogens and the skin mucosal microbiota as well as the fish skin immune responses in the presence of pathogens. Striped catfish (Pangasianodon hypophthalmus) were challenged with different concentrations of the bacterial pathogen Aeromonas hydrophila (AH), and the skin immune response and the mucosal microbiota were examined by quantitative PCR (qPCR) and 16S rRNA gene sequence analysis. We determined that the pathogen concentration needed to stimulate the skin immune response was associated with significant mucosal microbiota changes, and we reconfirmed these observations using an ex vivo fish skin model. Further analysis indicated that changes in the microbiota were attributed to a significant increase in opportunistic pathogens over AH. We concluded that the presence and increase of AH result in dysbiosis of the mucosal microbiota that can stimulate skin immune responses. We believe that our work sheds light on host-pathogen-commensal microbiota interactions and therefore contributes to aquaculture fish health.

IMPORTANCE The fish skin mucosal microbiota is essential in modulating the host response to the presence of pathogens. Our study provides a platform to study both the correlation and causation of the interactions among the pathogen, fish skin, and the skin mucosal microbiota. Based on these findings, we provide the first mechanistic information on how mucosal microbiota changes induced by the pathogen AH result in skin disturbance with immune stimulation in striped catfish in the natural state and a potential direction for early-infection screening. Thus, this study is highly significant in the prevention of fish disease.

Prevalence of bacterial infections and the use of multiplex PCR assay for rapid detection of pathogens in cultured fish in Ghana

The modern and rapid avenue for detecting pathogens provided by molecular genetic techniques including polymerase chain reaction (PCR) was explored in the present study to identify prevalent disease pathogens, from six aquaculture farms and in two commonly cultured fish in Ghana. The specific detection was carried out directly on clinical samples of naturally infected fish (O. niloticus and C. gariepinus) based on syber-mix reaction protocol in traditional PCR. Molecular diagnostic techniques allowed the detection of the six most common and important bacterial pathogens in aquaculture farms in Ghana. Also, three of the pathogens (Streptococcus agalactiae, Streptococcus iniae, and Staphylococcus aureus) were simultaneously isolated in a multiplex reaction. The results indicated 90-100% sensitivity and specificity for each of the six bacterial pathogens tested. Streptococcosis and motile aeromonad septicemia were found to be highly prevalent in most aquaculture farms in Ghana with severity in infections traced to the 85.7% and 14.9% co-infections with all six target pathogens in catfish and tilapia respectively. The prevalence rate of infections significantly correlated with variations in salinity, conductivity, and dissolved oxygen concentrations in the thermal stressed condition of the culture water. Multiplex techniques employed in this study represent one of the first to be used by a fish health laboratory in Ghana for rapid detection of pathogens in diseased fish and could be a useful alternative to the culture-based method for routine diagnosis of fish diseases in Ghana.

Non-lethal sampling for the detection of Renibacterium salmoninarum by qPCR for diagnosis of bacterial kidney disease

Bacterial kidney disease (BKD), caused by Renibacterium salmoninarum (Rs), can be transmitted both horizontally and vertically and there is no available cure or prophylaxis. The control of BKD requires continuous surveillance, which is challenging in aquaculture as well as in programs for conservation and restoration of salmonid fish strains. BKD is a notifiable disease in Sweden and is monitored through the mandatory health control program using a polyclonal ELISA for detection of the Rs p57 protein in kidney. Fish must be killed for sampling, an obvious disadvantage especially regarding valuable broodfish. The present study shows that gill-/cloacal swabs collected in vivo for real-time PCR (qPCR_gc ), allow a sensitive and specific detection of Rs. The sensitivity of qPCR_gc was estimated to 97.8% (credible interval (ci) 93.8%-100%) compared to 98.3% (ci 92.7%-100%) and 48.8% (ci 38.8%-58.8%) of kidney samples for qPCR (qPCR_k ) and ELISA (ELISA_k ) respectively, by use of the Bayesian Latent Class Analysis (BLCA). Since the goal of the program is eradication of BKD the most sensitive test is preferrable. Using qPCR_gc instead of ELISA_k will result in a lower false negative rate and can be useful for surveillance in aquaculture and in breeding programs with valuable fish. However, a higher false positive rate warrants confirmatory lethal testing before a previously Rs negative farm is subject to restrictions.

