Fig. 7

Graphical abstract. a The oral carnitine challenge test (OCCT) was used to classify human TMAO producer phenotypes and predict TMA/TMAO production and carnitine bioavailability from carnitine supplementation. The key gut microorganism responsible for conversion of carnitine to TMA need to be elucidated. b Distinct microbiome features, including Emergencia timonensis, were discovered between low- and high-TMAO producers; a machine learning model was created to predict the TMAO-producer phenotypes and was validated by using an external CVD patient cohort. A novel Ihubacter massiliensis was identified as a potential key bacterium for human TMA/TMAO production by using a humanized gnotobiotic mice model. E. timonensis and I. massiliensis together can explain 43% of high-TMAO producing status with 97% specificity