Figure 1. Interactions Among Host, Microbiota, and Metabolites.

In this simplified model, the gut microbiota metabolize substrate inputs from the host including diet and xenobiotics into metabolites that can enter the host’s bloodstream and affect the host peripherally. For example, therapeutic drugs can be inactivated, reducing their efficacy. Alternatively, drugs may converted to derivatives with non-target and possibly toxic effects. Changes in these input substrates, therefore, change the reservoir of available microbial substrates and alter the metabolomic profile of the gut, yielding variable effects on the host. The new host phenotype can, in turn, have a feedback effect on the microbial community.