Structural and Functional Aspects of the Spleen in Molly Fish Poecilia sphenops (Valenciennes, 1846): Synergistic Interactions of Stem Cells, Neurons, and Immune Cells

In fish, the spleen is the prime secondary lymphoid organ. It has a role in the induction of adaptive immune responses, in addition to its significance in the elimination of immune complexes. This study was conducted on 18 randomly obtained adult molly fish (Poecilia sphenops) of both sexes using histological, immunohistochemical, and ultrastructural studies to highlight the cellular components of the spleen and their potential role in the immune system. The spleen of molly fish was characterized by the presence of well-distinct melanomacrophage centers, and other basic structures present in higher vertebrates including red and white pulps, blood vessels, and ellipsoids. Some mitotic cells could also be identified in the red pulp. Mast cells with characteristic metachromatic granules could be seen among the splenic cells. Rodlet cells were randomly distributed in the spleen and were also observed around the ellipsoids. The white pulp of the spleen expressed APG5. The expressions were well distinct in the melanomacrophages, leukocytes, and macrophages. Myostatin was expressed in leukocytes and epithelial reticular cells. IL-1β showed immunoreactivity in monocytes and macrophages around the ellipsoids. NF-κB and TGF-β were expressed in macrophages and epithelial reticular cells. Nrf2 expression was detected in stem cells and rodlet cells. Sox-9 had a higher expression in epithelial reticular cells and stem cells. The high frequency of immune cells in the spleen confirmed its role in the regulation of both innate and adaptive immunity, cell proliferation, and apoptosis.

Transport and Barrier Functions in Rainbow Trout Trunk Skin Are Regulated by Environmental Salinity

The mechanisms underpinning ionic transport and barrier function have been relatively well characterised in amphibians and fish. In teleost fish, these processes have mostly been characterised in the gill and intestine. In contrast, these processes remain much less clear for the trunk skin of fish. In this study, we measured barrier function and active transport in the trunk skin of the rainbow trout, using the Ussing chamber technique. The effects of epithelial damage, skin region, salinity, and pharmacological inhibition were tested. Skin barrier function decreased significantly after the infliction of a superficial wound through the removal of scales. Wound healing was already underway after 3 h and, after 24 h, there was no significant difference in barrier function towards ions between the wounded and control skin. In relation to salinity, skin permeability decreased drastically following exposure to freshwater, and increased following exposure to seawater. Changes in epithelial permeability were accompanied by salinity-dependent changes in transepithelial potential and short-circuit current. The results of this study support the idea that barrier function in rainbow trout trunk skin is regulated by tight junctions that rapidly respond to changes in salinity. The changes in transepithelial permeability and short circuit current also suggest the presence of an active transport component. Immunostaining and selective inhibition suggest that one active transport component is an apical V-ATPase. However, further research is required to determine the exact role of this transporter in the context of the trunk skin.

Water Quality and Microbiological Contamination across the Fish Marketing Chain: A Case Study in the Peruvian Amazon (Lagoon Yarinacocha)

The contamination of the surface water of lagoons is a common problem in developing countries, and can affect fishing activities. A case study was conducted on water quality and microbiological contamination of the fishing marketing chain in the Peruvian Amazon (Laguna de Yarinacocha). The microbiological, physical–chemical and parasitological parameters of the surface water were evaluated in three points of the lagoon near the landing stage; and microbiological parameters of facilities, handlers and three species of fish (Prochilodus nigricans, Mylossoma duriventre and Siluriforme spp.). In the water, there were coliform counts ≥ 23 (Most probable number—MPN)/100 mL, Escherichia coli ≥ 3.6 MPN/100 mL, and Pseudomona spp. up to 2.2 MPN/100 mL; high turbidity and variable amounts of parasites. In facilities and handlers, high levels of coliforms, mainly Escherichia coli, and Staphylococcus aureus and Escherichia coli, were found in M. duriventre meat. A poor quality of the surface water of the lagoon is concluded that compromises part of the fishing marketing chain, mainly facilities and manipulators. Furthermore, the levels of Staphylococcus aureus and Escherichia coli in fish meat show poor handling practices and possible risk of contamination by water sources.

Modulatory effects of longan seed powder on growth performance, immune response, and immune-antioxidant related gene expression in Nile tilapia (Oreochromis niloticus) raised under biofloc system

This study evaluates the effects of longan seed powder (LS) on the growth performance, immunological response, and immune-antioxidant related gene expression of Nile tilapia (Oreochromis niloticus). Three hundred fish (13.82 ± 0.06 g) were divided into five experiments and fed 5 diets, including the basal diet (control without LS) and basal diet containing 10 (LS10), 20 (LS20), 40 (LS40), and 80 (LS80) g kg^-1 LS for eight weeks. A completely randomized design (CRD) with three replications was utilised. The growth performance and immune response were measured at weeks 4 and 8 post feeding, while the gene expressions were determined at the end of the feeding trial. The results revealed that administration of LS could significantly (P < 0.05) improve specific growth rate (SGR), weight gain (WG), and feed conversion ratio (FCR) in Nile tilapia as compared to the control group. However, no significant differences (P > 0.05) were observed in survival rates among treatments. LS-supplemented diets showed enhanced serum peroxidase activity (SPA), serum lysozyme activity (SLA), skin mucus lysozyme activity (MLA), and skin mucus peroxidase activity (MPA) at weeks 4 and 8 post-feeding, with the highest values observed in the LS20 diet (P < 0.05). Additionally, LS-supplemented diets significantly up-regulated (P < 0.05) immune and antioxidant related gene expressions (IL1, IL8, LBP, GSTa, GPX, and GSR) in the liver and intestine, with highest values observed in the LS20 treatment. The present results confirmed the beneficial effects of LS as a functional feed additive and immunostimulant for Nile Tilapia culture in a biofloc system.

