Time-related changes of the intestinal morphology of Atlantic salmon, Salmo salar L., at two different soybean meal inclusion levels

Effects of Oregano (Origanum vulgare) Essential Oil Supplementation on Growth Performance of Zebrafish (Danio rerio) Fed a High-Inclusion Soybean Meal Diet

Soybean meal (SBM) has become a common dietary replacement for fish meal (FM) in aquafeed. However, at high inclusions, SBM has been shown to have negative impacts presenting as reduced feed intake and intestinal inflammation. Medicinal plant extracts, namely essential oils, have been used to promote growth performance and immune response. The objective of this study was to investigate the potential therapeutic effects of oregano (Origanum vulgare) essential oil (OEO) inclusion on utilization of a high-inclusion SBM diet using zebrafish as a model. Five diets were used in this study: reference-FM-based diet, control-55.7% inclusion SBM diet, and three experimental SBM-based diets OEO1, OEO2, and OEO3 that were supplemented with 1%, 2%, or 3% of oregano oil, respectively. The FM group had overall better growth performance when compared with the other treatment groups; however, the OEO3 mean weight and feed conversion ratio were not significantly different from the FM group (p > 0.05) and were significantly improved compared with the SBM group (p < 0.05). Similarly, OEO2 total length was not significantly different from FM (p > 0.05) but significantly higher than the SBM group (p < 0.05). Expression of inflammation-related genes did not significantly differ between the OEO groups and the SBM-only group. However, the OEO2 and OEO3 groups displayed improved growth performance compared with the SBM group, suggesting that inclusion of OEO at or above 2% inclusion may help to alleviate common symptoms induced by a high-inclusion SBM diet.

Microplastics weaken the digestion and absorption functions in the golden pompano (Trachinotus blochii) by affecting the intestinal structure, bacteria and metabolites

Microplastics are difficult to degrade and widespread environmental pollutants. Coastal areas are hardest hit of microplastic pollution as they receive significant amounts of microplastics discharged from inland sources. Golden pompano (Trachinotus blochii) is a high commercial valuable marine aquaculture fish species, most of the golden pompano are raised in coastal areas, which means they are at significant risk of exposure to microplastics. Therefore, we exposed golden pompano to 10 μg/L, 100 μg/L and 1000 μg/L of 5 μm spherical polystyrene microplastics and conducted a 14-day stress experiment. Histopathology results showed the intestinal villi shrank. The 16s sequencing analysis revealed that microplastics significantly impacted the abundance and community structure of intestinal microorganisms, which may affect the metabolic function of the gastrointestinal tract. Metabolomics sequencing of the intestinal contents showed that microplastics caused disruptions in lipid, glucose, and amino acid metabolism, thus compromising the normal digestion and absorption functions in the intestinal system. In addition, the activation of various pathways, including the intestinal endocrine system, proline metabolism, and signal transduction, which can lead to the occurrence of several diseases. This study combined various methods to investigate the adverse effects of microplastics on intestinal digestion and absorption, and provided new insights into the toxic mechanisms of microplastics.

Chronic social isolation affects feeding behavior of juvenile zebrafish (Danio rerio)

Many organisms exhibit social behaviors and are part of some scheme of social structure. Zebrafish are highly social, shoaling fish and therefore, social isolation may have notable impacts on their physiology and behavior. The objective of this study was to evaluate the effects of social isolation on feed intake, monoaminergic system related gene expression, and intestinal health of juvenile zebrafish fed a high-inclusion soybean meal based diet. At 20 days post-fertilization zebrafish were randomly assigned to chronic isolation (1 fish per 1.5 L tank) or social housing (6 fish per 9 L tank) with 18 tanks per treatment group (n = 18). Dividers were placed between all tanks to prevent visual cues between fish. Zebrafish were fed a commercial fishmeal based diet until 35 days post-fertilization and then fed the experimental high-inclusion soybean meal based diet until 50 days post-fertilization. At the end of the experiment (51 days post-fertilization), the mean total length, weight, and weight gain were not significantly different between treatment groups. Feed intake and feed conversion ratio were significantly higher in chronic isolation fish than in social housing fish. Expression of monoaminergic and appetite-related genes were not significantly different between groups. The chronic isolation group showed higher expression of the inflammatory gene il-1b, however, average intestinal villi width was significantly smaller and average length-to-width ratio was significantly higher in chronic isolation fish, suggesting morphological signs of inflammation were not present at the time of sampling. These results indicate that chronic isolation positively affects feed intake of juvenile zebrafish and suggest that isolation may be useful in promoting feed intake of less-palatable diets such as those based on soybean meal.

How Do Alternative Protein Resources Affect the Intestine Morphology and Microbiota of Atlantic Salmon?

The availability and cost of fishmeal constitute a bottleneck in Atlantic salmon production expansion. Fishmeal is produced from wild fish species and constitutes the major feed ingredient in carnivorous species such as the Atlantic salmon. These natural stocks are at risk of depletion and it is therefore of major importance to find alternative protein sources that meet the nutritional requirements of the Atlantic salmon, without compromising the animals' health. Terrestrial animal by-products have been used in aquaculture feed, but their use is limited by the lack of several essential amino acids and consumer acceptance. In the case of plant ingredients, it is necessary to take into account both their concentration and the extraction methodologies, since, if not dosed correctly, they can cause macro- and microscopic alterations of the structure of the gastrointestinal tract and can also negatively modulate the microbiota composition. These alterations may compromise the digestive functions, growth of the animal, and, ultimately, its well-being. An updated revision of alternative protein sources is provided, with the respective impact on the intestine health in terms of both morphology and microbiota composition. Such information may constitute the premise for the choice and development of Atlantic salmon feeds that guarantee fish health and growth performance without having a significant impact on the surrounding environment, both in terms of depletion of the fish's natural stocks and in terms of pressure on the terrestrial agriculture. The sustainability of aquaculture should be a priority when choosing next-generation ingredients.

Spirulina-enriched Substrate to Rear Black Soldier Fly (Hermetia illucens) Prepupae as Alternative Aquafeed Ingredient for Rainbow Trout (Oncorhynchus mykiss) Diets: Possible Effects on Zootechnical Performances, Gut and Liver Health Status, and Fillet Quality

In the present study, an organic substrate (coffee silverskin) enriched with spirulina (Arthrospira platensis; 15% w/w), as a source of lipids and bioactive molecules, was used to rear the black soldier fly (Hermetia illucens) prepupae. Three grossly isonitrogenous, isoproteic, isolipidic and isoenergetic experimental diets for rainbow trout (Oncorhynchus mykiss) juveniles were then produced: a control diet (HM0) mostly including fish meal and fish oil, and two other test diets named HM3 and HM20, in which 3 or 20% of the marine ingredients were substituted with full fat black soldier fly prepupae meal (HM), respectively. Experimental diets were provided for 6 weeks, and at the end of the trial the physiological responses and marketable traits of the fish were investigated using a multidisciplinary approach. Generally, all test diets were well accepted, and fish growth, gut and liver health status, and marketable characteristics were not impaired by the experimental diets. However, an increased immuno-related gene expression along with a slight reduction of fillet redness and yellowness was evident in fish from the HM20 group.

