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The intestinal tract is home to trillions of microbial cells known as gut microbiota,which can perform many beneficial functions to the host,such as absorbing and metabolizing nutrients.In mammals,gut microbiota participated in the lipid metabolism,including synthesizing secondary bile acid to favor dietary lipid absorption,increasing energy homeostasis and body lipid storage,modulating dietary choline bioavailability.It has been proved that intestinal bacteria in zebrafish can regulate the intestinal absorption and metabolism of fatty acids,and different bacteria show different performance in the lipid droplets formation.However,the exact role of intestinal microbiota in lipid metabolism in fish remains unclear.Germ-free mice have been widely used in the research of lipid metabolism in mammals,while application of germ-free zebrafish(GF zebrafish)in the lipid metabolism research has just begun.Our lab constructed GF zebrafish by washing fertilized eggs with antibiotics.The hatched juvenile fish were reared in gnotobiotic zebrafish medium and the sterility was checked every day by bacterial culturing tests.GF zerbrafish can survive one to two weeks,but it can live longer if they were inoculated with commensal bacteria.The role of commensals in lipid absorption was studied.GF zebrafish and conventional zebrafish were fed with egg yolk and two hours later,zebrafish were collected to detect the quantity and size of lipid droplet in the intestinal epithelial by Transmission Electron Microscope(TEM).The results indicated that lipid droplets in the intestinal epithelial of GF zebrafish was smaller and fewer than those in conventional zebrafish.RNAseq was applied to identify the key genes involved in lipid metabolism.In short,we successfully established GF zebrafish and verified that the lipid accumulation is largely dependent on the existence of intestinal microbiota,thus our model could be a powerful platform for the future study of fish lipid metabolism.