Rheological Properties and Drag Reduction Performance of Puffer Epidermal Mucus

Most species of fish are covered with mucus, which provides the effect of reduction in swimming drag. In this paper, three concentrations of puffer epidermal mucus were obtained from the epidermal mucosa of puffer. The rheological properties and the drag reduction performance of the puffer epidermal mucus were characterized via a rheometer experimental and numerical simulation method. The relationship between the rheological properties and the drag reduction performance was analyzed and discussed, and the drag reduction mechanism of the puffer epidermal mucus was further explored. The results showed that the best drag reduction rate was 6.2% when the inflow velocity and concentration of puffer epidermal mucus were 0.1 m/s and 18.2 g/L, respectively. The rheological properties of puffer epidermal mucus are viscoelastic, and the mucus forms a sliding surface, which reduces the frictional drag of the fluid. In conclusion, this paper may provide a reference for the development of drag-reducing agents and drag-reducing research studies on other fish mucus.

Antibacterial activity of supernatants of Lactoccocus lactis, Lactobacillus rhamnosus, Pediococcus pentosaceus and curcumin against Aeromonas hydrophila. In vitro study

Secretions of beneficial intestinal bacteria can inhibit the growth and biofilm formation of a wide range of microorganisms. Curcumin has shown broad spectrum antioxidant, anti-inflammatory, and antimicrobial potential. It is important to evaluate the influence of these secretions with bioactive peptides, in combination with curcumin, to limit growth and inhibit biofilm formation of pathogenic bacteria of importance in aquaculture. In the present study, the supernatants of Lactoccocus lactis NZ9000, Lactobacillus rhamnosus GG and Pediococcus pentosaceus NCDO 990, and curcumin (0,1,10,25 and 50 μM) were used to evaluate their efficacy in growth, inhibition biofilm and membrane permeability of Aeromonas hydrophila CAIM 347 (A. hydrophila). The supernatants of probiotics and curcumin 1,10 and 25 μM exerted similar effects in reducing the growth of A. hydrophila at 12 h of interaction. The supernatants of the probiotics and curcumin 25 and 50 μM exerted similar effects in reducing the biofilm of A. hydrophila. There is a significant increase in the membrane permeability of A. hydrophila in interaction with 50 μM curcumin at two hours of incubation and with the supernatants separately in the same period. Different modes of action of curcumin and bacteriocins separately were demonstrated as effective substitutes for antibiotics in containing A. hydrophila and avoiding the application of antibiotics. The techniques implemented in this study provide evidence that there is no synergy between treatments at the selected concentrations and times.

Mucus piRNAs profiles of Vibrio harveyi-infected Cynoglossus semilaevis: A hint for fish disease monitoring

The half-smooth tongue sole, Cynoglossus semilaevis, is an important cultured flatfish species. Vibrio harveyi is a common pathogen to this fish, which may result in great economic loss to C. semilaevis culture industry. piRNAs, a non-coding RNAs with 26-32 nt, have been regarded as promising biomarkers for cancer diagnosis and fish diseases. Here, we extracted the RNA from mucus of C. semilaevis and constructed the differential expression profiles of piRNAs between the sick fish (MS) and healthy fish (MC). We identified 45,696 differentially expressed piRNAs including 22,735 up-regulated piRNAs and 22,961 down-regulated piRNAs in MS group compared with MC group. The GO enrichment and KEGG pathway enrichment analyses of the differential piRNAs were carried out. The result showed immunity-related target genes mainly involved in immune system process, response to stimulus, cell killing, immune system, infectious diseases and cell growth and death. The 10 most differentially expressed piRNAs were chosen to perform the qRT-PCR, while only seven piRNAs were consistent with the sequence result. Compared with MC group, the expression levels of piR-mmu-72173>piR-rno-62831>piR-xtr-704880, piR-dme-15546979, piR-mmu-49941660, piR-mmu-29283297 and piR-mmu-1758399 were significantly lower, and piR-gga-10574 and piR-gga-134812 were significantly higher in MS group. These piRNAs may be potential biomarkers during the V. harveyi infection of C. semilaevis. This study could provide a new method to identify the infection status of C. semilaevis and understand better about the innate and adaptive immune system in C. semilaevis during bacterial infection.

