3.4 | Assessment of biological activity of the model mixed bacterial community
To test if the model gut microbial community grown in the in vitro gut model exhibited selected biological activities associated with gut microbiota in vivo, the ability of the culture to metabolize bile acids was assayed. In the first experiment, six bioreactor modules operating in the batch culture mode with pH control were inoculated with the model microbiota. Taurocholic acid (TCA), a common conjugated bile acid, was added to the culture medium at 0, 10, or 100 mM concentrations. The growth of the model microbiota treated with TCA was substantially less compared to the untreated model microbiota. Nevertheless, bacterial growth was observed even at the highest TCA concentration used in the experiment (Figure 5A). At 10 mM TCA treatment, the proportion ofStreptobacteria spp. in the community was reduced while that ofLactobacillus spp. was increased (Figure 5B), which is likely due to its ability to metabolize TCA (Gilliland & Speck, 1977). HPLC analysis of the bile acid species in the culture medium showed the appearance of cholic acid, a product of TCA deconjugation by bacteria, after 24 h of culture (Figure 5C). In the second experiment, the model microbiota was treated with chenodeoxycholic acid (CDCA), an unconjugated bile acid. CDCA was more potent than TCA in inhibiting the growth of the model microbiota (Figure 5D). As shown in Figure 5E, ursodeoxycholic acid, the 7β-hydroxyl group epimer of the 7α-hydroxyl group-containing CDCA produced by bacterial metabolism, was present in the culture after 24 h. These results demonstrated that the model microbiota grown in the in vitro digestive tract model could exhibit biological activities normally associated with healthy gut microbiota in vivo.