Validation of a three-stage compound continuous culture system for investigating the effect of retention time on the ecology and metabolism of bacteria in the human colon

A three-stage compound continuous culture system was used to study the effect of retention time (27.1 and 66.7 h) on the catabolism of organic carbon and nitrogen sources in mixed populations of human colonic bacteria. The fermentation system was designed to reproduce spatial, temporal, nutritional, and physicochemical characteristics of the microbiota in the proximal (vessel 1) and distal (vessels 2 and 3) colons, and was validated on the basis of chemical and microbiological measurements on intestinal contents obtained from human sudden death victims. Results showed that the majority of carbohydrate breakdown and short-chain fatty acid production occurred in V1. Conversely, dissimilatory amino acid metabolism, as evidenced by formation of branched-chain fatty acids and phenolic compounds, occurred primarily in V2 and V3. Fermentation of aromatic amino acids was strongly affected by system retention lime (R), with concentrations of phenolic metabolites being three times higher in V3, at 66.7 h, compared to 27.1 h. Bacteriological measurements of intestinal contents, in which nine groups of marker organisms were studied, showed that, with the exception of bifidobacteria, no major differences in relative bacterial cell numbers were evident in the proximal and distal colons. These organisms were also studied in the continuous culture system, where marked reductions in Escherichia coli were observed in V2 and V3, especially at R = 27.1 h. Increasing R to 66.7 h reduced numbers of Clostridium perfringens, anaerobic Gram-positive cocci, and total anaerobe counts. Correlations between in vivo chemical and bacteriological measurements and data obtained in vitro demonstrate that the three-stage fermentation system provided a useful model for studying the physiology and ecology of large intestinal microorganisms under different nutritional and environmental conditions.