Untargeted Metabolomics on Skin Mucus Extract of Channa argus against Staphylococcus aureus: Antimicrobial Activity and Mechanism

Microbial contamination is one of the most common food safety issues that lead to food spoilage and foodborne illness, which readily affects the health of the masses as well as gives rise to huge economic losses. In this study, Channa argus was used as a source of antimicrobial agent that was then analyzed by untargeted metabolomics for its antibacterial mechanism against Staphylococcus aureus. The results indicated that the skin mucus extract of C. argus had great inhibitory action on the growth of S. aureus, and the morphology of S. aureus cells treated with the skin mucus extract exhibited severe morphological damage under scanning electron microscopy. In addition, metabolomics analysis revealed that skin mucus extract stress inhibited the primary metabolic pathways of S. aureus by inducing the tricarboxylic acid cycle and amino acid biosynthesis, which further affected the normal physiological functions of biofilms. In conclusion, the antimicrobial effect of the skin mucus extract is achieved by disrupting cell membrane functions to induce an intracellular metabolic imbalance. Hence, these results conduce to amass novel insights into the antimicrobial mechanism of the skin mucus extract of C. argus against S. aureus.

Impact of dietary porcine blood by-products in meagre (Argyrosomus regius) physiology, evaluated by welfare biomarkers and the antibacterial properties of the skin mucus

Tools are required for quick and easy preliminary evaluation of functional feeds efficiency on fisheries. The analysis of skin mucus biomarkers is a recent alternative approach providing a faster feed-back from the laboratory which is characterized by being less invasive, more rapid and with reduced costs. The effect of replacing fishmeal and fish protein hydrolysates by means of two porcine by-products, the porcine spray-dried plasma (SDPP) and pig protein hydrolysate (PPH), in compound diets (50.4% crude protein, 16.2% crude protein, 22.1 MJ/kg feed) was evaluated in juvenile meagre (Argyrosomus regius) during a two-months period. To determine the impact of these dietary replacements, growth and food performance were measured together with digestive enzymes activities and filet proximal composition. Additionally, skin mucus was collected and characterized by determining main mucus biomarkers (protein, glucose, lactate, cortisol, and antioxidant capacity) and its antibacterial properties, measured by the quick in vitro co-culture challenges. In comparison to the control group, the inclusion of PPH and SDPP, in meagre diets reduced growth (7.4-8.8% in body weight), increased feed conversion ratios (9.0-10.0%), results that were attributed to a reduction in feed intake values (24.2-33.0%) (P < 0.05). Porcine blood by-products did not modify the activity of gastric and pancreatic digestive enzymes as well as those involved in nutrient absorption (alkaline phosphatase) nor liver oxidative stress condition (P > 0.05). In contrast, a reduction in fillet lipid content associated to an increase in fillet protein levels were found in fish fed SDPP and PPH diets (P < 0.05). As compared to the control diet, the dietary replacement did not alter the levels of the skin mucus biomarkers related to stress (cortisol and antioxidant capacity) or nutritional status (soluble protein, glucose and lactate) (P > 0.05). Interestingly, regardless of the worst performance in somatic growth, meagre fed diets containing both tested porcine by-products showed a significantly improved antibacterial capacity of their skin mucus. This enhancement was more prominent for fish fed with the PPH diet, which may be attributed to a higher content of immunomodulatory bioactive compounds in PPH. Further research will be necessary to provide insights on how the inclusion of SDPP and PPH, at the expense of dietary fishmeal and fish protein hydrolysates, affects feed intake and growth performance in meagre. However, the use of skin mucus biomarkers has been demonstrated to be an excellent methodology for a preliminary characterization of the functional feeds, in particular for their prophylactic properties by the study of mucus antibacterial activity.

Transport Stress Induces Skin Innate Immunity Response in Hybrid Yellow Catfish (Tachysurus fulvidraco♀ × P. vachellii♂) Through TLR/NLR Signaling Pathways and Regulation of Mucus Secretion

