Effect of Supplementing Exogenous Glucanase or/and Mannanase to Diets Containing Torula Yeast on Growth Performance, Biochemical Indices, Liver and Intestinal Morphology, and Intestinal Microbiota and Metabolism of Largemouth Bass (Micropterus salmoides).

In the current study, we investigated the effect of a basic diet (where 20% of fishmeal was replaced by torula yeast, referred to as the control group), supplementation with β-glucanase (1000 U·kg, referred to as the TYG group), β-mannanase (510 U·kg, referred to as the group), and their combination (TYGM group), on the growth and health of juvenile largemouth bass (Micropterus salmoides). After an 8-week feeding experiment, the results revealed that juveniles in the TYM and TYGM groups exhibited significantly higher specific growth rates and hepatic antioxidant capacity, along with notably reduced levels of alkaline phosphatase, alanine aminotransferase, and aspartate aminotransferase activities in their serum. Histomorphological assessment indicated that dietary glucanase and/or mannanase could mitigate vacuolization and nuclear deviation in the liver, while also increasing villus width and height. Furthermore, 16S rRNA sequence analysis revealed a significant decrease in Mycoplasma levels in the TYM and TYGM groups, along with a notable increase in Cetobacterium content in the TYGM group compared to the other groups. Additionally, untargeted metabolomics analysis showed that the differentially expressed metabolites were primarily correlated with lipid metabolism, including steroid hormone biosynthesis (cholesterol sulfate), primary bile acid biosynthesis (cerebrosterol), and sphingolipid metabolism (phytosphingosine) between the control and TYGM groups. In conclusion, our study demonstrated that dietary glucanase + mannanase could partially alleviate the adverse impacts on the growth and health of juveniles caused by high levels of torula yeast in the diet.