Effects of Dietary Supplementation with Three Different Probiotics on Growth Performance, Antioxidant Capacity, and Intestinal Microbiota in Grass Carp (Ctenopharyngodon idella)

The growing demand for sustainable aquaculture has intensified research on probiotics as antibiotic alternatives. This study aims to evaluate the effects of three probiotic supplements-1 × 10¹⁰ CFU/g of <i>Bacillus subtilis</i> (BS), <i>Clostridium butyricum</i> (CB), or <i>Enterococcus faecalis</i> (EF)-on growth performance, antioxidant capacity, intestinal structure, and gut microbiota in grass carp (<i>Ctenopharyngodon idella</i>; initial body weight: 42.52 ± 4.17 g) for 28 d. Compared to the non-supplemented (NC) control group, all probiotic-supplemented groups significantly enhanced final body weight, weight gain rate, specific growth rate, and crude protein content, and reduced feed conversion ratio (<i>p</i> < 0.05). Probiotic supplementation upregulated the intestinal <i>ctrb1</i> gene expression and increased villus length. Serum superoxide dismutase (SOD) and catalase activity were elevated in the BS group, whereas only SOD was increased in the CB group (<i>p</i> < 0.05). Gut microbiota analysis revealed reduced Proteobacteria abundance in all probiotic-supplemented groups. Compared with the NC group, the BS group enriched Bacteroidetes and <i>Prevotella_7</i>, while the CB group promoted the abundance of Actinobacteria, <i>Lactobacillus,</i> and <i>Clostridium_sensu_stricto_1</i>. The EF group increased the abundance of Fusobacteria, <i>Cetobacterium</i>, and <i>Bacteroides</i> (<i>p</i> < 0.05). These findings demonstrate that dietary supplementation with probiotics enhances growth performance by modulating antioxidant responses, intestinal morphology, and microbial community balance.