Adequate levels of dietary sulphur amino acids impart improved liver and gut health in juvenile yellowtail kingfish (Seriola lalandi)

The sulphur amino acids methionine (Met) and cysteine (Cys) and their derivative taurine (Tau) are metabolically active molecules with interlinked roles in nutritional requirements. Deficiencies in these nutrients are linked to poor growth and health; however, the impacts of these deficiencies on organ structure and function are largely unknown. This study examined the effects of dietary Met, Cys and Tau fed at different levels on yellowtail kingfish (YTK) liver histology and surface colour, plasma biochemistry and posterior intestine histology. Samples were collected from two dose-response feeding trials that quantified (1) the Tau requirement and sparing effect of Met by feeding YTK diets containing one of seven levels of Tau at one of two levels of Met and (2) the Met requirement and sparing effect of Cys by feeding YTK diets containing one of five levels of Met at one of two levels of Cys. YTK fed inadequate levels of dietary Met, Cys and Tau exhibited thicker bile ducts, less red livers, more intestinal acidic goblet cell mucus and supranuclear vacuoles and less posterior intestinal absorptive surface area. Further, thicker bile ducts correlated with less red livers (a*, R), whereas increased hepatic fat correlated with a liver yellowing (b*). Our results indicate a shift towards histological properties and functions indicative of improved intrahepatic biliary condition, posterior intestinal nutrient absorption and homoeostasis of YTK fed adequate amounts of Met, Cys and Tau. These findings may assist in formulating aquafeed for optimised gastrointestinal and liver functions and maintaining good health in YTK.

Towards the Identification of a Suitable Commercial Diet for Carpione (Salmo carpio, Linnaeus 1758): A Multidisciplinary Study on Fish Performances, Animal Welfare and Quality Traits

Simple Summary

Carpione (Salmo carpio, Linnaeus 1758) is an endangered precious endemism of Lake Garda (Northern Italy), the largest Italian lake. To date, several bottlenecks about its culture remain unsolved, including the identification of a proper growth-out diet. In the present study, four different grossly isolipidic, isoproteic, and isoenergetic diets containing ingredients from different origins were used for S. carpio culture. Specifically, a diet largely based on marine ingredients, and currently used for carpione farming, was used as control. Three other diets were formulated in order to include relevant percentages of vegetable ingredients or processed animal proteins (at two different inclusion levels). After a three-month feeding trial, fish zootechnical performances, welfare, and flesh quality were evaluated through a multidisciplinary approach, including histology, gene expression, chemical analysis, and Fourier transform infrared spectroscopy (FTIR). This study provided the first insights on carpione physiological responses to different commercial dietary formulations.

Abstract

Carpione (Salmo carpio, Linnaeus 1758) is an endangered precious endemism of Lake Garda (Northern Italy), the largest Italian lake. To date, several bottlenecks about its culture remain unsolved, including the identification of a proper growth-out diet. The aim of the present study was to test four different grossly isolipidic, isoproteic, and isoenergetic diets in which the main ingredients had a different origin. Specifically, a diet currently used by local farmers for carpione culture, largely based on marine ingredients, was used as control (CTRL), while the other three diets were formulated by partially replacing marine ingredients with plant ones (VEG) or with different percentages of processed animal proteins (PAP1 and PAP2). The feeding trial was run in triplicate, over a three-month period. No significant differences in growth performance among the experimental groups were observed. However, remarkable histological alterations and inflammatory markers upregulation were observed in VEG group, while PAP inclusion played a role in attenuating inflammation and improving nutrient uptake. Fillet analyses highlighted significant differences in marketable traits and flesh fatty acid composition among the experimental groups, including the reduction of polyunsaturated fatty acids related to PAPs inclusion. In conclusion, PAPs used in the present study promoted S. carpio gut health and absorption capacity, while further studies are required to maintain proper quality traits of the final product.

Antigen-Presenting Cells and T Cells Interact in a Specific Area of the Intestinal Mucosa Defined by the Ccl25-Ccr9 Axis in Medaka

Organized intestinal mucosal immune response appears to be restricted to tetrapods. In teleost fish, there is no evidence for the existence of a particular intestinal region that facilitates the interaction of antigen-presenting cells (APCs) and T cells, such as secondary lymphoid organs. Indeed, despite their importance in the defense against pathogens, the location and manner of APC-T cell interaction within the fish gut is unknown. Here, using non-invasive live imaging of newly developed transgenic reporter lines, we addressed the spatial organization and behavior of APCs and T cells in the intestine of medaka fish both during homeostasis and inflammation. We report that Ccr9a⁺ T cells are recruited to a band in the lamina propria next to the muscularis mucosa in which Ccl25-expressing cells are present. Ccr9a⁺ T cells contact APCs for several minutes, in a process mediated by connexin 43. This type of interaction was observed in homeostasis and inflammation, with the interaction being longer and more frequent during inflammation. Thus, our results demonstrate that the mucosal immune response in the intestine of medaka is organized and endowed with a specific region with specialized microenvironment and function.

Successful Inclusion of High Vegetable Protein Sources in Feed for Rainbow Trout without Decrement in Intestinal Health

The aquaculture of carnivorous fish is in continuous expansion, which leads to the need to reduce the dependence on fishmeal (FM). Plant proteins (PP) represent a suitable protein alternative to FM and are increasingly used in fish feed. However, PP may lead to stunted growth and enteritis. In the current study, the effect of high FM substitution by PP sources on the growth, mortality and intestinal health of rainbow trout (Oncorhynchus mykiss) was evaluated in terms of the histological intestine parameters and expression of genes related to inflammation (IL-1β, IL-8 and TGF-β) and immune responses (Transferrin, IgT and IFN-γ). The results show that a total substitution registered lower growth and survival rates, probably due to a disruption to the animal's health. Confirming this hypothesis, fish fed FM0 showed histological changes in the intestine and gene changes related to inflammatory responses, which in the long-term could have triggered an immunosuppression. The FM10 diet presented not only a similar expression to FM20 (control diet), but also similar growth and survival. Therefore, 90% of FM substitution was demonstrated as being feasible in this species using a PP blend of wheat gluten (WG) and soybean meal (SBM) as a protein source.

Cross-Talk Between Intestinal Microbiota and Host Gene Expression in Gilthead Sea Bream (Sparus aurata) Juveniles: Insights in Fish Feeds for Increased Circularity and Resource Utilization

