Yeast enzyme hydrolysis slurry supplementation improves growth, intestinal health, and metabolic responses in juvenile largemouth bass (Micropterus salmoides) fed soybean meal-based diets with partial fishmeal replacement

BACKGROUND: Yeast enzyme hydrolysis slurry (YS) has the potential to optimize feed utilization efficiency and improve the health of farmed animals, as it contains abundant bioactive components like small-molecule peptides and amino acids. However, its function and application effects in juvenile largemouth bass (Micropterus salmoides) are unclear. METHODS: Three hundred and twenty largemouth bass (8.20 ± 0.05 g) were randomly divided into four groups (4 replicates of 20 fish). Four isonitrogenous (52%) and isolipidic (10%) diets were formulated: FM group (positive control), SBM group (soybean meal replaced 30% of fish meal protein, negative control), and the SBM group supplemented with 1% YS (SBM + 1% YS) and 2% YS (SBM + 2% YS), respectively. After a 56-day feeding period, the fish were assessed for growth, intestinal health, and metabolic regulation-related indices. RESULTS: Our study found that weight gain rate (P = 0.032) and specific growth rate (P = 0.030) in the SBM + 1% YS and SBM + 2% YS groups were significantly higher than those in the SBM group. Relative to the SBM group, YS-supplemented groups exhibited marked elevations in intestinal folds, goblet cell numbers, serum acid and alkaline phosphatase activities, catalase and superoxide dismutase activities, as well as the activities of key digestive enzymes (lipase, α-amylase, pepsin, chymotrypsin), accompanied by downregulated mRNA expression of anorexigenic genes cholecystokinin and leptin. Meanwhile, these groups showed significantly lower serum D-lactate, diamine oxidase, lipopolysaccharide levels and malondialdehyde content. The abundance of beneficial genus Cetobacterium increased while the abundance of pathogenic genus Edwardsiella (P = 0.0265) significantly reduced in SBM + 1% YS and SBM comparison groups. Metabolomics analysis revealed that protein digestion and absorption (P = 0.0041), and amino acid metabolism pathways (P = 0.0052) were significantly enriched in the comparison between SBM + 1%YS and SBM groups. Correlation analysis further indicated that differential metabolites such as arginine and methionine exhibite a strong negative association with Edwardsiella. CONCLUSION: Yeast enzyme hydrolysis slurry in soybean meal-based diets with partial fishmeal replacement enhanced the antioxidant capacity, reduced intestinal permeability, altered the abundances of intestinal microbiota and associated core metabolites. These positive changes collectively contributed to improved growth performance in largemouth bass.