Dietary Formic Acid Supplementation Affects the Growth and Health Status and Alters the Gut Transcriptome and Microbiota in Largemouth Bass (Micropterus salmoides).

This study investigated the effects of dietary formic acid on growth performance, antioxidant capacity, serum biochemistry, digestive enzyme activity, gut microbiota composition, and transcriptomic profiles of largemouth bass (Micropterus salmoides). Four isonitrogenous (52.67% crude protein) and isolipidic (9.31% crude lipid) experimental diets were formulated: control (CG, 0.00% formic acid), FA1 (0.2%), FA2 (0.6%), and FA3 (1.0%). A total of 480 juvenile fish (initial body weight: 17.31 ± 0.10 g) were randomly allocated into four groups consisting of three replicate tanks (30 fish per tank). The fish were fed these diets for 9 weeks. The results showed that dietary formic acid significantly increased final body weight (FBW), weight gain (WG) and specific growth rate (SGR) (P < 0.05). Dietary supplementation with 0.2% formic acid significantly increased muscle and whole-body crude protein and lipid content, while improving mineral composition, fatty acid profiles, and amino acid profiles (P < 0.05). Digestive enzymes activity, including pepsin and lipase in stomach and trypsin, lipase and Na+/K+-ATPase in intestine, was enhanced (P < 0.05). Additionally, it elevated antioxidant activities (T-AOC, T-SOD, and GSH-Px) and serum immune parameters (LZM, ACP, and AKP activities, as well as TP and GLB contents) in M. salmoides. In terms of intestinal microorganisms, it was revealed that exogenous formic acid significantly changed intestinal microbial composition: decreased the abundance of potentially pathogenic bacteria such as Plesiomonas and Pseudomonas (P < 0.05); and increased the abundance of probiotics such as Bacillus, Acinetobacter, and Cetobacterium (P < 0.05). Furthermore, functional analysis revealed that the abundance of microbial functions related to nutrient metabolism increased significantly in FA1 group (P < 0.05). Compared with CG group fish, both the most up-regulated DEGs and the most down-regulated DEGs were related to the cell cycle, intestinal immune network for IgA production, and mTOR signaling pathway. These findings suggested that exogenous formic acid influences host intestinal gene expression and the structure of the gut microbiota, and contributes to digestion and absorption capacity and health in M. salmoides.