[Action of microbial flora of the digestive tract on the metabolism of bile acids in the rat (author's transl)].

An isotopic balance is established in rats receiving a regular feed intake of [4-14-C]cholesterol so that various chemical species of bile acids have the same specific activity. This property is used to study bile acids distribution in the rat liver, digestive tract and fecal excretion. Bile acids are separated by thin-layer chromatography, radioactivity is determined by liquid scintillation, and the mass by 3-hydroxysteroid dehydrogenase action. The resulting comparative study made between the germ-free rat (axenic rat) and the rat exposed to microbes ("holoxenic" or conventional rat) receiving a semi-synthetetic feed, shows the influence excercised on the metabolism by the microbial flora of the digestive tract. This study confirms that the axenic rat compared to its holoxenic homologue has a higher bile acids pool and a lower fecal excretion. At all levels of the digestive tract (small intestine and the whole caecum and large intestine), probably as well as in the liver, the total amount of bile acids which is observed in the axenic rat is about twice the amount observed in the holoxenic rat, but fecal excretion is decreased by 20%. Values obtained by this method are higher than those previously observed by other authors using gas-liquid chromatography or [14-C]cholic acid isotopic dilution. This study also confirms that cholic and beta-muricholic acids are the main bile acids in the axenic rat and in addition establishes that in this animal bile acids composition is complex and varies from the small intestine to the feces. Besides cholic, alpha- and beta-muricholic, chenodeoxycholic and ursodeoxycholic acids, unidentified chemical species constitute 21% of the whole in the feces. Comparing the compositions observed in axenic and holoxenic rats in this experiment, it could not be determined if the relative activity of the two pathways of bile acid biosynthesis is deeply or only slightly changed by the presence of microbial flora. This is because of a large fraction of unknown composants in the feces of the axenic rat and the extreme complexity in the feces of the holoxenic rat.