Bacillus subtilis FZU103 Promotes Growth in Micropterus salmoides, Accompanied by Modulation of Gut Microbiota, Enhanced Liver Antioxidants and Digestive Enzyme Activity

Probiotics hold great potential in aquaculture, as they can effectively modulate gut microbiota and improve fish health, thereby enhancing farming efficiency. Translating this potential into practical application critically relies on screening high-efficacy probiotic strains. This study evaluated the growth-promoting and health-enhancing effects of probiotic candidates <i>Lactobacillus rhamnosus</i> GG (LGG), <i>Lactobacillus plantarum</i> FZU310 (LP-FZU310) and <i>Bacillus subtilis</i> FZU103 (BS-FZU103) in largemouth bass (<i>Micropterus salmoides</i>). After feeding different probiotics for 30 days, the growth, antioxidant, and intestinal enzyme indicators of <i>M. salmoides</i> were detected. BS-FZU103 demonstrated superior efficacy among the tested strains, showing significant differences in both specific growth rate (SGR) (<i>p</i> < 0.05) and condition factor (CF) (<i>p</i> < 0.05). It also markedly enhanced hepatic antioxidant status, elevating superoxide dismutase and glutathione peroxidase activities while reducing malondialdehyde levels by 80%. Improved liver integrity was indicated by significant decreases in serum alanine aminotransferase, aspartate aminotransferase and alkaline phosphatase. Digestively, BS-FZU103 specifically increased intestinal amylase activity by 14.7%, without affecting protease or lipase, suggesting enhanced carbohydrate digestion. 16S rRNA sequencing revealed BS-FZU103 remodeled gut microbiota, increasing <i>Proteobacteria</i> abundance at the phylum level and enriching <i>Bacillus</i> while reducing <i>Clostridium sensu stricto 1</i> at the genus level. Functional prediction based on PICRUSt2 indicated an enhanced metabolic potential of the gut microbiota, with inferred upregulation of pathways related to carbohydrate transport and metabolism (e.g., ABC transporters) and intestinal enzymatic activities. Collectively, BS-FZU103 is associated with metabolic modulation, promoting <i>M. salmoides</i> growth through gut microbiota remodeling, hepatic antioxidant fortification, and targeted augmentation of carbohydrate utilization efficiency.