Microbially-catalyzed conjugation of GABA and tyramine to bile acids.

Bile acids (BAs) are modified in multiple ways by host enzymes and the microbiota to produce a chemically diverse set of molecules that assist in the digestive process and impact many physiological functions. This study reports the discovery of bacterial species that conjugate the neuroactive molecules, GABA and tyramine, to primary and secondary BAs. We further present evidence that BA-GABA and BA-tyramine conjugates are present in the human gut, and document a shifting BA-GABA profile in a human pouchitis patient before, during and after inflammation and antibiotic treatment. GABA and tyramine are potent neuroactive molecules and common metabolic products of the gut microbiota. GABA- and tyramine-conjugated BAs may influence receptor-mediated regulatory mechanisms within the gastrointestinal tract and absorption of these molecules and their entry into the enterohepatic circulation may impact host physiology at distal tissue sites. This discovery defines new conjugated bile acids in the human gut.