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Xylooligosaccharides (XOSs) are being explored as prebiotics in fish diets; however, their effects and modes of action have not been well evaluated. Reports have shown that dietary XOS has the potential to improve the proliferation of beneficial gut microbes, and their metabolites, and enhance disease resistance in several fish species. In contrast, other studies report no substantial changes in immune and growth parameters compared to control groups. Like all prebiotics, the mode of action of XOS is based on their selective stimulation of beneficial gut microbiota, which will outcompete and prevent pathogen proliferation in the gut, and produce metabolites that modulate host immune responses. The reports of improved growth performance of XOS fed fish may be due to improved intestinal microbiome, enhanced glycolysis activity and elevated gastrointestinal enzymatic activities. Dietary XOSs have different effects on fish performance depending on the fish species and the structure of XOSs (degree of XOS polymerization and substitution). Nevertheless, further research is essential to determine the optimal dosage, degree of polymerization, and substitution levels required to improve each fish species' gut health and growth performance. This review highlights the prebiotic effects of XOSs, their mechanism of action, and knowledge gaps. [ABSTRACT FROM AUTHOR]

Previous studies have shown that the gut microbiota of fish differs depending on host feeding habits and these populations may also be influenced by dietary ingredients. In this study, the bacterial populations of the intestinal tract of the opportunistic omnivore jundiá Rhamdia quelen and the typical omnivore tilapia Oreochromis niloticus were investigated in two experiments. In experiment I, the levels of amylolytic, cellulolytic, lipolytic, proteolytic and total culturable bacteria were compared in the proximal, mid and distal intestine using selective agar. Higher levels of amylolytic, cellulolytic, lipolytic, proteolytic and total culturable bacteria were observed in the tilapia than jundiá, in all intestinal regions. Jundiá contained higher levels of proteolytic bacteria and lipid digesting bacteria in the distal intestinal portion as compared with the mid intestine; both fish species also presented more amylolytic bacteria in the distal intestine. In experiment II, the amylolytic intestinal microbiota between the two species was compared after administration of diets containing different carbohydrate sources. Jundiá fed broken rice presented higher total culturable bacterial levels; however, dietary cassava bagasse and ground corn significantly elevated the population of amylolytic bacteria in tilapia ( P > 0.05). PCR- DGGE was also used to assess the bacterial communities in experiment II. A Cetobacterium spp. was detected in jundiá fed diets containing broken rice, and tilapia fed cassava bagasse, dextrin, broken rice and ground corn. Microbial differentiation was further demonstrated between jundiá and tilapia, because an uncultured bacterium was unique in tilapia and an uncultured spirochete was observed only in jundiá; the presence of these bacterial species was also influenced by dietary carbohydrate sources. [ABSTRACT FROM AUTHOR]

The article presents a study on the influence of different feeding schemes on enzyme activities of gut microbiota in Atlantic cod (Gadus morhua L.). It examines enzyme producing bacteria that were isolated from the gastrointestinal (GI) tract and determines the most promising enzyme-producing bacteria through 16S ribosomal RNA (rRNA). It suggests that the diversity of bacteria appears to be influenced by the feeding regimes.

As no information is available regarding the efficacy of using rice protein concentrate ( RPC) to replace fishmeal ( FM) in diets for European sea bass ( Dicentrarchus labrax), a trial was conducted to determine the optimum inclusion for European sea bass juveniles. Diets were formulated to replace 25%, 50% and 75% of fishmeal with RPC. Additional high level inclusion diets (50% and 75%) were supplemented with lysine and methionine to determine if these were limiting factors. Fish (18.0 ± 0.06 g) were fed the experimental diets for 12 weeks. The final mean weight and specific growth rate ( SGR) were significantly higher in fish fed the control diet (diet FM) and low level RPC (diet RPC25) than in fish fed the other diets. The supplementation of the amino acids had a positive effect on growth, elevating the final weight of the high level RPC ( RPC75 + AA) group above that of the non-supplemented group ( RPC75). However, this was still not comparable to fish fed FM and RPC25 diets. No significant differences in body composition were observed. A digestibility study demonstrated an inverse relationship between elevating inclusion levels of RPC and apparent digestibility coefficients ( ADCs) for dry matter and lipid, with significantly lower values than the control being observed with RPC inclusions above 25%. The mesenteric fat index followed this trend. Circulating leucocyte levels, leucocyte ratios and serum lysozyme activity remained unaffected by dietary treatment. However, compared with the control group, fish fed RPC75, RPC50 + AA RPC75 + AA displayed significantly lower haematocrit values. The present study demonstrates that the inclusion of RPC at 140 g kg−1 (effectively replacing 25% of FM content) does not compromise European sea bass growth performance, body quality or basic haematological parameters. It is suggested that future studies including higher levels test the efficacy of additional amino acid supplementation (e.g. tryptophan). [ABSTRACT FROM AUTHOR]

The article focuses on the influence of probiotic adhesion to intestinal mucosa on the activity and morphology of rainbow trout enterocytes. Probiotics can adhere to the intestinal mucus and mucosa. Pediococcus acidilactici-like bacteria on the distal intestinal epithelium of rainbow trout post feeding on Pediococcus acidilactici-supplemented diets.