Fish-derived Loigolactobacillus coryniformis LJR-37 enhances growth performance and disease resistance in tench in association with altered gut microbial ecology and predicted metabolic potential

Introduction The extensive use of antibiotics in aquaculture has intensified concerns regarding antimicrobial resistance and environmental sustainability, highlighting the need for effective probiotic alternatives. Therefore, this study aimed to isolate a fish-derived lactic acid bacterium from fish and evaluate its probiotic potential, growth-promoting effects, and protective effects against Aeromonas hydrophila (A. hydrophila) infection in tench. Methods A fish-derived lactic acid bacterium, Loigolactobacillus coryniformis ( L. coryniformis ) LJR-37, was isolated from crucian carp and characterized using whole-genome sequencing and in vitro assays. Its in vivo effects were further investigated in tench ( Tinca tinca ) through a 28-day feeding trial, followed by an A. hydrophila challenge. Results Genome analysis and phenotypic assays demonstrated that LJR-37 possessed favorable probiotic traits, including in vitro survival under acidic and bile-salt conditions, antagonistic activity against A. hydrophila , and biofilm-forming ability. Moreover, LJR-37 showed no hemolytic activity, and genome-wide screening against the VFDB and CARD databases detected no sequence with high similarity to known virulence or acquired antibiotic resistance genes. Dietary supplementation with LJR-37 improved growth performance, preserved intestinal morphology, and enhanced digestive enzyme activity. Metagenomic analysis further showed that LJR-37 supplementation was associated with higher relative abundances of Lactobacillus -related taxa and lower relative abundances of opportunistic pathogens, including Aeromonas and Acinetobacter , and an altered predicted functional profile, with higher relative representation of carbohydrate utilization and biosynthesis pathways and lower relative representation of degradative pathways. Following the A. hydrophila challenge, fish receiving LJR-37 exhibited higher survival, attenuated inflammatory responses, enhanced immune function, and alleviated intestinal tissue damage. Discussion Collectively, these findings indicate that the fish-derived probiotic LJR-37 promoted growth and may improve resistance to A. hydrophila , with these effects accompanied by changes in gut microbial composition and predicted functional potential. This study provides evidence at the level of association supporting the application of fish-derived probiotics as sustainable alternatives to antibiotics in aquaculture disease prevention.