Host Gut-Derived Probiotic, Exiguobacterium acetylicum G1-33, Improves Growth, Immunity, and Resistance to Vibrio harveyi in Hybrid Grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂)

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 <i>Exiguobacterium acetylicum</i> G1-33 to hybrid grouper (<i>Epinephelus fuscoguttatus</i> ♀ × <i>Epinephelus lanceolatus</i> ♂) 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 <i>E. acetylicum</i> G1-33 (10⁶, 10⁸, and 10¹⁰ CFU/g), while a control group received commercial feed. The results showed that supplementation with 10⁶ and 10⁸ CFU/g of <i>E. acetylicum</i> G1-33 enhanced gut morphology, upregulated growth-related genes (<i>ghr1</i>, <i>igf-2</i>, <i>s6k1</i>, <i>tor</i>), and promoted growth, with supplementation with 10⁸ CFU/g resulting in the most notable enhancement. However, supplementation with 10¹⁰ CFU/g inhibited growth, possibly because of changes in intestinal morphology. Additionally, supplementation with <i>E. acetylicum</i> G1-33 upregulated the expression of immune-related genes (<i>c3</i>, <i>myd88</i>, <i>Cu/Zn-sod</i>, <i>tlr3</i>, and <i>tnf2</i>) 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 10⁶ and 10⁸ CFU/g <i>E. acetylicum</i> G1-33 enhanced the widespread expression of immune-related genes in the head kidney and liver, respectively, and improved resistance to <i>Vibrio harveyi</i>, whereas supplementation with 10¹⁰ CFU/g weakened this resistance. In conclusion, <i>E. acetylicum</i> G1-33, particularly at 10⁸ CFU/g, emerged as an effective probiotic, optimizing growth performance and immunity in hybrid grouper. This research is pioneering in its application of <i>E. acetylicum</i> in mariculture, potentially broadening the range of probiotic strategies in aquaculture.