Supplementation with Bacillus velezensis GY1 Improves Growth Performance, Intestine Morphology, and Intestine Microbial Diversity in Largemouth Bass (Micropterus salmoides)

Largemouth bass (Micropterus salmoides) is a major freshwater farmed species in China, making substantial economic contributions owing to its desirable flesh quality, rapid growth, and strong acceptance of formulated feeds. This study assessed the effects of dietary supplementation (intermittent and continuous feeding group, n = 3) with the probiotic Bacillus velezensis (1.0 × 109 CFU/g) on growth performance, intestinal morphology, digestive enzyme activities, and gut microbial diversity in this fish. The results indicated that continuous B. velezensis administration significantly increased percentage weight gain, specific growth rate, and survival, while concurrently reducing feed conversion ratio relative to the basal diet (p < 0.05). Intermittent feeding also resulted in a markedly higher survival rate and a lower FCR. Histological examination of intestinal sections revealed elevated villus height, villus width, and muscularis thickness in the B. velezensis supplemented groups, with the intermittent regimen producing more pronounced effects. Compared with the control, the intermittent group exhibited substantially decreased the pepsin and lipase activities of the serum. In the continuous group, serum globulin levels rose noticeably, accompanied by significant declines in total complement and superoxide dismutase. Moreover, continuous B. velezensis supplementation markedly upregulated the expression levels of TGFβ, myd88, IL34, IL8, and TNFα in the head-kidney of the largemouth bass, whereas intermittent supplementation significantly increased TGFβ, Gpx, Nrf2, and Acp transcript levels. Dietary inclusion of B. velezensis was also found to enhance intestinal microbial richness and greatly increase the abundance of potentially beneficial bacteria. Collectively, these results indicate that B. velezensis supplementation exerts beneficial effects on intestinal digestive capacity, growth, and innate immunity, thereby lowering disease susceptibility in largemouth bass.