Effects Of Gut Microbiota On Bile Acid Profile And Bile Acid Metabolism In Piglets

Bile acid is the main functional component of bile.Primary bile acid is synthesized from the cholesterol in hepatocytes,stored in the gallbladder,and released into the intestine after being eaten by the body,which is further metabolized by intestinal microbiota to produce secondary bile acid.In addition to promoting the absorption of fatsoluble nutrients in food,bile acids also regulate many metabolic processes,such as the balance of glucose,lipid and energy metabolism.In recent years,bile acid has been widely used as a new feed additive in animal breeding,especially in piglet diets,and has obtained relatively good economic benefits.The transformation of bile acids requires the participation of intestinal microorganisms to metabolize primary bile acids into secondary bile acids.Taking gut microbiota and bile acids as the entry point,this study explored the synthesis and metabolic characteristics of endogenous bile acids in piglets,the influence of gut microbiota on the bile acid spectrum of piglets and the regulation of gut microbiota on bile acid metabolism.This study can provide scientific basis for the study on the metabolic mechanism of bile acid and its molecular regulation.Sterile cesarean section was performed on pregnant Bama donor sows without specific pathogens under good feeding and management conditions to obtain germfree(GF)newborn piglets.Eleven GF newborn piglets were randomly selected and divided into aseptic pig group(GF group,n=5)and fecal microbiota transplant group(FMT group,n=6).The GF group remained sterile during the experiment,while GF piglets in the FMT group received FMT delivery by oral(1ml fecal suspension per time)at 7 days of age and FMT for 3 continuous days.Through the 42 days experiment,each piglet was kept in an aseptic isolator,undergoing unified feeding and management.Sterile diets were the same for different individuals,and piglets had ad libitum access to drinking sterile distilled water.At 42 days old,liver,bile,serum and intestinal contents were collected for the quantification of the bile acid profile,and liver and intestinal tissues were collected for the quantification of bile acid metabolism-related genes.The results show that:(1)Compared with GF piglets,total bile acid levels in cecum contents,colon contents and feces of FMT piglets were significantly increased(P<0.01),the contents of jejunum also had the same increasing trend;The proportion of secondary bile acids in jejunum,cecum,colon contents and feces in FMT piglets was significantly increased(P<0.05);The level and proportion of taurine conjugated bile acid in the liver and bile of FMT piglets were significantly reduced(liver:P<0.05;bile:P<0.01),the proportion of glycine-conjugated bile acids(liver: 84.72%;bile: 70.42%)in total bile acids increased significantly(liver: P<0.05;bile: P<0.01),the level of glycine-conjugated bile acids in bile of FMT piglets were significantly increased(P<0.01);The proportion of taurine-conjugated bile acid(16.78%)in serum of FMT piglets was significantly reduced(P<0.05),the proportion of free bile(61.87%)increased significantly(P<0.01);The proportion of taurine-conjugated bile acid in jejunum contents,cecum contents,colon contents and fecal of FMT piglets were significantly reduced(P<0.01);The proportion of free bile acid in cecum contents,colon contents and fecal of FMT piglets were significantly increased(P<0.05),the level of free bile acid in colon contents and fecal of FMT piglets were significantly increased(P<0.05).The proportion of glycine-conjugated bile acid(60.83%)in the jejunum contents of FMT piglets also showed an increasing trend(P>0.05).(2)Compared with GF piglets,the mRNA level of CYP7A1,the main enzyme in the classical pathway of bile acid synthesis in FMT piglets,was significantly reduced(P<0.05),the protein expression levels of CYP7B1 and CYP27A1,the major enzymes in the alternative pathway,were significantly reduced(P<0.01).Compared with GF piglets,BAT gene in liver of FMT piglets was significantly down-regulated.Compared with GF piglets,MRP2 mRNA expression level of bile acid efflux protein in FMT piglets was significantly lower than that in GF piglets(P<0.05),there was the same trend in protein expression level,but no significant difference(P<0.05).The protein expression levels of OST and BSEP in the liver efflux of FMT piglets were significantly reduced(P<0.05).Compared with GF piglets,the expression level of ASBT mRNA in ileum of FMT piglets was significantly down-regulated(P<0.01),MRP3 mRNA expression levels in i leum,cecum and colon of FMT piglets were significantly up-regulated(P<0.05);FMT and GF piglets showed greater intestinal differences in FXR/TGR5 signaling pathway than liver,and FMT piglets significantly down-regulated FMR mRNA expression compared with GF piglets(P<0.01),protein expression level also showed a downward trend(P>0.05),mRNA expression levels of FGFR4 and KL? mRNA in FMT liver were down-regulated compared with GF piglets(P<0.01),FGFR4 also significantly decreased at the protein level(P<0.05).In summary,the translocation and colonization of intestinal microorganisms increased the content and excretion of intestinal bile acids in piglets,increased the proportion of secondary bile acids,and changed the bile acid spectrum of piglets.Moreover,the presence of intestinal microorganisms inhibited the synthesis of bile acids in piglets' liver,promoted the accumulation of bile acids in the liver and affected the hepatointestinal circulation of bile acids.