New types of fish feed based on processed animal proteins (PAPs), insect meal, yeast, and microbial biomasses have been used with success in gilthead sea bream. However, some drawback effects on feed conversion and inflammatory systemic markers were reported in different degrees with PAP- and non-PAP-based feed formulations. Here, we focused on the effects of control and two experimental diets on gut mucosal-adherent microbiota, and how it correlated with host transcriptomics at the local (intestine) and systemic (liver and head kidney) levels. The use of tissue-specific PCR-arrays of 93 genes in total rendered 13, 12, and 9 differentially expressed (DE) genes in the intestine, liver, and head kidney, respectively. Illumina sequencing of gut microbiota yielded a mean of 125,350 reads per sample, assigned to 1,281 operational taxonomic unit (OTUs). Bacterial richness and alpha diversity were lower in fish fed with the PAP diet, and discriminant analysis displayed 135 OTUs driving the separation between groups with 43 taxa correlating with 27 DE genes. The highest expression of intestinal pcna and alpi was achieved in PAP fish with intermediate values in non-PAP, being the pro-inflammatory action of alpi associated with the presence of Psychrobacter piscatorii. The intestinal muc13 gene was down-regulated in non-PAP fish, with this gene being negatively correlated with anaerobic (Chloroflexi and Anoxybacillus) and metal-reducing (Pelosinus and Psychrosinus) bacteria. Other inflammatory markers (igm, il8, tnfα) were up-regulated in PAP fish, positively correlating the intestinal igm gene with the inflammasome activator Escherichia/Shigella, whereas the systemic expression of il8 and tnfα was negatively correlated with the Bacilli class in PAP fish and positively correlated with Paracoccus yeei in non-PAP fish. Overall changes in the expression pattern of il10, galectins (lgals1, lgals8), and toll-like receptors (tlr2, tlr5, tlr9) reinforced the anti-inflammatory profile of fish fed with the non-PAP diet, with these gene markers being associated with a wide range of OTUs. A gut microbiota-liver axis was also established, linking the microbial generation of short chain fatty acids with the fueling of scd1- and elovl6-mediated lipogenesis. In summary, by correlating the microbiome with host gene expression, we offer new insights in the evaluation of fish diets promoting gut and metabolism homeostasis, and ultimately, the health of farmed fish.

Does dietary incorporation level of pea protein isolate influence the digestive system morphology in rainbow trout (Oncorhynchus mykiss)?

In the present study, fish meal (FM) was replaced by pea (Pisum sativum) protein (PP) in diet for Rainbow trout (Oncorhynchus mykiss) at levels of 0% (PP0), 25% (PP25), 50% (PP50), 75% (PP75) and 100% (PP100), and the effect of dietary PP level on the digestive system tracts and liver was investigated by micromorphological and histopathological evaluations. Morphometric measurements (mm 100g fish^-1 ) of the liver width and stomach length in rainbow trout were found to be significantly larger (p <0.05) in fish with high-level pea protein as the main protein source (PP75, PP100) compared to the low-level PP replacement group (PP25). No significant differences were found in morphometric measurements for pyloric caecum and intestines among treatment groups, whereas the number of the caecum of fish fed the PP25 diets significantly increased over the control (PP0) (p<0.05). In the histological examination of the liver, mild hydropic and vacuolar degeneration was observed in all experimental groups except PP0 and PP25. The measurements of pyloric caecum fold height, enterocyte length and width of tunica muscularis of the high-level pea protein groups of PP75 and PP100 were significantly higher (p <0.05) compared to the control group. In conclusion, 25% substitution of PP can be suggested for FM in trout diets, because the findings of the present study provided evidence that the digestive system improved by increasing the number of pyloric caecum at this replacement level.

Nutritive value of guar and copra meal after fermentation with yeast Saccharomyces cerevisiae in the diet of Nile tilapia, Oreochromis niloticus

Solid-state fermentation increases nutritional bioavailability of plant ingredients by reducing the antinutritional factors and complexity of the nutrients. The present experiment was conducted to replace fish meal (FM) with a mixture (3:1) of yeast fermented guar and copra meal (FGCM) in the diet of Nile tilapia. Five iso-nitrogenous (30% CP) and iso-lipidic (6% CL) diets were formulated replacing FM at 0% (FGCM0), 25% (FGCM 1), 50% (FGCM 2), 75% (FGCM 3), and 100% (FGCM 4) with FGCM mixture. Growth, nutrient utilisation, digestibility, digestive enzyme activity, haematological parameters, and antioxidant capacity were analysed in five treatments after a feeding trial of 75 days. Bioprocessing increased the protein and amino acid content and reduced the antinutritional factors in the guar and copra meal. The weight gain was significantly increased in 25% replaced diet. The feed conversion ratio of tilapia fed with FGCM 1 was similar to that of control and significantly lower (P < 0.05) among the treatments. The group fed with FGCM 1 exhibited significantly higher body protein and lipid content. The activity of digestive enzymes and digestibility of nutrients reduced significantly (P < 0.05) with the replacement of fishmeal beyond 25% with the FGCM mixture. Similarly, the antioxidant enzymes and blood parameters were also negatively affected when more than 25% FM was replaced with FGCM mixture. The cubic regression analysis found that 17.45% FM can be replaced with FGCM mixture for optimum weight gain in tilapia.

Skin damage caused by scale loss modifies the intestine of chronically stressed gilthead sea bream (Sparus aurata, L.)

The present study was designed to test if the damage caused by scale loss provokes a change in other innate immune barriers such as the intestine and how chronic stress affects this response. Sea bream (Sparus aurata) were kept in tanks at low density (16 kg m^-3, LD) or exposed to a chronic high density (45 kg m^-3, HD) stress for 4 weeks. Scales were then removed (approximately 50%) from the left flank in the LD and HD fish. Intestine samples (n = 8/group) were examined before and at 12 h, 3 days and 7 days after scale removal. Changes in the morphology of the intestine revealed that chronic stress and scale loss was associated with intestinal inflammation. Specifically, enterocyte height and the width of the lamina propria, submucosa and muscle layer were significantly increased (p < 0.05) 3 days after skin damage in fish under chronic stress (HD) compared to other treatments (LDWgut3d or HDgut0h). This was associated with a significant up-regulation (p < 0.05) in the intestine of gene transcripts for cell proliferation (pcna) and anti-inflammatory cytokine tgfβ1 and down-regulation of gene transcripts for the pro-inflammatory cytokines tnf-α and il1β (p < 0.05) in HD and LD fish 3 days after scale removal compared to the undamaged control (LDgut0h). Furthermore, a significant up-regulation of kit, a marker of mast cells, in the intestine of HDWgut3d and LDWgut3d fish suggests they may mediate the crosstalk between immune barriers. Skin damage induced an increase in cortisol levels in the anterior intestine in HDWgut12 h fish and significant (p < 0.05) down-regulation of mr expression, irrespective of stress. These results suggest glucocorticoid levels and signalling in the intestine of fish are modified by superficial cutaneous wounds and it likely modulates intestine inflammation.

Hermetia illucens and Poultry by-Product Meals as Alternatives to Plant Protein Sources in Gilthead Seabream (Sparus aurata) Diet: A Multidisciplinary Study on Fish Gut Status

Simple Summary

Sustainability and fish welfare have been receiving increasing attention in the aquaculture sector, with an emphasis on the search for new, sustainable, and healthy aquafeed ingredients. For many years, plant ingredients have been widely used in aquafeed formulation; however, negative side effects on gut welfare have often been reported in several carnivorous fish species. From this perspective, alternative ingredients such as poultry by-products and insect meal are receiving attention due to their low ecological footprint and high nutritional value. In the present study, these two ingredients were used, singly or in combination, to formulate practical diets for gilthead seabream (Sparus aurata). After a twelve-week feeding trial, a multidisciplinary laboratory approach including histological, molecular, and spectroscopic techniques was adopted in order to investigate fish physiological responses to the new test diets. The results obtained showed excellent zootechnical performances and ameliorated gut health in fish fed dietary inclusions of poultry by-products and insect meal compared to those fed a vegetable-based diet. In addition, the modulation of nutrient absorption in relation to the ingredients used was highlighted by means of spectroscopic tools. The results obtained demonstrated that poultry by-products and insect meal can be successfully used to replace plant-derived ingredients in diets for gilthead seabream without negatively affecting fish welfare.

