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This process consumes bilirubin and generates multiple downstream derivatives, such as urobilinogen and stercobilinogen. Levels of bilirubin and its derivatives are associated with susceptibility to inflammatory and metabolic disorders, but the microbial species and enzymes that metabolize bilirubin have remained largely unknown. Here, demonstrate that metabolism of bilirubin to urobilinogen requires two separate reactions that can occur in either order and identify novel enzymes and pathway intermediates required for conversion. We find that bilirubin reductase (BilR), an enzyme that was recently discovered and proposed to convert bilirubin to urobilinogen, is specific for reducing the methine bridges of bilinoids, converting bilirubin to the novel intermediate divinylurobilinogen and mesobilirubin to urobilinogen. Using transcriptomic profiling, we identify the bilinoid vinyl reductase (BilV) responsible for reducing the vinyl groups of bilirubin and divinylurobilinogen. BilV is a flavin-dependent oxidoreductase of the Old Yellow Enzyme (OYE) superfamily with a broad distribution across human gut bacteria that overlaps with but does not completely mirror the distribution of BilR. These findings establish the complete pathway for bacterial conversion of bilirubin to urobilinogen, enabling defined studies to interrogate how this metabolism contributes to human health and disease.<\/jats:p>","DOI":"10.64898\/2026.06.10.731317","type":"posted-content","created":{"date-parts":[[2026,6,11]],"date-time":"2026-06-11T06:00:20Z","timestamp":1781157620000},"source":"Crossref","is-referenced-by-count":0,"title":["Identification of the complete pathway for conversion of bilirubin to urobilinogen by human gut bacteria"],"prefix":"10.64898","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-5284-3809","authenticated-orcid":false,"given":"Baylee J.","family":"Russell","sequence":"first","affiliation":[{"name":"Department of Microbiology, Harvard Medical School, Boston, MA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1507-741X","authenticated-orcid":false,"given":"Erik","family":"Hasenoehrl","sequence":"additional","affiliation":[{"name":"Division of Infectious Diseases, Department of Pediatrics, Boston Children\u2019s Hospital, Boston, MA, USA"},{"name":"Division of Gastroenterology, Hepatology and Endoscopy, Department of Medicine, Mass General Brigham, Boston, MA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3557-5838","authenticated-orcid":false,"given":"Victoria M.","family":"Marando","sequence":"additional","affiliation":[{"name":"Department of Microbiology, Harvard Medical School, Boston, MA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0442-3743","authenticated-orcid":false,"given":"Joy","family":"Lu","sequence":"additional","affiliation":[{"name":"Division of Infectious Diseases, Department of Pediatrics, Boston Children\u2019s Hospital, Boston, MA, USA"},{"name":"Division of Gastroenterology, Hepatology and Endoscopy, Department of Medicine, Mass General Brigham, Boston, MA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0006-2902-7065","authenticated-orcid":false,"given":"Jessica M.","family":"Chen","sequence":"additional","affiliation":[{"name":"Department of Microbiology, Harvard Medical School, Boston, MA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Michael J.","family":"James","sequence":"additional","affiliation":[{"name":"Department of Microbiology, Harvard Medical School, Boston, MA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Mudita","family":"Goyal","sequence":"additional","affiliation":[{"name":"Department of Microbiology, Harvard Medical School, Boston, MA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0545-914X","authenticated-orcid":false,"given":"Suzanne","family":"Walker","sequence":"additional","affiliation":[{"name":"Department of Microbiology, Harvard Medical School, Boston, MA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3233-0675","authenticated-orcid":false,"given":"Seth","family":"Rakoff-Nahoum","sequence":"additional","affiliation":[{"name":"Department of Microbiology, Harvard Medical School, Boston, MA, USA"},{"name":"Division of Infectious Diseases, Department of Pediatrics, Boston Children\u2019s Hospital, Boston, MA, USA"},{"name":"Division of Gastroenterology, Department of Pediatrics, Boston Children\u2019s Hospital, Boston, MA, USA"},{"name":"Broad Institute, Cambridge, MA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1369-4908","authenticated-orcid":false,"given":"Marco","family":"Jost","sequence":"additional","affiliation":[{"name":"Department of Microbiology, Harvard Medical School, Boston, MA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"54368","reference":[{"key":"2026061314150546000_2026.06.10.731317v1.1","doi-asserted-by":"crossref","first-page":"1246","DOI":"10.1038\/s41590-025-02227-2","article-title":"The role of gut microbial metabolites in the T cell lifecycle","volume":"26","year":"2025","journal-title":"Nat. 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