Metabolism of hydrogen gases and bile acids in the gut microbiome

The human gut microbiome refers to a highly diverse microbial ecosystem, which has a symbiotic relationship with the host. Molecular hydrogen (H 2 ) and carbon dioxide (CO 2 ) are generated by fermentative metabolism in anaerobic ecosystems. H 2 generation and oxidation coupled to CO 2 reduction to methane or acetate help maintain the structure of the gut microbiome. Bile acids are synthesized by hepatocytes from cholesterol in the liver and are important regulators of host metabolism. In this Review, we discuss how gut bacteria metabolize hydrogen gases and bile acids in the intestinal tract and the consequences on host physiology. Finally, we focus on bile acid metabolism by the Actinobacterium Eggerthella lenta . Eggerthella lenta appears to couple hydroxyl group oxidations to reductive acetogenesis under a CO 2 or N 2 atmosphere, but not under H 2 . Hence, at low H 2 levels, E. lenta is proposed to use NADH from bile acid hydroxyl group oxidations to reduce CO 2 to acetate.