Effects of Algal-Derived β-Glucan on the Growth Performance, Intestinal Health, and Aeromonas veronii Resistance of Ricefield Eel (Monopterus albus).

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Ricefield eel is an important economic fish in China. However, large-scale intensive breeding has increased the incidence of diseases in eels. In this study, we conducted an 8-week feeding trial to investigate the effects of β-glucan on the growth performance, intestinal health, and Aeromonas veronii resistance of Monopterus albus (M. albus). Three hundred healthy fish (initial body weight: 12.38 ± 0.50 g) were randomly divided into five groups: A1 (basal diet) was considered the control group, whereas A2, A3, A4, and A5 were the experimental groups. The fish in the experimental groups were fed a basal diet supplemented with 250, 500, 1000, and 2000 mg/kg β-glucan, respectively. The addition of 0.025%-0.2% β-glucan resulted in a notable enhancement of eel growth performance, with the most significant improvement observed in eels supplemented with 0.1% β-glucan (p < 0.05). Furthermore, 0.025%-0.2% β-glucan could significantly enhance the antioxidant properties of the eel intestinal tract (p < 0.05), and the addition of 0.1% β-glucan significantly improved trypsin (TPS), amylase (AMS), and lipase (LPS) activities in the intestine (p < 0.05). In terms of intestinal histology, the A3, A4, and A5 groups exhibited significantly greater villus height compared to the control group (p < 0.05). Concentrations of β-glucan at 0.1% and 0.2% enhanced the composition of the intestinal flora; specifically, the relative abundance of Proteobacteria increased, while the relative abundance of Firmicutes decreased. Moreover, the addition of 0.05%-0.2% β-glucan significantly improved the relative survival rate (SR) of A. veronii-infected eels and significantly decreased the bacterial load of the liver, spleen, and kidney (p < 0.05). In comparison to eels that did not receive β-glucan supplementation, eels supplemented with 0.2% β-glucan exhibited decreased intestinal structural damage. In summary, the addition of 0.1%-0.2% β-glucan can promote eel growth, improve intestinal digestion and antioxidant capacity, regulate intestinal flora, and enhance intestinal physical function and anti-infection ability.