Dietary supplementation with emodin affects growth and gut health by modulating the gut microbiota of common carp (Cyprinus carpio)

The effects of dietary emodin supplementation on the growth, gut health, and gut microbiota of common carp (Cyprinus carpio) were evaluated in this study. The basal diet (CONTROL group) was supplemented with 5 g/kg (EMODIN 5 group), 10 g/kg (EMODIN 10 group) and 20 g/kg (EMODIN 20 group) emodin for 6 weeks. Compared with the CONTROL alone group, the EMODIN 10 and EMODIN 20 groups of common carp had a significantly increased weight gain rate (WG%), specific growth rate (SGR%), and condition factor ratio (CF%) and decreased feed conversion ratio (FCR). The activities of intestinal digestive enzymes, including α-amylase (AMS), protease (PRT) and lipase (LPS), were also significantly increased in the emodin group (EMODIN 10 and EMODIN 20). Emodin also enhanced intestinal barrier function by increasing the length of intestinal villi, the number of goblet cells, and expression of the occludin gene. The dominant gut probiotics Cetobacterium and Actinobacteriotaᵤnclassified were significantly enriched in the emodin group, while Aeromonas abundance was depleted, indicating that the intestinal barrier health of the emodin-treated carp improved. We also observed elevated alanine aminotransferase (ALT), aspartate aminotransferase (AST), catalase (CAT) and superoxide dismutase (T-SOD) activities, and malondialdehyde (MDA) levels were reduced in the serum of the treatment group, which means that the fish's immunity was improved. There was functional enrichment of gut microbiota immune system pathways in the emodin group, which highlights the importance of gut immunity. Network pharmacology analysis showed that the NF-κB signaling pathway was significantly enriched in the intestine-immunity axis. Accordingly, RTPCR showed that emodin affected the intestinal expression of NF-κB signaling pathway-related immune factors in artificially infected fish, including decreased NF-κB, IL-6 and TNF-α expression and increased TLR22 expression. Correlation analysis showed that downregulated NF-κB expression upon emodin addition (EMODIN 20) was related to increased Cetobacterium abundance. Therefore, dietary emodin supplementation may enhance intestinal immune defense by regulating the intestinal microbiota, which promotes common carp growth.