Background/AimsHuman gut microbiota harbors numerous metabolic properties essential for the host's health. Increased intestinal transit time affects a part of the population and is notably observed with human aging, which also corresponds to modifications of the gut microbiota. Thus we tested the metabolic and compositional changes of a human gut microbiota induced by an increased transit time simulated in vitro. MethodsThe in vitro system, Environmental Control System for Intestinal Microbiota, was used to simulate the environmental conditions of 3 different anatomical parts of the human colon in a continuous process. The retention times of the chemostat conditions were established to correspond to a typical transit time of 48 hours next increased to 96 hours. The bacterial communities, short chain fatty acids and metabolite fingerprints were determined. ResultsIncrease of transit time resulted in a decrease of biomass and of diversity in the more distal compartments. Short chain fatty acid analyses and metabolite fingerprinting revealed increased activity corresponding to carbohydrate fermentation in the proximal compartments while protein fermentations were increased in the lower parts. ConclusionsThis study provides the evidence that the increase of transit time, independently of other factors, affects the composition and metabolism of the gut microbiota. The transit time is one of the factors that explain some of the modifications seen in the gut microbiota of the elderly, as well as patients with slow transit time.

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Ethical and technical difficulties for in vivo studies on gut microbiotas argue for the development of alternative in vitro models: here, we describe a system simulating the proximal part of a human colon both nutritionally and physico-chemically with a procedure aimed to limit experimental variations over the time (Proximal Environmental Control System For Intestinal Microbiota--P-ECSIM). The continuous culture system P-ECSIM is first inoculated by a -20 °C glycerol stock established from the batch culture of a stool-inoculated medium. The anaerobic atmosphere is self-maintained by the gases produced in the ordinary metabolism of fermentations. The monitoring of metabolic activities and microbial constitutions indicates that different steady states are obtained according to the dilution rate. Finally, the glycerol conservation of the batch culture-derived inoculum gives a similar differential response between the two dilution rates (D = 0.08 h⁻¹ and D = 0.04 h⁻¹) after a 1-year storage time as well for their metabolism and constitution in steady states, but with a lower abundance. Molecular fingerprints of the microbiota reveal however alterations over the time. Further efforts are needed concerning the preservation of standardized inoculums in order to improve the process for intra- and inter-lab comparison. Combined with appropriate analytical techniques, this system provides an efficient alternative means of studying functionally human microbiota in its constitution, metabolism and adaptation to environmental changes, particularly nutritional.