Abstract

The attempt to replace marine-derived ingredients for aquafeed formulation with plant-derived ones has met some limitations due to their negative side effects on many fish species. In this context, finding new, sustainable ingredients able to promote fish welfare is currently under exploration. In the present study, poultry by-products and Hermetia illucens meal were used to replace the vegetable protein fraction in diets totally deprived of fish meal intended for gilthead seabream (Sparus aurata). After a 12-week feeding trial, a multidisciplinary approach including histological, molecular, and spectroscopic techniques was adopted to investigate intestine and liver responses to the different dietary formulations. Regardless of the alternative ingredient used, the reduction in dietary vegetable proteins resulted in a lower incidence of intestine histological alterations and inflammatory responses. In addition, the dietary inclusion of insect meal positively affected the reduction in the molecular inflammatory markers analyzed. Spectroscopic analyses showed that poultry by-product meal improved lipid absorption in the intestine, while insect meal induced increased liver lipid deposition in fish. The results obtained demonstrated that both poultry by-products and H. illucens meal can successfully be used to replace plant-derived ingredients in diets for gilthead seabream, promoting healthy aquaculture.

Physiological responses of Siberian sturgeon (Acipenser baerii) juveniles fed on full-fat insect-based diet in an aquaponic system

Over the last years, the potential use of Black Soldier Fly meal (BSF) as a new and sustainable aquafeed ingredient has been largely explored in several fish species. However, only fragmentary information is available about the use of BSF meal-based diets in sturgeon nutrition. In consideration of a circular economy concept and a more sustainable aquaculture development, the present research represents the first comprehensive multidisciplinary study on the physiological effects of a BSF diet during sturgeon culture in an aquaponic system. Siberian sturgeon (Acipenser baerii) juveniles were fed over a 60-days feeding trial on a control diet (Hi0) and a diet containing 50% of full-fat BSF meal respect to fish meal (Hi50). Physiological responses of fish were investigated using several analytical approaches, such as gas chromatography-mass spectrometry, histology, Fourier Transformed Infrared Spectroscopy (FTIR), microbiome sequencing and Real-time PCR. While aquaponic systems performed optimally during the trial, Hi50 group fish showed lower diet acceptance that resulted in growth and survival reduction, a decrease in hepatic lipids and glycogen content (FTIR), a higher hepatic hsp70.1 gene expression and a worsening in gut histological morphometric parameters. The low feed acceptance showed by Hi50 group sturgeon highlighted the necessity to improve the palatability of BSF-based diet designed for sturgeon culture.

Gut immunity in European sea bass (Dicentrarchus labrax): a review

In this review, we summarize and discuss the trends and supporting findings in scientific literature on the gut mucosa immune role in European sea bass (Dicentrarchus labrax L.). Overall, the purpose is to provide an updated overview of the gastrointestinal tract functional regionalization and defence barriers. A description of the available information regarding immune cells found in two immunologically-relevant intestinal compartments, namely epithelium and lamina propria, is provided. Attention has been also paid to mucosal immunoglobulins and to the latest research investigating gut microbiota and dietary manipulation impacts. Finally, we review oral vaccination strategies, as a safe method for sea bass vaccine delivery.

The Role of Feed in Aquatic Laboratory Animal Nutrition and the Potential Impact on Animal Models and Study Reproducibility

Feed plays a central role in the physiological development of terrestrial and aquatic animals. Historically, the feeding practice of aquatic research species derived from aquaculture, farmed, or ornamental trades. These diets are highly variable, with limited quality control, and have been typically selected to provide the fastest growth or highest fecundity. These variations of quality and composition of diets may affect animal/colony health and can introduce confounding experimental variables into animal-based studies that impact research reproducibility.

Intestinal Transcriptome Analysis Highlights Key Differentially Expressed Genes Involved in Nutrient Metabolism and Digestion in Yellowtail Kingfish (Seriola lalandi) Fed Terrestrial Animal and Plant Proteins

This study investigated the effects of dietary terrestrial animal and plant proteins on the intestinal transcriptomes of yellowtail kingfish (YTK), Seriola lalandi, an ecologically and economically important marine species in Australia. Five diets containing fish meal (FM), poultry by-product meal (PBM), blood meal (BLM), faba bean meal (FBM) and corn gluten meal (CGM) were formulated and fed over a period of 4 weeks. The Illumina RNA-sequencing (RNA-Seq) results identified a suite of differentially expressed genes involved in nutrient metabolism and protein digestion pathways, reinforced by quantitative polymerase chain reaction (qPCR) results. These findings provide molecular support to the notion that PBM and FBM are useful raw materials in commercial diets for YTK. Using the same evidence, we have demonstrated that BLM and CGM may be less useful and their incorporation into commercial aquafeeds for this species should be done cautiously. The differentially expressed genes showed a subtle difference and high correlation with apparent nutrient digestibility of raw materials. Further, our results indicate that transcriptome profiling provides a useful tool to evaluate alternative protein sources for use in aquaculture feeds.

Beneficial influences of dietary Aspergillus awamori fermented soybean meal on oxidative homoeostasis and inflammatory response in turbot (Scophthalmus maximus L.)

High levels of soybean meal (SBM) in aquafeed leads to detrimental inflammatory response and oxidative stress in fish. In the present study, fermentation with Aspergillus awamori was conducted to explore the potential effects on improving the nutritional quality of soybean meal and the health status of turbot. A 63-day feeding trial (initial weight 8.53 ± 0.11 g) was carried out to evaluate the utilization of fermented soybean meal (FSM) by juvenile turbot. 0% (FM, control), 30% (S30, F30), 45% (S45, F45), and 60% (S60, F60) of fish meal were replaced with SBM or FSM, respectively. As the results showed, fermentation significantly reduced the contents of anti-nutritional factors in SBM, including raffinose (-98.8%), glycinin (-98.5%), β-conglycinin (-97.4%), trypsin inhibitors (-80%) and stachyose (-80%). A depression of fish growth performance and activities of superoxide dismutase and lysozyme were observed in S45 and S60 groups, while these inferiorities were only observed in F60 group. Meanwhile, fermentation also improved the heights of enterocytes and microvillus significantly in the F45 and F60 groups compared with those in SBM. An induced expression of anti-inflammatory cytokine transforming growth factor-β and depression of pro-inflammatory cytokines tumor necrosis factor-α and interleukin-1β in the distal intestine were observed in the F45 and F60 groups. Taken together, this study indicated that fermentation with Aspergillus awamori could improve the replacement level with soybean meal from 30% to 45% in turbot.

