Effect of Replacing Fish Meal Using Fermented Soybean Meal on Growth Performance, Intestine Bacterial Diversity, and Key Gene Expression of Largemouth Bass (Micropterus salmoides)

In China, aquaculture costs have increased because of the increase in fish meal (FM) prices. Plant proteins, such as soybean meal, have the potential to replace FM partially and thus reduce the cost of aquatic feed. In this study, soybean meal (SBM) was fermented using compound microorganisms (lactic acid bacteria, yeast, and Bacillus). Fermented soybean meal (FSBM) replaced FM in the diet. The effect of replacing FM on largemouth bass was comprehensively evaluated at three levels: macro (growth performance), microbial (bacterial diversity and metabolic), and gene (key gene expression) levels. The results showed that FSBM increased the crude protein content by 7.45% and decreased the phytic acid concentration by 48.66% compared with original SBM. Compared with the control, the weight gain rate and feed conversion ratio showed no significant difference (p > 0.05) when the replacement amount of FM was less than 30%. Compared with the 50% FSBM treatment, the 10% and 30% treatments showed more steatosis in the liver slices. Although the replacement of FSBM with less than 30% did not have a significant (p < 0.05) negative impact on body weight, FSBM substitution adversely affected the height and width of the intestinal villi. The expression levels of Hepcidin-1, Hepcidin-2, TGF-& beta;1, and IL-10 in 30% and 50% FSBM treatments were lower than those in the control, whereas the expression levels of SOD1 and SOD2 in 30% FSBM treatment were higher than those in the control group, and the expression level of SOD3a in 30% FSBM treatment was equal to the control. In addition, FSBM substitution could affect the composition of intestinal microorganisms and thus influence metabolic pathways, especially the biosynthesis of amino acids and plant secondary metabolites.