Dietary Bacillus subtilis Improves Growth Performance, Digestive Enzyme Activity, Antioxidant and Inflammatory Responses, and Gut Microbiota in Juvenile GIFT (Oreochromis niloticus)

This study investigated the effects of dietary Bacillus subtilis supplementation on growth performance, digestive enzyme activity, expression of antioxidant- and inflammation-related genes, and gut microbiota in juvenile GIFT (Oreochromis niloticus). The juvenile GIFT (total 540, initial body weight: 16.17 ± 1.32 g) were randomly divided into six groups and fed diets containing 0, 1 × 107, 1 × 108, 1 × 109, 1 × 1010, or 1 × 1011 CFU/kg B. subtilis for 60 days. Compared with the control group (0 CFU/kg B. subtilis), dietary B. subtilis significantly improved final body weight, weight gain rate, specific growth rate, and daily growth index and reduced feed conversion ratio (p < 0.05). Supplementation also significantly increased intestinal amylase, lipase, and trypsin activities (p < 0.05). In addition, the expression of several antioxidant-related genes was upregulated, whereas pro-inflammatory gene expression was generally downregulated in the intestine, liver, and head kidney. Gut microbiota analysis indicated that B. subtilis supplementation altered community composition, increased Firmicutes’ abundance, and reduced Proteobacteria abundance. Among all supplementation levels evaluated, the diet containing 1 × 109 CFU/kg B. subtilis produced the most favorable overall beneficial effects under the conditions of the present study.