The gut microbiome contributes to the overall metabolism of numerous compounds that circulate in and out of the gastrointestinal tract, including endogenous, dietary, and pharmaceutical molecules. Herein, three projects are presented aimed at uncovering bacterial species and genes responsible for the chemistry carried out by the gut microbiome. In the first, a previously unreported route for the production of dopamine by gut bacteria is posed. Dopamine, which is abundant in the gut, is converted to 3-methoxytyramine (3MT) by the human enzyme catechol O-methyltransferase (COMT). Communities of fecal bacteria are demonstrated to O-demethylate 3MT to produce dopamine. Two prominent acetogens found in the GI tract, Eubacterium limosum and Blautia producta, are shown to carry out this metabolism. This metabolism is shown to likely be carried out by cobalamin-dependent methyltransferases, as metabolism is reversibly inhibited by propyl iodide. In the second, I identify additional guaiacol metabolites that E. limosum and B. producta can O-demethylate to their respective guaiacols: vanillylmandelic acid, 3-O-methyldopa, and 2-methoxyestradiol. In the third, I determined that the gut bacterial protein called enterodiol lactonizing enzyme (Edl) is sufficient for converting enterodiol (END)—produced by gut bacterial metabolism of diet-derived molecules—into the phytoestrogen enterolactone (ENL). ENL has been reported to have anti-breast cancer effects in rodent models and humans. Thus, an absence of the bacterial genes required to produce ENL in an individual’s gut microbiome may serve as a biomarker for an increased risk for breast cancer.