The four colon simulation models presented here have structural and functional differences (Table 1), but the solutions used to reproduce the critical conditions that influence the microbiology of the colon are similar in all four models. Firstly the colonic microbiota is simulated in each model using fecal samples from a single donor or several donors in a pooled sample, because more realistic samples of gastrointestinal tract bacteria from the ileum or cecum of humans are very difficult to obtain both ethically and technically. Secondly all the colon simulators use similar growth media that originate from media originally published by Gibson et al. in 1988 (9) mimicking the ileal fluids obtained from sudden-death victims. Thirdly all the colon models have strictly anaerobic conditions, similar pH set-points representing the in vivo situation in the colon of healthy humans (19) and all the functions of these systems are computer-controlled.
The Reading model and the SHIME system are both run until a steady state in microbial growth is reached, while TIM 2 and the EnteroMix® model are run for a pre-determined time (2 or 5 days). The SHIME system is the only one of the above-mentioned four systems having a continuous line from stomach to distal colon, thus enabling the simulation of the whole GI-tract in one run. The simulated ileal fluid coming from TIM 1 can also be used indirectly as growth medium in TIM 2. The EnteroMix® model has the smallest working volumes (Table 1) in the vessels, enabling the simulation of small concentrations of the tested substrate. On the other hand the small volumes do not allow any samplings during the simulation run, which is possible in all the other models, because the volume of microbiota would be too heavily affected in the vessels. The EnteroMix® model is also the only model having parallel channels in the same simulator allowing four parallel simulations to be run at the same time with the same fecal inoculum.
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