A widespread hydrogenase supports fermentative growth of gut bacteria in healthy people.

Disruption of hydrogen (H2) cycling in the gut is linked to gastrointestinal disorders, infections and cancers. However, the mechanisms and microorganisms controlling H2 production in the gut remain unresolved. Here we show that gut H2 production is primarily driven by the microbial group B [FeFe]-hydrogenase. Metagenomics and metatranscriptomics of stool and tissue biopsy samples show that hydrogenase-encoding genes are widely present and transcribed in gut bacteria. Assessment of 19 taxonomically diverse gut isolates revealed that the group B [FeFe]-hydrogenases produce large amounts of H2 gas and support fermentative growth of Bacteroidetes and Firmicutes. Further biochemical and spectroscopic characterization of purified enzymes show that they are catalytically active, bind a di-iron active site and reoxidize ferredoxin derived from the pyruvate:ferredoxin oxidoreductase reaction. Group B hydrogenase-encoding genes are significantly depleted in favour of other fermentative hydrogenases in patients with Crohn's disease. Finally, metabolically flexible respiratory bacteria may be the dominant hydrogenotrophs in the gut, rather than acetogens, methanogens and sulfate reducers. These results uncover the enzymes and microorganisms controlling H2 cycling in the healthy human gut.