Alginate oligosaccharide modulates immune response, fat metabolism, and the gut bacterial community in grass carp (Ctenopharyngodon idellus).

Alginate oligosaccharide (AOS) is widely used in agriculture because of its many excellent biological properties. However, the possible beneficial effects of AOS and their underlying mechanisms are currently not well known in grass carp (Ctenopharyngodon idellus). Here, grass carp were fed diets supplemented with 5, 10, or 20 g/kg AOS for six weeks. HE and PAS staining showed that the diets of AOS significantly increased the number of goblet cells in the intestinal. According to transcriptome and quantitative real-time PCR (qRT-PCR) data, AOS-supplemented diets activated the expression of fat metabolism-related pathways and genes. The 16S rRNA sequencing results showed that supplementation with AOS affected the distribution and abundance of the gut bacterial assembly. qRT-PCR and activity assays revealed that the AOS diets significantly increased the antioxidant resistance in gut of grass carp, and down-regulated the expression of inflammatory and up-regulated anti-inflammatory cytokines. Finally, the Aeromonas hydrophila infection assay suggested that the mortality in the groups fed dietary AOS was slightly lower than that in the control. Therefore, supplementing the diet of grass carp with an appropriate amount of AOS can improve fat metabolism and immune responses and alter the intestinal bacterial community, which may help to fight bacterial infection.