Effects of Cetobacterium somerae fermentation product on gut and liver health of common carp (Cyprinus carpio) fed diet supplemented with ultra-micro ground mixed plant proteins

Improving the replacement percentage of fish meal by plant proteins is of value for the aquaculture industry. Cetobacterium is the dominant commensal bacterium in many fish species. However, the function and potential beneficial effects of Cetobacterium is unknown. In this study, carps were fed with a practical basal diet with partial fish meal replacement by plant proteins (Control), basal diet with improved fish meal replacement percentage by adding ultra-micro ground mixed plant proteins (uPP), or the uPP diet supplemented with the fermentation product of a commensal bacterial strain Cetobacterium somerae XMX-1 (uPP + XMX-1). The effects of C. somerae XMX-1 fermentation product on gut and liver health of carp fed uPP supplemented diet were evaluated. After 4-week feeding, growth performance, feed utilization, biometric parameters, inflammatory score, expression of inflammation-related and lipid metabolism related genes were evaluated. The levels of LBP, LPS, AST and ALT in serum were assessed. 16S rRNA gene pyrosequencing was used to evaluate intestinal microbiota. Results showed that dietary uPP and uPP + XMX-1 had no effect on survival rate and growth (p > 0.05). However, dietary uPP compromised gut and liver health of fish. The expressions of inflammation related genes in gut and liver and the levels of LBP, LPS, AST and ALT in serum were significantly increased in uPP group versus control (p < 0.05). Interestingly, uPP + XMX-1 significantly decreased the expressions of inflammation related genes in gut and liver, as well as the levels of LBP, LPS, AST and ALT in serum compared to the uPP group (p < 0.05). In addition, uPP + XMX-1 significantly reduced the formation of lipid droplets in liver and significantly up-regulated the expression of lipogenesis genes and down-regulated the expression of lipolysis genes (p < 0.05) in comparison to the uPP group. Taken together, our study suggests that dietary uPP can impair gut and liver health and induce fatty liver, whereas C. somerae XMX-1 fermentation product can improve gut and liver health of common carp and decrease lipid deposition in liver, suggesting that XMX-1 fermentation product can be used as feed additive to improve the efficacy of fish meal replacement by plant proteins.