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.