Dietary phytogenics and galactomannan oligosaccharides in low fish meal and fish oil-based diets for European sea bass (Dicentrarchus labrax) juveniles: Effects on gut health and implications on in vivo gut bacterial translocation

European sea bass were fed four low FM/FO (10%/6%) diets containing galactomannan oligosaccharides (GMOS), a mixture of garlic oil and labiatae plants oils (PHYTO), or a combination of both functional products (GMOSPHYTO) for 63 days before exposing the fish to an intestinal Vibrio anguillarum infection combined with crowding stress. In order to evaluate functional diets efficacy in terms of gut health maintenance, structural, cellular, and immune intestinal status were evaluated by optical and electron microscopy and gene expression analyses. A semi-automated software was adapted to determine variations in goblet cell area and mucosal mucus coverage during the challenge test. Feeding with functional diets did not affect growth performance; however, PHYTO and GMOS dietary inclusion reduced European sea bass susceptibility to V. anguillarum after 7 days of challenge testing. Rectum (post-ileorectal valve) showed longer (p = 0.001) folds than posterior gut (pre-ileorectal valve), whereas posterior gut had thicker submucosa (p = 0.001) and higher mucus coverage as a result of an increased cell density than rectum. Functional diets did not affect mucosal fold length or the grade of granulocytes and lymphocytes infiltration in either intestinal segment. However, the posterior gut fold area covered by goblet cells was smaller in fish fed GMOS (F = 14.53; p = 0.001) and PHYTO (F = 5.52; p = 0.019) than for the other diets. PHYTO (F = 3.95; p = 0.049) reduced posterior gut goblet cell size and increased rodlet cell density (F = 3.604; p = 0.068). Dietary GMOS reduced submucosal thickness (F = 51.31; p = 0.001) and increased rodlet cell density (F = 3.604; p = 0.068) in rectum. Structural TEM analyses revealed a normal intestinal morphological pattern, but the use of GMOS increased rectum microvilli length, whereas the use of PHYTO increased (p≤0.10) Ocln, N-Cad and Cad-17 posterior gut gene expression. After bacterial intestinal inoculation, posterior gut of fish fed PHYTO responded in a more controlled and belated way in terms of goblet cell size and mucus coverage in comparison to other treatments. For rectum, the pattern of response was similar for all dietary treatments, however fish fed GMOS maintained goblet cell size along the challenge test.

Macroalga-Derived Alginate Oligosaccharide Alters Intestinal Bacteria of Atlantic Salmon

Prebiotics are substrates intended to sculpt gut microbial communities as they are selectively utilized by the microorganisms to exert beneficial health effects on hosts. Macroalga-derived oligosaccharides are candidate prebiotics, and herein, we determined the effects of Laminaria sp.-derived alginate oligosaccharide (AlgOS) on the distal intestinal microbiota of Atlantic salmon (Salmo salar). Using a high-throughput 16S rRNA gene amplicon sequencing technique, we investigated the microbiota harbored in the intestinal content and mucus of the fish offered feeds supplemented with 0.5 and 2.5% AlgOS. We found that the prebiotic shifts the intestinal microbiota profile; alpha diversity was significantly reduced with 2.5% AlgOS while with 0.5% AlgOS the alteration occurred without impacting the bacterial diversity. Beta diversity analysis indicated the significant differences between control and prebiotic-fed groups. The low supplementation level of AlgOS facilitated the dominance of Proteobacteria (including Photobacterium phosphoreum, Aquabacterium parvum, Achromobacter insolitus), and Spirochaetes (Brevinema andersonii) in the content or mucus of the fish, and few of these bacteria (Aliivibrio logei, A. parvum, B. andersonii, A. insolitus) have genes associated with butyrate production. The results indicate that the low inclusion of AlgOS can plausibly induce a prebiotic effect on the distal intestinal microbiota of Atlantic salmon. These findings can generate further interest in the potential of macroalgae-derived oligosaccharides for food and feed applications.

Nutrition and water temperature regulate the expression of heat-shock proteins in golden pompano larvae (Trachinotus ovata, Limmaeus 1758)

Understanding fish larval development is of a great interest for aquaculture production efficiency. Identifying possible indicators of fish larvae stress could improve the production and limit the mortality rate that larval stage is subjected to. Heat-shock proteins (HSPs) and heat-shock factors (HSFs) are well known as indicators of response to many kinds of stressor (e.g., environmental, morphological, or pathological changes). In this study, golden pompano larvae were raised at different temperatures (23 °C, 26 °C, and 29 °C), as well as three different diets (Artemia nauplii unenriched, Artemia nauplii enriched with Nannochloropsis sp., and Artemia nauplii enriched with Algamac 3080), and the expression of HSP60, HSP70, HSF1, HSP2, and GRP94 were monitored. While stress genes were widely expressed in the larval tissues, HSP60 and HSP70 were principally from the gills and heart; HSF1 principally from the muscle, brain, and heart; and GRP94 principally from the head kidney and spleen. Golden pompano larvae were found to be more sensitive to thermal changes at later larval stage, and 29 °C was showed to likely be the best condition for golden pompano larval development. Nannochloropsis sp.-enriched Artemia nauplii treatment was found to be the most appropriate feed type with moderate relative expressions of HSP60, HSP70, HSF1, HSF2, and GRP94.

Symbiont-derived sphingolipids regulate inflammatory responses in rainbow trout (Oncorhynchus mykiss)

Farmed fish live in association with diverse bacterial communities that produce wide arrays of metabolites. In rainbow trout, the skin and the gills are colonized by Flectobacillus major, a bacterium known to produce sphingolipids (SLs). The goal of this study is to evaluate the ability of F. major SLs to regulate rainbow trout inflammatory responses. F. major SLs were delivered by themselves or in combination with Freund's Complete Adjuvant (FCA), an oil-based adjuvant known to cause severe abdominal inflammation when injected to fish. Trout injected with SL + FCA showed decreased severity of FCA toxic effects including necrosis, granuloma formation and presence of oil droplets. However, inclusion of SLs in the FCA preparation did not decrease infiltration of immune cells intramuscularly at the site of injection. Intraperitoneal or intravenous delivery of F. major SLs resulted in increased expression of IgT, IgM and TGFβ transcripts in the gills but not the head-kidney and had no effects on IL-10 expression. These results indicate the F. major SLs regulate rainbow trout inflammatory responses and indicate that this compound can have important applications in farmed fish health management.

Integrative Transcriptomic and microRNAomic Profiling Reveals Immune Mechanism for the Resilience to Soybean Meal Stress in Fish Gut and Liver

In aquafeeds, fish-meal has been commonly replaced with plant protein, which often causes enteritis. Currently, foodborne enteritis has few solutions in regards to prevention or cures. The recovery mechanism from enteritis in herbivorous fish may further help understand prevention or therapy. However, few reports could be found regarding the recovery or resilience to fish foodborne enteritis. In this study, grass carp was used as an animal model for soybean meal induced enteritis and it was found that the fish could adapt to the soybean meal at a moderate level of substitution. Resilience to soybean meal stress was found in the 40% soybean meal group for juvenile fish at growth performance, morphological and gene expression levels, after a 7-week feeding trial. Furthermore, the intestinal transcriptomic data, including transcriptome and miRNAome, was applied to demonstrate resilience mechanisms. The result of this study revealed that in juvenile grass carp after a 7-week feeding cycle with 40% soybean meal, the intestine recovered via enhancing both an immune tolerance and wound healing, the liver gradually adapted via re-balancing immune responses, such as phagosome and complement cascades. Also, many immune factors in the gut and liver were systemically revealed among stages of on-setting, remising, and recovering (or relief). In addition, miRNA regulation played a key role in switching immune states. Thus, the present data systemically demonstrated that the molecular adaptation mechanism of fish gut-liver immunity is involved in the resilience to soybean meal stress.

