Protective effects of mannan oligosaccharides on turbot Scophthalmus maximus suffering from soy enteropathy

Soybean meal is one of the most widely used alternative protein sources for aquafeed, but high level of soybean meal inclusion has been shown to induce poor growth performance and impair the gut health in fish, including turbot Scophthalmus maximus. Mannan oligosaccharides (MOS) have been used to improve soybean meal utilization in different fish species, but the results may appear contradictory. The present work was conducted to investigate the effects of MOS on soybean meal-fed turbot with an emphasis on gut health. Turbot (initial weight 7.6 g) were fed with three isonitrogenous and isolipidic diets for 8 weeks: FM, a basal diet with fish meal as the main protein source; SBM, a diet with 40% soybean meal mainly replacing fish meal; SBMMOS, a diet based on the SBM receipt supplemented with 0.2% mannan oligosaccharide. A typical soybean meal-induced enteritis (SBMIE) phenomenon, i.e., shorter and fewer microvilli and also changes in the autochthonous bacteria community were observed in fish fed the SBM diet. The MOS supplementation improved growth performance, feed utilization and counteracted the negative effects of SBM on gut health by increasing the activities of brush border enzymes, preventing alteration of the microvilli and mucosal folds and modulating the microbiota in a direction found in FM-fed fish. However, dietary MOS did not alleviate the infiltration of leucocytes in the lamina propria and submucosa, neither the high expression of inflammatory related cytokines caused by soybean meal. In conclusion, MOS was beneficial to the turbots suffering from soy enteropathy, but did not eliminate all symptoms of enteritis.