Nuclease treatment enhances the probiotic effect of Bacillus velezensis T23 on hepatic steatosis and inflammation induced by high-fat diet in zebrafish

Metabolic diseases caused by high-fat diet (HFD) have been a major problem currently in aquaculture. Nucleotides have been used as functional nutrients to improve the growth and health of fish，such as stimulating immune responses, such as immune cells and complement lytic activity. In this experiment, we supplemented solid fermentation product of Bacillus velezensis T23 and nuclease-treated Bacillus velezensis T23 (T23N) to HFD to evaluate their probiotic effects. The results showed that total antioxidant activity, lysozyme activity, complement component 3 and complement component 4 of zebrafish skin mucus were significantly elevated in both T23 and T23N groups. Compared with the HFD group, the T23 and T23N groups significantly reduced HFD-induced lipid metabolism disorders and inflammation, and did not affect the weight gain rate of zebrafish. Moreover, T23N enhanced the ability of T23 to alleviate lipid metabolism disorders and inflammation induced by HFD. The lipogenesis genes (PPARγ, Srebp-1c, Fas, and C/EBPα) of liver were significantly down-regulated in the T23N group compared with the T23 group (p < 0.05). Compared with the T23 group, the pro-inflammatory cytokines (IL-6, IL-1β) of liver were significantly decreased (p < 0.05), and the anti-inflammatory cytokine TGF-β was increased. T23 and T23N can reduce HFD-induced intestinal injury and intestinal inflammation by down-regulated the expression of pro-inflammatory factors (Nf-κb, TNF-α, IL-1β) (p < 0.05). 16s rDNA sequencing analysis of gut microbiota found that T23 and T23N decreased the richness and diversity of gut microbiota and significantly increased the abundance of Cetobacterium of the phylum Fusobacteriota. In conclusion, we found that T23 and nuclease-treated T23 were able to enhance nonspecific immunity, alleviate lipid metabolism disorders and inflammation, reduce intestinal injury and inflammation, and improve gut microbiota structure of zebrafish. Moreover, nuclease-treated T23 was able to enhance the ability of T23 to alleviate HFD-induced lipid metabolism disorders and inflammation.