The transport of live fish is a necessary step for commercial production. The skin of teleost fish is the first non-specific immune barrier against exogenous stimuli, and it plays an important protective role under transport stress. Thus, the aim of this study was to explore the skin responses to transport stress in hybrid yellow catfish (Tachysurus fulvidraco♀ × Pseudobagrus vachellii♂) through transcriptome and biochemical analyses. Water samples were collected during a simulated transport treatment. Biochemical indexes and/or gene expression in blood, skin, and mucus in fish in control groups and transport-stress groups (0 h, 2 h, 4 h, 8 h, 16 h) were assayed. The levels of total ammonia-nitrogen and nitrite-nitrogen in the water increased with increasing transport time. Comparison of skin transcriptomes between the control group and the group subjected to 16 h of transport revealed 1547 differentially expressed genes (868 up-regulated and 679 down-regulated). The results of the transcriptome analysis were validated by analyses of the expression levels of selected genes by qRT-PCR. The results indicated that the toll-like receptors and nod-like receptors signaling pathways mediate the skin's immune response to transport stress: tlr9, mfn2, and ikbke were significantly up-regulated and nfkbia and map3k7cl were significantly down-regulated under transport stress. With increasing transport time, lysozyme activity and the immunoglobulin M content in skin mucus first increased and then decreased. The number of mucous cells peaked at 8 h of transport stress, and then decreased. The mucus cells changed from types II and IV to types I, II, III, and IV. The amounts of red and white blood cells and the levels of hemoglobin and hematocrit first increased and then decreased during 16 h of transport stress. Together, the results showed that the skin responds to transport stress by activating the immune signaling pathway and regulating mucus secretion. These findings have important biological significance for selecting strains that tolerate transport, as well as economic significance for optimizing the transport conditions for scaleless fish.

Dietary Administration of Novel Multistrain Probiotics from Healthy Grouper Intestines Promotes the Intestinal Immune Response against NNV Infection

Epinephelus lanceolatus (giant grouper) is a high-value cultured species in the Asia-Pacific region. However, nervous necrosis virus (NNV) is an infectious viral disease that affects over 120 species of marine cultured species and causes high mortality, ranging from 90-100% in the grouper industry. Probiotics isolated from the intestines of healthy individuals have provided insight into novel approaches involved in the defense against viral pathogens. In this study, we isolated three strains of bacteria as candidate probiotics from healthy grouper intestines and a 28-day feeding trial was performed. At day 21, the nervous necrosis virus (NNV) challenge test was conducted for 7 days to evaluate the antiviral effect of candidate probiotics. The results showed that candidate probiotics could improve growth conditions, such as weight gain (WG) and specific growth rate (SGR), and increase the utilization of feed. Furthermore, the candidate probiotic mixture had the ability to protect against NNV, which could decrease the mortality rate by 100% in giant grouper after NNV challenge. Subsequently, we analyzed the mechanism of the candidate probiotic mixture's defense against NNV. A volcano plot revealed 203 (control vs. NNV), 126 (NNV vs. probiotics - NNV), and 5 (control vs. probiotics - NNV) differentially expressed transcripts in intestinal tissue. Moreover, principal components analysis (PCA) and cluster analysis heatmap showed large differences among the three groups. Functional pathway analysis showed that the candidate probiotic mixture could induce the innate and adaptive immunity of the host to defend against virus pathogens. Therefore, we hope that potential candidate probiotics could be successfully applied to the industry to achieve sustainable aquaculture.

Comparative Mucous miRomics in Cynoglossus semilaevis Related to Vibrio harveyi Caused Infection

Epidermal mucus is an important barrier and regulating mediator in fish. MicroRNAs (miRNAs) are proved to be involved in various biological processes, also as promising biomarkers for disease diagnosis. Vibrio harveyi has long been a noticeable bacterial pathogen in Cynoglossus semilaevis aquaculture. To find the evidence whether there are indicating miRNAs in mucus and whether the miRNAs are related to infections caused by V. harveyi, miRNA profiles of mucus from V. harveyi infected fish and healthy controls were screened by small RNA sequencing and verified by quantitative real-time PCR. This is the first report about miRNA profiling of flatfish mucus, aiming at illustrating the pathogenesis of V. harveyi caused infection and developing disease-related biomarkers. The results revealed significant differences in expression levels of some miRNAs between infected fish and healthy ones. Three hundred differentially expressed miRNAs were obtained after filtering through FC > 2 or FC < 0.5 and most of the differential miRNAs were downregulated. After verification through qRT-PCR, four unique miRNAs, dre-miR-451, dre-miR-184, dre-miR-205-5p > ssa-miR-205b-5p, and dre-miR-181a-5p > ssa-miR-181a-5p, were identified as V. harveyi infection-related signatures, consistent with sequencing trend. The expression levels of these four miRNAs in the infected fish were all significantly lower than controls. These miRNAs in mucus could be used to differentiate diseased and healthy fish in a non-invasive way with practical value for large-scale disease screening. They also provided new insights into the mechanism underlying the bacterial infections in fish.

Elucidating bacterial adhesion to mucosal surface by an original AFM approach

Background

Fish skin represents an ancient vertebrate mucosal surface, sharing characteristics with other mucosal surfaces including those of the intestine. The skin mucosa is continuously exposed to microbes in the surrounding water and is therefore important in the first line defense against environmental pathogens by preventing bacteria from accessing the underlying surfaces. Understanding the microbe-host interactions at the fish skin mucosa is highly relevant in order to understand and control infection, commensalism, colonization, persistence, infection, and disease. Here we investigate the interactions between the pathogenic bacteria Aeromonas salmonicida (A. salmonicida) and Yersinia ruckeri (Y. ruckeri), respectively, and the skin mucosal surface of Atlantic salmon fry using AFM force spectroscopy.

