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In recent years, the widespread use of antibiotics in intensive grouper mariculture has resulted in the ineffectiveness of antibiotic treatment, leading to an increasing incidence of diseases caused by bacteria, viruses, and parasites, causing serious economic losses. Hence, it is crucial to develop alternative strategies to antibiotics for healthy and sustainable development of the mariculture industry. Here, we aimed to screen host gut-derived probiotics and evaluate its effects on growth and immunity of grouper. In this study, 43 bacterial strains were isolated from the intestine of the hybrid grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂), and a potential probiotic strain G1-26, which can efficiently secrete amylase, protease, and lipase, was obtained using different screening mediums. Based on 16S rDNA sequencing, the potential probiotic strain G1-26 was identified as Vibrio fluvialis. The results of a biological characteristic evaluation showed that V. fluvialis G1-26 could grow at 25–45 °C, pH 5.5–7.5, salinity 10–40, and bile salt concentration 0–0.030%, and produce amylase, lipase, and protease under different culture conditions. Additionally, V. fluvialis G1-26 is sensitive to many antibiotics and does not exhibit aquatic biotoxicity. Subsequently, hybrid groupers were fed diets containing V. fluvialis G1-26 at different concentrations (0, 106, 108, and 1010 CFU/g) for 60 d. The results showed that V. fluvialis G1-26 at 108 CFU/g did not significantly affect the growth performance of the hybrid grouper (P > 0.05). V. fluvialis G1-26 supplementation at 108 and 1010 CFU/g significantly promoted the relative expression of immune-related genes in hybrid groupers (TLR3 , TLR5 , IL-1β , IL-8 , IL-10 , CTL , LysC , TNF-2 , and MHC-2) and improved the activities of alkaline phosphatase, acid phosphatase, total superoxide dismutase, and total protein in the liver. In conclusion, V. fluvialis G1-26, a potential probiotic strain isolated from the intestine of the hybrid grouper, can be used as an effective immunopotentiator at an optimal dose of 108 CFU/g diet. Our results provide a scientific basis for the development and utilization of probiotics in the grouper mariculture industry. • 43 bacterial strains were isolated from the intestine of the hybrid grouper. • A strain G1-26 identified as Vibrio fluvialis with strong digestive enzyme production ability was isolated. • The G1-26 feeding at 108 and 1010 CFU/g could enhance the immune performance and the optimal dosage is 108 CFU/g diet. [ABSTRACT FROM AUTHOR]

Probiotics have attracted considerable attention as potential antibiotic substitutes in aquaculture. However, their functional genomic characterisation remains incomplete. Therefore, this study aimed to evaluate the probiotic characteristics of Bacillus cereus G1-11 isolated from the gut of the hybrid grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂) by integrating genomics and phenotypic characteristics. Genome mining revealed gene clusters that are responsible for acid and bile tolerance as well as host intestinal adhesion and colonisation. Moreover, the G1-11 genome encodes proteins that associated with the bacterial antioxidant system and antimicrobial substances. Additionally, it contains gene clusters related to nutrient absorption, essential amino acids, and vitamin synthesis. The results showed that the G1-11 strain has the ability improved the intestinal barrier function of the host and enhanced the nutritional value of the feed. The genome analysis of G1-11 was complemented by in vitro experiments, which showed its ability to survive during gastrointestinal transit, as well as its antioxidant and antibacterial capabilities. The results indicate that B. cereus G1-11, as a probiotic, has the potential to enhance the growth and immunity of hybrid groupers. • Bacillus cereus G1-11 isolated from hybrid grouper gut was tested as a probiotic. • G1-11 has gene clusters for gut adhesion, colonisation, and immune functions. • G1-11 contains molecular markers associated with amino acid and vitamin synthesis. • Genes encoding secondary metabolites and bacteriocins were found in G1-11 genome. • G1-11 is resilient to gut transport and has antioxidant and antibacterial effects. [ABSTRACT FROM AUTHOR]