The effect of dietary supplementation of Lactobacillus rhamnosus GCC-3 fermentation product on gut and liver health, and resistance against bacterial infection of the genetically improved farmed tilapia (GIFT, Oreochromis niloticus)

High-fat diet can promote the growth of fish and save protein and its application is becoming more and more common in aquaculture. However, high-fat feed causes negative effects on fish health. The purpose of this study was to evaluate the effects of Lactobacillus rhamnosus GCC-3 fermentation product on gut and liver health and resistance against bacterial infection in genetically improved farmed tilapia (GIFT, Oreochromis niloticus) fed high fat diet. A total of 120 tilapia were randomly divided into three groups: control group, 1% GCC-3 group and 2% GCC-3 group (basal diet supplemented with 0, 10 and 20 g GCC-3/kg feed respectively). After 7 weeks of feeding, the growth performance and gut and liver health parameters of tilapia were evaluated. The gut microbiota and virome were detected by sequencing. Fish were also challenged with a combination of Aeromonas veronii and Aeromonas hydrophila by bath immersion. GCC-3 had no effect on growth performance. For gut health, the expression of HIF-1 alpha was significantly increased in 1% GCC-3 group (P < 0.05). The expression of intestinal IL1 beta, IL-6 and TNF alpha was significantly down-regulated in 1% and 2% GCC-3 groups (P < 0.05), and the intestinal expression of TGF-beta was significantly increased (P < 0.05). GCC-3 also increased the intestinal expression of tight junction genes Tjp2a and Claudin. For liver health, GCC-3 significantly decreased liver TAG (P < 0.05). Consistently, the hepatic expression of lipid synthesis genes SREBP1, FAS and ATGL was significantly decreased in 2% GCC-3 group, while the expression of genes related to fatty acid oxidation PPAR alpha and CPT1a increased significantly in 1% and 2% GCC-3 groups, respectively. GCC-3 increased the abundance of Cetobacterium in the gut microbiota, and led to alteration in the virome composition. Finally, the 2% GCC-3 group significantly improved the resistance of tilapia against A. veronii and A. hydrophila infection. To sum up, our results show that the dietary addition of GCC-3 fermentation product can improve the gut and liver health as well as resistance against pathogenic bacteria of tilapia.