Results

The results obtained revealed that when retracting probes functionalized with bacteria from surfaces coated with immobilized mucins, isolated from salmon mucosal surfaces, rupture events reflecting the disruption of adhesive interactions were observed, with rupture strengths centered around 200 pN. However, when retracting probes functionalized with bacteria from the intact mucosal surface of salmon fish fry no adhesive interactions could be detected. Furthermore, rheological measurements revealed a near fluid-like behavior for the fish fry skin mucus. Taken together, the experimental data indicate that the adhesion between the mucin molecules within the mucous layer may be significantly weaker than the interaction between the bacteria and the mucin molecules. The bacteria, immobilized on the AFM probe, do bind to individual mucins in the mucosal layer, but are released from the near fluid mucus with little resistance upon retraction of the AFM probe, to which they are immobilized.

Conclusion

The data provided in the current paper reveal that A. salmonicida and Y. ruckeri do bind to the immobilized mucins. However, when retracting the bacteria from intact mucosal surfaces, no adhesive interactions are detected. These observations suggest a mechanism underlying the protective function of the mucosal surface based on the clearing of potential threats by adhering them to loosely attached mucus that is subsequently released from the fish skin.

The ability of Neoparamoeba perurans to bind to and digest non-fish-derived mucin: Insights into the amoeba's mechanism of action to overcome gill mucus production

Amoebic gill disease (AGD) is caused by the marine amoeba Neoparamoeba perurans, a facultative parasite. Despite the significant impact this disease has on production of Atlantic salmon worldwide, the mechanisms involved in host-parasite interaction remains unknown. Excessive gill mucus secretion is reported as a host defence mechanism to prevent microbial colonization in the gill epithelium. Despite this response, N. perurans still attaches and proliferates. The present study aimed to investigate the interaction between N. perurans and mucin, the most abundant component in mucus. An in vitro adhesion assay using bovine submaxillary mucin (BSM) demonstrated that amoeba binding to mucin-coated substrate was significantly higher than to the BSA control. This binding interaction is likely glycan-mediated as pre-incubation with galactose, galactosamine, N-acetylgalactosamine and fucose reduced mucin adhesion to control levels. The ability of N. perurans to secrete proteases that target mucin was also investigated. Protease activity was detected in the amoeba culture media in the presence of BSM, but not when protease inhibitor was added. Mucin degradation was visually assessed on protein gels. This study provides preliminary evidence that N. perurans has developed mechanisms to interact with and evade mucus by binding to mucin glycan receptors and secreting proteases with mucolytic activity.

Proteomics of mucosal exosomes of Cynoglossus semilaevis altered when infected by Vibrio harveyi

The cargo of exosomes contains proteins with various functions, which might be promising biomarkers for disease diagnosis and prognosis. To explore the impact of the Vibrio harveyi pathogen on Cynoglossus semilaevis from a different perspective and develop promising biomarkers for infection, the exosomes from epidermal mucus of healthy controls（EC）and sick fish（ES）were extracted and identified, coupled with proteomic screening through iTRAQ followed with LC-MS/MS. 1531 credible proteins were obtained relating to structural, metabolic and immunological functions. 359 different expressed proteins (DEPs) (FC > 2 or FC < 0.5) were found, with 161 up-regulated and 198 down-regulated in ES. Based on the database of C. semilaevis on Uniprot, 71 proteins were characterized as concrete names, including 19 up-regulated proteins and 52 down-regulated proteins, and were selected as subjects for further studies. Ferritin, Toll-like receptor 5S protein and Calcium-transporting ATPase were upregulated, while Histone H2B and Eukaryotic translation initiation factor 5A were downregulated, consistent with the expression levels of related mRNAs in skin tissue verified by qRT-PCR. The integrated analysis between miRomics and proteomics also provided possible regulatory relationships mediated by mucous exosomes during infection. The signature proteins in mucosal exosomes could make sense in the explanation of the infection defending mechanism and the development of biomarkers which can differentiate diseased and healthy C. semilaevis individuals.

