Response of grass carp to conjugated linoleic acid-supplemented diets free of fish oil and fish meal: growth performance, intestinal microbiota and associated immunological responses. Response of grass carp to conjugated linoleic acid-supplemented diets free of fish oil and fish meal: growth performance, intestinal microbiota and associated immunological responses

Conjugated linoleic acid (CLA)-induced liver lipid-lowering effect on grass carp in our previous finding may be of considerable interest in aquaculture. However, the mechanisms underlying the beneficial effects of CLA in fish are unknown. Pivotal studies in mammals have demonstrated that manipulation of the intestinal microbiota and its metabolic pathways can affect host’s adiposity and metabolism. We hypothesized that the mechanism of dietary CLA on lipid metabolism could be mediated by intestinal microbiota through intestine-microbiota-related inflammatory responses. Therefore, a 65-day growth trial was conducted to verify the liver lipid-lowering effect of dietary CLA, when the fish were fed with diets free of fish oil and fish meal in grass carp, which the interactive factor-fish oil in lipid metabolism were incorporated into the diets in the previous study. In addition, the composition and the relative abundance of microbiota and the mRNA expressions of toll-like receptor (TLR)3-TLR9 were also investigated. Our results showed that a minimum value of 2.5% CLA incorporated in grass carp diets induced a reduced growth rate (P < 0.05). Compared to that fed with the control diet, a significant decrease in the lipid content in the liver was observed in fish fed with 2% to 3% CLA (P < 0.05). The bacterial richness and diversity in mucosa of the whole intestine tended to decrease in fish treated with dietary CLA. The dominant phyla in all groups are Proteobacteria, Fusobacteria and Bacteroidetes. The relative abundance of Bacteroidetes, Firmicutes and Firmicutes/Bacteroidetes ratio showed dynamic changes among groups. Dietary CLA can significantly activate the mRNA expressions of TLR4, TLR5, TLR8 and TLR9 in different intestinal segments. Our results raise the possibility that the activation of TLR3-TLR9 genes could be mediated by the intestinal microbiota provoked intestinal inflammation associated with the dietary CLA in grass carp and the lipid-lowering effects of dietary CLA could be could be mediated by intestinal microbiota through intestine-microbiota-related inflammatory responses.