Abstract

Bile acids (BAs), key molecules with fat digestion and homeostatic regulation, are synthesized in the liver and circulate through our upper gastrointestinal tract following a meal via enterohepatic circulation. Conjugation with glycine or taurine is the final step in BA synthesis, performed by the host-associated bile acid-CoA:amino acid N-acyltransferase. BA conjugation was believed to be limited to the host while microbial action on BAs was limited to amino acid deconjugation and hydroxyl group modification (dehydroxylation, oxidation, and epimerization). However, it was recently discovered that gut anaerobes, particularly Enterocloster bolteae of the Lachnospiraceae, can conjugate a myriad of amino acids to BAs. These microbially conjugated bile acids (MCBAs) represent a fifth mechanism of BA transformation, one that had eluded characterization across over 50 years of research into gut anaerobes and BAs. Here, we investigated BA conjugation capabilities within the family Lachnospiraceae to identify phylogenetic relationships with this unique biochemical function. Members of 10 genera within Lachnospiraceae were cultured for 16 hours in the presence of cholate followed by methanol extraction and untargeted LC-MS/MS analysis. Data were submitted to the Global Natural Products Social Molecular Networking database for spectral identification. Here, we identify genera closely related to Enterocloster both responsible for and incapable of MCBA production. Genera not associated with the gut (i.e., Lacrimispora) did not conjugate cholate. MCBA production was not ubiquitous among gut-associated genera, as Blautia species did not have observable MCBA production. However, Enterocloster, Ruminococcus, and Lachnoclostridium species showed robust conjugation including 14 different amino acids and the amino acid precursor citrulline. MCBA producers also displayed a distinct preference for glycine as a substrate, implicating Lachnospiraceae in the production of molecular mimics of “host” conjugated bile acids. Little is known about the effects of MCBAs on the host or their microbiome, further demonstrating a need to elucidate the mechanisms behind this novel, widespread, transformation.