Environmental Salinity Modifies Mucus Exudation and Energy Use in European Sea Bass Juveniles

The European sea bass (Dicentrarchus labrax) is a euryhaline marine teleost that can often be found in brackish and freshwater or even in hypersaline environments. Here, we exposed sea bass juveniles to sustained salinity challenges for 15 days, simulating one hypoosmotic (3‰), one isosmotic (12‰) and one hyperosmotic (50‰) environment, in addition to control (35‰). We analyzed parameters of skin mucus exudation and mucus biomarkers, as a minimally invasive tool, and plasma biomarkers. Additionally, Na⁺/K⁺-ATPase activity was measured, as well as the gill mucous cell distribution, type and shape. The volume of exuded mucus increased significantly under all the salinity challenges, increasing by 130% at 50‰ condition. Significantly greater amounts of soluble protein (3.9 ± 0.6 mg at 50‰ vs. 1.1 ± 0.2 mg at 35‰, p < 0.05) and lactate (4.0 ± 1.0 µg at 50‰ vs. 1.2 ± 0.3 µg at 35‰, p < 0.05) were released, with clear energy expenditure. Gill ATPase activity was significantly higher at the extreme salinities, and the gill mucous cell distribution was rearranged, with more acid and neutral mucin mucous cells at 50‰. Skin mucus osmolality suggested an osmoregulatory function as an ion-trap layer in hypoosmotic conditions, retaining osmosis-related ions. Overall, when sea bass cope with different salinities, the hyperosmotic condition (50‰) demanded more energy than the extreme hypoosmotic condition.

Alteration of the Immune Response and the Microbiota of the Skin during a Natural Infection by Vibrio harveyi in European Seabass (Dicentrarchus labrax)

Disease outbreaks continue to represent one of the main bottlenecks for the sustainable development of the aquaculture industry. In marine aquaculture, many species from the Vibrio genus are serious opportunistic pathogens responsible for significant losses to producers. In this study, the effects on the immune response and the skin microbiota of European sea bass (Dicentrarchus labrax) were studied after a natural disease outbreak caused by V. harveyi. Data obtained from infected and non-infected fish were studied and compared. Regarding the local immune response (skin mucus) a decrease in the protease activity was observed in infected fish. Meanwhile, at a systemic level, a decrease in protease and lysozyme activity was reported while peroxidase activity showed a significant increase in serum from infected fish. A clear dysbiosis was observed in the skin mucus microbiota of infected fish in comparison with non-infected fish. Moreover, V. harveyi, was identified as a biomarker for the infected group and Rubritalea for healthy fish. This study highlights the importance of characterizing the mucosal surfaces and microbial composition of the skin mucus (as a non-invasive technique) to detect potential disease outbreaks in fish farms.

Evidence for the Use of Mucus Swabs to Detect Renibacterium salmoninarum in Brook Trout

Efforts to advance fish health diagnostics have been highlighted in many studies to improve the detection of pathogens in aquaculture facilities and wild fish populations. Typically, the detection of a pathogen has required sacrificing fish; however, many hatcheries have valuable and sometimes irreplaceable broodstocks, and lethal sampling is undesirable. Therefore, the development of non-lethal detection methods is a high priority. The goal of our study was to compare non-lethal sampling methods with standardized lethal kidney tissue sampling that is used to detect Renibacterium salmoninarum infections in salmonids. We collected anal, buccal, and mucus swabs (non-lethal qPCR) and kidney tissue samples (lethal DFAT) from 72 adult brook trout (Salvelinus fontinalis) reared at the Colorado Parks and Wildlife Pitkin Brood Unit and tested each sample to assess R. salmoninarum infections. Standard kidney tissue detected R. salmoninarum 1.59 times more often than mucus swabs, compared to 10.43 and 13.16 times more often than buccal or anal swabs, respectively, indicating mucus swabs were the most effective and may be a useful non-lethal method. Our study highlights the potential of non-lethal mucus swabs to sample for R. salmoninarum and suggests future studies are needed to refine this technique for use in aquaculture facilities and wild populations of inland salmonids.

Gentamicin Combined With Hypoionic Shock Rapidly Eradicates Aquaculture Bacteria in vitro and in vivo

Bacterial pathogens are a major cause of infectious diseases in aquatic animals. The abuse of antibiotics in the aquatic industry has led to the proliferation of antibiotic resistance. It is therefore essential to develop more effective and safer strategies to increase the efficacy and extend the life span of the antibiotics used in aquaculture. In this study, we show that six aquaculture bacterial pathogens (i.e., Aeromonas hydrophila, Vibrio alginolyticus, Edwardsiella tarda, Streptococcus iniae, Vibrio harveyi, and Vibrio fluvialis) in the stationary phase can be rapidly killed after immersion in gentamicin- or neomycin-containing, ion-free solutions for a few minutes. Such hypoionic shock treatment enhances the bacterial uptake of gentamicin in an ATP-dependent manner. Importantly, we demonstrate, as a proof of concept, that gentamicin under hypoionic shock conditions can effectively kill A. hydrophila in vivo in a skin infection model of zebrafish (Danio rerio), completely curing the infected fish. Given that pathogenic bacteria generally adhere to the skin surface and gills of aquatic animals, our strategy is of potential significance for bacterial infection control, especially for small-scale economic fish farming and ornamental fish farming. Further, the combined treatment can be completed within 5 min with a relatively small volume of solution, thus minimizing the amount of residual antibiotics in both animals and the environment.

