Effects of Dietary Bacillus velezensis LSG2-5 on Growth, Immunity, Antioxidant Capacity, and Disease Resistance of Amur minnow (Rhynchocypris lagowskii Dybowski)

The study was aimed at investigating the effects of Bacillus velezensis LSG2-5 isolated from host’s intestinal tract on growth performance, immune response, antioxidant capacity, and disease resistance of Amur minnow (Rhynchocypris lagowskii Dybowski). 535 fish were divided into 5 groups with 3 replicates per group. Diets containing different doses (0, 106, 107, 108, and 109 CFU/g, respectively) of B. velezensis LSG2-5 were used to feed fish ( $9.95\pm 0.05$ g) for 56 days. Results showed that compared with control, weight gain rate and specific growth rate of fish were significantly increased by dietary 107-109 CFU/g B. velezensis ( $p<.05$ ). While for the feed conversion ratio, all treatment groups were significantly decreased ( $p<.05$ ). In serum, the contents of lysozyme and immunoglobulin M were significantly increased by dietary 106-109 CFU/g B. velezensis ( $p<.05$ ); the activities of component 3, component 4, and alkaline phosphatase were significantly increased by dietary 108 CFU/g B. velezensis ( $p<.05$ ). The activities of catalase, glutathione peroxidase, total antioxidant capacity, and superoxide dismutase in hepatopancreas were significantly increased by dietary 108-109 CFU/g B. velezensis ( $p<.05$ ). The activities of catalase, glutathione, glutathione peroxidase, total antioxidant capacity, and superoxide dismutase in the intestinal tract of 108 CFU/g group were significantly increased ( $p<.05$ ). However, malondialdehyde levels in the hepatopancreas and intestines were significantly decreased by dietary 107-108 CFU/g B. velezensis ( $p<.05$ ). Dietary 107-109 CFU/g B. velezensis significantly upregulated catalase, glutathione peroxidase, and copper/zinc superoxide dismutase mRNA levels ( $p<.05$ ). The levels of transforming growth factor β and interleukin-10 mRNA in the hepatopancreas of fish fed with B. velezensis of 108 CFU/g were significantly increased ( $p<.05$ ), and these in the intestinal tract of 106-109 CFU/g groups were upregulated ( $p<.05$ ). Feeding dietary 107-108 CFU/g B. velezensis significantly downregulated interleukin-8, interleukin-1β, and tumour necrosis factor α mRNA levels in hepatopancreas and intestines ( $p<.05$ ). After 56 days of feeding, 225 fish ( $23.20\pm 0.59$ g) were challenged with Aeromonas hydrophila (108 CFU/g), and survival rates were recorded for the next 14 days. Meanwhile, the serum immune parameters of fish were determined. Results showed that compared with control, the survival rates of the treatment groups were significantly increased ( $p<.05$ ). Lysozyme, component 3, component 4, immunoglobulin M, acid phosphatase, and alkaline phosphatase levels in serum by dietary106-109 CFU/g B. velezensis showed significant promoting effects ( $p<.05$ ). In conclusion, dietary 107-109 CFU/g B. velezensis boosts the growth, immunity, antioxidant capacity, and against A. hydrophila infection of Amur minnow. Regression analysis suggested that the optimal dose of B. velezensis LSG2-5 in the Amur minnow diet was $5.00\times {10}^8$ CFU/g.