Effects of dietary chitosan oligosaccharide on juvenile grass carp: Insight from the intestinal microbiota and metabolism.

Nowadays, chitosan oligosaccharide (COS) served as a feed additive has been gradually applied to aquaculture. In aims to reveal the effects of COS on the growth intestinal microbiota and metabolism of Grass carp, this experiment using the feed with 3% COS supplement was conducted. The results indicated that the growth indicators (WGR and SGR) of Grass carp were promoted significantly (p < 0.05). Moreover, the non-specific immunity in the liver of carp increased owing to the rising LZM activity by the effect of COS. Meanwhile, the intestinal microbiota diversity indices declined and the dominance rose on account of COS. In addition, the intestinal health of Grass carp was improved greatly, reflecting in the 5.48 times higher level of '(Fusobacteriota+Firmicutes+Bacteroidota)/Proteobacteria' compared to the control. Furthermore, the genus of Cetobacterium, Plesiomonas and Brevinema were enriched in the COS treatment, and the intestinal microbiota profile was improved through increasing the abundance of beneficial bacteria (Cetobacterium). Besides, a series of lysophosphatidylcholine (LPC) (LPC 16:2, LPC 14:0, LPC 19:2, LPC 19:1, LPC O-16:2), lysophosphatidylethanolamine (LPE) (LPE 16:2 and LPE O-16:2) and a butanoic acid derivative named 3-[4-methyl-1-(2-methylpropanoyl)-3-oxocyclohexyl]butanoic acid were up-regulated, and two long chain fatty acyl-CoA (LPG) (LPG 20:2 and LPG 18:0) and LPS 20:3 belonging to lipopolysaccharide (LPS) down-regulated in the COS treatment. Therefore the result implied that the intestinal health of Grass carp would be protected via reducing the LPS induced the intestinal oxidative stress and inflammatory injury in fish. The metabolic pathways enriched in the profile between the COS treatment and control were mainly concentrated on lipid and derivative metabolism, amino acid metabolism, carbohydrate metabolism and ones associated to the signal transduction. Based on the correlation analysis, butanoic acid was identified as the metabolic marker related to Cetobacterium and Plesiomonas, while LPC O-16:2 was determined as the biomarker associated to Brevinema. Therefore, the application of COS gives promising results and will be a green feed additive in aquaculture.