Granulomatous enteritis in rainbow trout (Oncorhynchus mykiss) associated with soya bean meal regardless of water dissolved oxygen level

This study investigated morphological changes associated with soya bean meal-induced enteritis (SBMIE) in distal intestine (DI) of rainbow trout (Oncorhynchus mykiss) fed a soya bean meal (SBM)-based diet and exposed to normoxia or hypoxia created by optimal and low water flow rates, respectively. A 28-day adaption period was followed by a 42-day challenge period where 600 fish were subjected to dietary challenge and/or hypoxia. Twelve tanks each containing 50 juvenile trout were assigned randomly in triplicate to each treatment. Histopathological and immunohistochemical evaluation revealed pathological features that have not previously been described in association with SBMIE. Vacuolar degeneration of epithelial cells mainly at the base of mucosal folds, epithelial cysts, epithelial dysplasia, necrosis, shedding of necrotic cells, and granulomatous inflammation including infiltration of enlarged, sometimes finely vacuolated or "foamy" macrophages, multinucleated giant cells and increased proliferation of fibroblasts were observed. Acid-fast bacteria were not detected in enlarged macrophages; however, these cells contained AB-PAS- and sometimes cytokeratin-positive material, which was interpreted to be of epithelial/goblet cell origin. Hypoxia did not affect the morphological changes in DI. These results suggest that SBM was associated with a granulomatous form of enteritis in DI of rainbow trout regardless of water oxygen level.

Alternative Protein Sources in the Diet Modulate Microbiota and Functionality in the Distal Intestine of Atlantic Salmon (Salmo salar)

The present study aimed to investigate whether alternative dietary protein sources modulate the microbial communities in the distal intestine (DI) of Atlantic salmon, and whether alterations in microbiota profiles are reflected in modifications in host intestinal function and health status. A 48-day feeding trial was conducted, in which groups of fish received one of five diets: a reference diet in which fishmeal (diet FM) was the only protein source and four experimental diets with commercially relevant compositions containing alternative ingredients as partial replacements of fishmeal, i.e., poultry meal (diet PM), a mix of soybean meal and wheat gluten (diet SBMWG), a mix of soy protein concentrate and poultry meal (diet SPCPM), and guar meal and wheat gluten (diet GMWG). Samples were taken of DI digesta and mucosa for microbial profiling using high-throughput sequencing and from DI whole tissue for immunohistochemistry and expression profiling of marker genes for gut health. Regardless of diet, there were significant differences between the microbial populations in the digesta and the mucosa in the salmon DI. Microbial richness was higher in the digesta than the mucosa. The digesta-associated bacterial communities were more affected by the diet than the mucosa-associated microbiota. Interestingly, both legume-based diets (SBMWG and GMWG) presented high relative abundance of lactic acid bacteria in addition to alteration in the expression of a salmon gene related to cell proliferation (pcna). It was, however, not possible to ascertain the cause-effect relationship between changes in bacterial communities and the host's intestinal responses to the diets.IMPORTANCE The intestine of cultivated Atlantic salmon shows symptoms of compromised function, which are most likely caused by imbalances related to the use of new feed ingredients. Intestinal microbiota profiling may become in the future a valuable endpoint measurement in order to assess fish intestinal health status and effects of diet. The present study aimed to gain information about whether alternative dietary protein sources modulate the microbial communities in the Atlantic salmon intestine and whether alterations in microbiota profiles are reflected in alterations in host intestinal function and health status. We demonstrate here that there are substantial differences between the intestinal digesta and mucosa in the presence and abundance of bacteria. The digesta-associated microbiota showed clear dependence on the diet composition, whereas mucosa-associated microbiota appeared to be less affected by diet composition. Most important, the study identified bacterial groups associated with diet-induced gut dysfunction that may be utilized as microbial markers of gut health status in fish.

The zebrafish as a model to study intestinal inflammation

Starting out as a model for developmental biology, during the last decade, zebrafish have also gained the attention of the immunologists and oncologists. Due to its small size, high fecundity and full annotation of its genome, the zebrafish is an attractive model system. The fact that fish are transparent early in life combined with the growing list of immune cell reporter fish, enables in vivo tracking of immune responses in a complete organism. Since zebrafish develop ex utero from a fertilized egg, immune development can be monitored from the start of life. Given that several gut functions and immune genes are conserved between zebrafish and mammals, the zebrafish is an interesting model organism to investigate fundamental processes underlying intestinal inflammation and injury. This review will first provide some background on zebrafish intestinal development, bacterial colonization and immunity, showing the similarities and differences compared to mammals. This will be followed by an overview of the existing models for intestinal disease, and concluded by future perspectives in light of the newest technologies and insights.

Experimental transmission of segmented filamentous bacteria (SFB) in rainbow trout Oncorhynchus mykiss

Rainbow trout gastroenteritis (RTGE) has been the cause of acute mortality in farmed rainbow trout in Europe since 1992. Epidemiological analysis has indicated a strong association with high production levels and suggested an infectious aetiology. The condition is characterised by the presence of large numbers of segmented filamentous bacteria (SFB) in the intestine, but the role of these in the disease has not been confirmed, in part because the organisms cannot be cultured. Therefore, other approaches need to be developed to investigate the role of SFB in RTGE. Faecal material from clinically affected RTGE trout, either untreated or heat-inactivated, was administered to fish from a susceptible stock, to determine whether the SFB could be transferred artificially and survive in or colonise the new host. Using histology and nested PCR, SFB were detected in the pyloric caeca of fish 23 to 30 d after challenge with untreated faeces. Histological changes in the intestine and the presence of an unidentified Gram-negative coccus were also significantly associated with exposure to untreated faeces. Upregulation of IFN-γ, IL-17A/F and IL-22 gene expression in proximal intestine suggested a low-level immune response to the challenge.

Replacement of dietary soy- with air classified faba bean protein concentrate alters the hepatic transcriptome in Atlantic salmon (Salmo salar) parr

The production of carnivorous fish such as Atlantic salmon (Salmo salar) is dependent on the availability of high quality proteins for feed formulations. For a number of nutritional, strategic and economic reasons, the use of plant proteins has steadily increased over the years, however a major limitation is associated with the presence of anti-nutritional factors and the nutritional profile of the protein concentrate. Investigating novel raw materials involves understanding the physiological consequences associated with the dietary inclusion of protein concentrates. The primary aim of the present study was to assess the metabolic response of salmon to increasing inclusion of air-classified faba bean protein concentrate (BPC) in feeds as a replacement for soy protein concentrate (SPC). Specifically, we tested treatments with identical contents of fishmeal (222.4gkg(-1)) and progressively higher inclusion of BPC (0gkg(-1), 111.8gkg(-1), 223.6gkg(-1), 335.4gkg(-1), 447.2gkg(-1)) substituting SPC. This study demonstrated a dose-dependent metabolic response to a plant ingredient and was the first to compare the nutrigenomic transcriptional responses after substitution of terrestrial feed ingredients such as BPC and SPC without withdrawal of marine ingredients. It was found that after eight weeks a major physiological response in liver was only evident above 335.4gkg(-1) BPC and included decreased expression of metabolic pathways, and increased expression of genes regulating transcription and translation processes and the innate immune response. Furthermore, we showed that the nutritional stress caused by BPC resembled, at least at hepatic transcriptional level, that caused by soybean meal (included as a positive control in our experimental design). The outcomes of the present study suggested that Atlantic salmon parr might efficiently utilize moderate substitution of dietary SPC with BPC, with the optimum inclusion level being around 120gkg(-1)in the type of feeds tested here.

