To assess the applicability of the model to probe the effect of APIs and API analogues on colonocyte and bacterial metabolism, as well as diffusion of the drug through the mucosal layer, a fluorinated tetrapeptide (p-F-Phe-Ala-His-Trp) was pipetted on top of the solution (1 mg in 10 µL media), following inoculation. Changes in pH across the 24hour period were less pronounced, with the pH across different depths of the NMR tube dropping from 7.1 to 5.6 (vs 7.1 to 5.1 without the peptide, Figure vs 3A). Decreased production of both propionate and butyrate were observed in the presence of the tetrapeptide (Figure ). The tetrapeptide was shown to have no significant (p>0.05) effect on colonocyte viability, but was shown to decrease bacterial viability (Figures ). The ability to trace the diffusion of the tetrapeptide from the top of the NMR tube (intraluminal space) through the mucosa and all the way down to the bottom of the tube (colonocyte level) provides a model for drug transit in a complex environment such as the human gut (Figure and Figure ).