Extracellular vesicles piwi-interacting RNAs from skin mucus for identification of infected Cynoglossus semilaevis with Vibrio harveyi

Extracellular vesicles play a regulatory role in intracellular and intercellular transmission through a variety of biological information molecules, including mRNA, small RNAs and proteins. piRNAs are one kind of regulatory small RNAs in the vesicles at the post transcriptional level. Hereby, we isolated the extracellular vesicles from skin mucus and screened the piRNA profiles of these vesicles, aiming at developing biomarkers related to bacterial infections in Cynoglossus semilaevis. The different profilings of piRNAs in mucous extracellular vesicles of C. semilaevis were compared through small RNA sequencing, between fish infected with Vibrio harveyi and healthy ones. The number of clean reads on the alignment of exosome sick (ES) group was 105, 345 and that of exosome control (EC) group was 455, 144. GO and KEGG pathway enrichment analysis showed that most of the target genes were involved in cellular process, response to stimulus, biological regulation, immune system process and signal transduction, signal molecular and interaction, transport and catabolism. The 45 final candidate piRNAs related to immunity or infectious diseases included 20 piRNAs with high expression in the ES group and 25 piRNAs with a low expression in the ES group. After verification by qRT-PCR, there was significant difference of five piRNAs expression level between infected fish and healthy fish, in line with the sequencing. The expression level of piR-mmu-16401212, piR-mmu-26829319 and piR-gga-244092 in infected fish were significantly lower than that of control group, while piR-gga-71717 and piR-gga-99034 were higher, which implying that these piRNAs in mucous extracellular vesicles can be used to identify diseased fish from normal ones. This work supplied a novel class of biomarker for infection diagnosis in fish, and it will be benefit for screening disease resistant breeding of C. semilaevis.

Tetracycline-Resistant Bacteria Selected from Water and Zebrafish after Antibiotic Exposure

The emergence of antibiotic-resistant pathogens due to worldwide antibiotic use is raising concern in several settings, including aquaculture. In this work, the selection of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) was evaluated after exposure of zebrafish to oxytetracycline (OTC) for two months, followed by a recovery period. The selection of ARB in water and fish was determined using selective media. The abundance of tetA genes was estimated through qPCR. Higher prevalence of ARB was measured in all samples exposed to the antibiotic when compared to control samples, although statistical significance was only achieved five days after exposure. Isolates recovered from samples exposed to the antibiotic were affiliated with Pseudomonas and Stenotrophomonas. Various antibiotic susceptibility profiles were detected and 37% of the isolates displayed multidrug resistance (MDR). The selection of the tetA gene was confirmed by qPCR at the highest OTC concentration tested. Two MDR isolates, tested using zebrafish embryos, caused significant mortality, indicating a potential impact on fish health and survival. Overall, our work highlights the potential impact of antibiotic contamination in the selection of potential pathogenic ARB and ARGS.

Carvacrol, Thymol, and Garlic Essential Oil Promote Skin Innate Immunity in Gilthead Seabream (Sparus aurata) Through the Multifactorial Modulation of the Secretory Pathway and Enhancement of Mucus Protective Capacity

One of the main targets for the use of phytogenics in aquafeeds is the mucosal tissues as they constitute a physical and biochemical shield against environmental and pathogenic threats, comprising elements from both the innate and acquired immunity. In the present study, the modulation of the skin transcriptional immune response, the bacterial growth capacity in skin mucus, and the overall health condition of gilthead seabream (Sparus aurata) juveniles fed a dietary supplementation of garlic essential oil, carvacrol, and thymol were assessed. The enrichment analysis of the skin transcriptional profile of fish fed the phytogenic-supplemented diet revealed the regulation of genes associated to cellular components involved in the secretory pathway, suggesting the stimulation, and recruitment of phagocytic cells. Genes recognized by their involvement in non-specific immune response were also identified in the analysis. The promotion of the secretion of non-specific immune molecules into the skin mucus was proposed to be involved in the in vitro decreased growth capacity of pathogenic bacteria in the mucus of fish fed the phytogenic-supplemented diet. Although the mucus antioxidant capacity was not affected by the phytogenics supplementation, the regulation of genes coding for oxidative stress enzymes suggested the reduction of the skin oxidative stress. Additionally, the decreased levels of cortisol in mucus indicated a reduction in the fish allostatic load due to the properties of the tested additive. Altogether, the dietary garlic, carvacrol, and thymol appear to promote the gilthead seabream skin innate immunity and the mucus protective capacity, decreasing its susceptibility to be colonized by pathogenic bacteria.