Nutrigenomic profiling of transcriptional processes affected in liver and distal intestine in response to a soybean meal-induced nutritional stress in Atlantic salmon (Salmo salar)

The aim of the present study was to generate an experimental model to characterize the nutrigenomic profile of a plant-derived nutritional stress. Atlantic salmon (Salmo salar) was used as the model species. The nutritional stress was induced by inclusion of dietary defatted soybean meal (SBM), as this ingredient had been previously demonstrated to induce enteropathy in the distal intestine and reduce growth in salmon. Triplicate groups of Atlantic salmon were fed concentrations of 0, 100, 200 and 300 g kg(-1) SBM for 12 weeks and reduced growth performance was used as the indicator of nutritional stress. The transcriptome was analyzed in two tissues, liver and distal intestine, with the hypothesis being that the liver transcriptome would be characterized by gene expression responses related to overall growth and health performance, whereas intestinal gene expression would be dominated by specific responses to SBM. A set of 133 genes was differentially expressed in liver including 44 genes in common with the intestinal response. The liver-specific response included up-regulation of genes involved in protein digestion, energy metabolism and immune functions, whereas genes in other metabolic pathways were generally anabolic and down-regulated. These responses may be more related to general nutritional stress than to SBM per se. The transcriptomic profile in the distal intestine was consistent with the enteritis response as described previously. This study provides a comprehensive report on the profiles of liver and distal intestine transcriptomes, specifically highlighting the role of the liver in fish undergoing SBM-induced nutritional stress.

Impact of Fishmeal Replacement in Diets for Gilthead Sea Bream (Sparus aurata) on the Gastrointestinal Microbiota Determined by Pyrosequencing the 16S rRNA Gene

Recent studies have demonstrated the impact of diet on microbiota composition, but the essential need for the optimization of production rates and costs forces farms and aquaculture production to carry out continuous dietary tests. In order to understand the effect of total fishmeal replacement by vegetable-based feed in the sea bream (Sparus aurata), the microbial composition of the stomach, foregut, midgut and hindgut was analysed using high-throughput 16S rDNA sequencing, also considering parameters of growth, survival and nutrient utilisation indices.A total of 91,539 16S rRNA filtered-sequences were analysed, with an average number of 3661.56 taxonomically assigned, high-quality sequences per sample. The dominant phyla throughout the whole gastrointestinal tract were Actinobacteria, Protebacteria and Firmicutes. A lower diversity in the stomach in comparison to the other intestinal sections was observed. The microbial composition of the Recirculating Aquaculture System was totally different to that of the sea bream gastrointestinal tract. Total fishmeal replacement had an important impact on microbial profiles but not on diversity. Streptococcus (p-value: 0.043) and Photobacterium (p-value: 0.025) were highly represented in fish fed with fishmeal and vegetable-meal diets, respectively. In the stomach samples with the vegetable diet, reads of chloroplasts and mitochondria from vegetable dietary ingredients were rather abundant. Principal Coordinate Analysis showed a clear differentiation between diets in the microbiota present in the gut, supporting the presence of specific bacterial consortia associated with the diet.Although differences in growth and nutritive parameters were not observed, a negative effect of the vegetable diet on the survival rate was determined. Further studies are required to shed more light on the relationship between the immune system and sea bream gastrointestinal tract microbiota and should consider the modulation of the microbiota to improve the survival rate and nutritive efficacy when using plant-based diets.

Diet type dictates the gut microbiota and the immune response against Yersinia ruckeri in rainbow trout (Oncorhynchus mykiss)

This study investigated the influence of the rainbow trout (Oncorhynchus mykiss) commensal intestinal microbiota in connection to an experimental Yersina ruckeri infection, the causative agent of enteric redmouth disease. One marine and one plant diet was administered to two different groups of rainbow trout. The plant-based diet gave rise to an intestinal microbiota dominated by the genera Streptococcus, Leuconostoc and Weissella from phylum Firmicutes whereas phylum Proteobacteria/Bacteroidetes/Actinobacteria dominated the community in the marine fed fish. In connection to the Y. ruckeri bath challenge there was no effect of the diet type on the cumulative survival, but the number of Y. ruckeri positive fish as measured by plate count and the number of fish with a 'high' number of reads belonging to genus Yersinia as measured by 16S rRNA next-generation sequencing was higher for marine diet fed fish. Furthermore, the two experimental groups of fish showed a differential immune response, where Y. ruckeri challenged marine fed fish had a higher transcription of IL-1β and MBL-2 relative to challenged plant diet fed fish. The data suggest that the plant diet gave rise to a prebiotic effect favouring the presence of bacterial taxons proving protective in connection to bath challenge by Y. ruckeri.

Effect of the dietary inclusion of soybean components on the innate immune system in zebrafish

Some components of plant-based meals, such as saponins and vegetal proteins, have been proposed as inducers of intestinal inflammation in some fish. However, the molecular and cellular bases for this phenomenon have not been reported. In this work, zebrafish were used as a model to evaluate the effects of individual soybean meal components, such as saponins and soy proteins. Zebrafish larvae fed a fish meal feed containing soy components were assessed according to low and high inclusion levels. The granulocytes associated with the digestive tract and the induction of genes related to the immune system were quantitated as markers of the effects of the dietary components. A significant increase in the number of granulocytes was observed after feeding fish diets containing high saponin or soy protein contents. These dietary components also induced the expression of genes related to the innate immune system, including myeloid-specific peroxidase, as well as the complement protein and cytokines. These results reveal the influence of dietary components on the stimulation of the immune system. These observations could be significant to understanding the contributions of saponin and soy protein to the onset of enteritis in aqua-cultured fish, and this knowledge may aid in defining the role of the innate immune system in other inflammatory diseases involving dietary components in mammals.

Characterization of the endocrine, digestive and morphological adjustments of the intestine in response to food deprivation and torpor in cunner, Tautogolabrus adspersus

The cunner, Tautogolabrus adspersus, is a marine teleost endemic to the cold waters of the Northwest Atlantic Ocean. The cunner is non-migratory and is known for its remarkable ability to endure the freezing winter months with little to no food by entering a torpid/dormant state. To evaluate the physiological strategies employed by the cunner's intestinal tract to withstand food deprivation, fish were sampled for their gut after a four-week period of acute food deprivation during their summer (active/feeding) state, as well as after 4months of overwinter fasting. Digestive capacity was evaluated by measuring digestive enzyme activity and related mRNA transcript expression for trypsin, alkaline phosphatase, alanine aminopeptidase and lipase. In order to assess how gut hormones affect/are affected by acute fasting and torpor, we examined the intestinal mRNA expression of several putative appetite regulators, i.e. CCK, apelin, orexin and mTOR. Short-term summer fasting induced a reduction in the activity, but not the transcript expression, of all digestive enzymes examined as well as a reduction in gut apelin mRNA. Torpor induced a reduction in the activity of all enzymes with the exception of alanine aminopeptidase, and a decrease in mRNA levels of alanine aminopeptidase, orexin, CCK and mTOR. Our results suggest that both acute fasting and long-term fasting induce a reduction in the intestinal function of cunner, as evidenced by an overall decrease in the activities of digestive enzymes and mRNA expression of several factors involved in feeding and digestion.

