Host Gut-Derived Probiotic, G1-33, Improves Growth, Immunity, and Resistance to in Hybrid Grouper ( ♀ × ♂).

Several exogenous probiotics are applicable in fish culture; however, challenges in isolation and verification have hindered the full utilization of numerous host probiotics. Therefore, this study aimed to apply the host probiotic G1-33 to hybrid grouper ( ♀ × ♂) cultures and explore its mechanism of action. In total, 360 hybrid grouper were divided into four groups, which were fed the following for 60 days: three received commercial feed with varying concentrations of G1-33 (10, 10, and 10 CFU/g), while a control group received commercial feed. The results showed that supplementation with 10 and 10 CFU/g of G1-33 enhanced gut morphology, upregulated growth-related genes (, , , ), and promoted growth, with supplementation with 10 CFU/g resulting in the most notable enhancement. However, supplementation with 10 CFU/g inhibited growth, possibly because of changes in intestinal morphology. Additionally, supplementation with G1-33 upregulated the expression of immune-related genes (, , , , and ) in the liver and head kidney but led to an increase in malondialdehyde content, as well as a decrease in alkaline phosphatase and acid phosphatase activities, in the liver and serum, indicating increased oxidative stress. Moreover, supplementation with 10 and 10 CFU/g G1-33 enhanced the widespread expression of immune-related genes in the head kidney and liver, respectively, and improved resistance to , whereas supplementation with 10 CFU/g weakened this resistance. In conclusion, G1-33, particularly at 10 CFU/g, emerged as an effective probiotic, optimizing growth performance and immunity in hybrid grouper. This research is pioneering in its application of in mariculture, potentially broadening the range of probiotic strategies in aquaculture.