Soybean meal induces intestinal inflammation in zebrafish larvae

The necessary replacement of fish meal with other protein source in diets of commercially important fish has prompted the study of the effect of the inclusion of different vegetable proteins sources on growth performance and on the gastro-intestinal tract. Currently, soybean meal is the primary protein source as a fish meal replacement because of its low price and high availability. Likewise, it is been documented that the ingestion of soybean meal by several fish species, such as salmonids and carp, triggers a type of intestinal inflammation called enteritis. In this paper, we analyzed the effects of the ingestion of soybean meal and two of its components, soy protein and soy saponin, on zebrafish to establish the basis for using zebrafish larvae as a model for fish nutrition. We took advantage of the existence of different transgenic lines, which allowed us to perform in vivo analysis. Our results indicated that larvae that were feed with soybean meal developed a clear intestinal inflammation as early as two day after beginning the diet. Moreover, we determined that is not the soy protein present in the diet but the soy saponin that is primarily responsible for triggering the immune response. These findings support the use of zebrafish screening assays to identify novel ingredients that would to improved current fish diets or would formulate new ones.

Transcriptional regulation of IL-17A and other inflammatory markers during the development of soybean meal-induced enteropathy in the distal intestine of Atlantic salmon (Salmo salar L.)

Progression of soybean meal (SBM)-induced enteropathy in Atlantic salmon (Salmo salar L.) distal intestine (DI) was studied to investigate pathophysiological mechanisms and immune responses. Seawater-adapted salmon were fed an extracted SBM-containing diet (200 g kg(-1)) from day 1-21 and compared with fish fed a fishmeal-based diet (day 0). Histological evaluation of the DI revealed signs of inflammation from day 5, which progressively increased in severity and affected more fish with increasing SBM exposure time. The expression profiles of 16 genes were analyzed by quantitative PCR. The pro-inflammatory cytokines interleukin 17A (IL-17A), IL-1β, interferon α (IFNα) and IFNγ, as well as IL-17A receptor, T-cell receptor γ (TCRγ), cluster of differentiation 4α (CD4α), CD8β, transforming growth factor β (TGFβ), trypsin, protease-activated receptor 2 (PAR2) and myeloid differentiation primary response gene 88 (MyD88) were significantly up-regulated during early and/or late inflammation stages, whereas interferon-γ-inducible lysosomal thiol reductase (GILT) was downregulated. Up-regulation of TCRγ from day seven suggests proliferation of intraepithelial γδ T cells. IL-17A, up-regulated by 218-fold during early inflammation, indicates involvement of T helper 17 cells in the pathogenesis of the SBM-induced inflammatory response.

PCR-TTGE analysis of 16S rRNA from rainbow trout (Oncorhynchus mykiss) gut microbiota reveals host-specific communities of active bacteria

This study assessed the relative contributions of host genetics and diet in shaping the gut microbiota of rainbow trout. Full sibling fish from four unrelated families, each consisting of individuals derived from the mating of one male and one female belonging to a breeding program, were fed diets containing either vegetable proteins or vegetable oils for two months in comparison to a control diet consisting of only fish protein and fish oil. Two parallel approaches were applied on the same samples: transcriptionally active bacterial populations were examined based on RNA analysis and were compared with bacterial populations obtained from DNA analysis. Comparison of temporal temperature gradient gel electrophoresis (TTGE) profiles from DNA and RNA showed important differences, indicating that active bacterial populations were better described by RNA analysis. Results showed that some bacterial groups were significantly (P<0.05) associated with specific families, indicating that microbiota composition may be influenced by the host. In addition, the effect of diet on microbiota composition was dependent on the trout family.

Teleost intestinal immunology

Teleosts clearly have a more diffuse gut associated lymphoid system, which is morphological and functional clearly different from the mammalian GALT. All immune cells necessary for a local immune response are abundantly present in the gut mucosa of the species studied and local immune responses can be monitored after intestinal immunization. Fish do not produce IgA, but a special mucosal IgM isotype seems to be secreted and may (partly) be the recently described IgZ/IgT. Fish produce a pIgR in their mucosal tissues but it is smaller (2 ILD) than the 4-5 ILD pIgR of higher vertebrates. Whether teleost pIgR is transcytosed and cleaved off in the same way needs further investigation, especially because a secretory component (SC) is only reported in one species. Teleosts also have high numbers of IEL, most of them are CD3-ɛ+/CD8-α+ and have cytotoxic and/or regulatory function. Possibly many of these cells are TCRγδ cells and they may be involved in the oral tolerance induction observed in fish. Innate immune cells can be observed in the teleost gut from first feeding onwards, but B cells appear much later in mucosal compartments compared to systemic sites. Conspicuous is the very early presence of putative T cells or their precursors in the fish gut, which together with the rag-1 expression of intestinal lymphoid cells may be an indication for an extra-thymic development of certain T cells. Teleosts can develop enteritis in their antigen transporting second gut segment and epithelial cells, IEL and eosinophils/basophils seem to play a crucial role in this intestinal inflammation model. Teleost intestine can be exploited for oral vaccination strategies and probiotic immune stimulation. A variety of encapsulation methods, to protect vaccines against degradation in the foregut, are reported with promising results but in most cases they appear not to be cost effective yet. Microbiota in fish are clearly different from terrestrial animals. In the past decade a fast increasing number of papers is dedicated to the oral administration of a variety of probiotics that can have a strong health beneficial effect, but much more attention has to be paid to the immune mechanisms behind these effects. The recent development of gnotobiotic fish models may be very helpful to study the immune effects of microbiota and probiotics in teleosts.

Heat shock proteins (chaperones) in fish and shellfish and their potential role in relation to fish health: a review

Heat shock proteins (HSPs), also known as stress proteins and extrinsic chaperones, are a suite of highly conserved proteins of varying molecular weight (c. 16-100 kDa) produced in all cellular organisms when they are exposed to stress. They develop following up-regulation of specific genes, whose transcription is mediated by the interaction of heat shock factors with heat shock elements in gene promoter regions. HSPs function as helper molecules or chaperones for all protein and lipid metabolic activities of the cell, and it is now recognized that the up-regulation in response to stress is universal to all cells and not restricted to heat stress. Thus, other stressors such as anoxia, ischaemia, toxins, protein degradation, hypoxia, acidosis and microbial damage will also lead to their up-regulation. They play a fundamental role in the regulation of normal protein synthesis within the cell. HSP families, such as HSP90 and HSP70, are critical to the folding and assembly of other cellular proteins and are also involved in regulation of kinetic partitioning between folding, translocation and aggregation within the cell. HSPs also have a wider role in relation to the function of the immune system, apoptosis and various facets of the inflammatory process. In aquatic animals, they have been shown to play an important role in health, in relation to the host response to environmental pollutants, to food toxins and in particular in the development of inflammation and the specific and non-specific immune responses to bacterial and viral infections in both finfish and shrimp. With the recent development of non-traumatic methods for enhancing HSP levels in fish and shrimp populations via heat, via provision of exogenous HSPs or by oral or water administration of HSP stimulants, they have also, in addition to the health effects, been demonstrated to be valuable in contributing to reducing trauma and physical stress in relation to husbandry events such as transportation and vaccination.