Metabolism of mixed human colonic bacteria in a continuous culture mimicking the human cecal contents.

The purpose of this study was to develop an in vitro model of the proximal colon that would permit investigation of the function of colonic bacteria. Five continuous cultures of fecal bacteria were maintained anaerobically in steady state for at least 21 days in a medium simulating ileostomy effluent. The pH was maintained at 6 by the controlled infusion of sodium bicarbonate. Fresh medium (13 ml) was pumped into the culture for 2 min every hour and excess culture was simultaneously removed. The redox potential of all cultures remained below -300 mV, whereas the osmolality was maintained hypertonic compared with the original inoculum or medium. The cultures readily produced volatile fatty acids at a rate of 30.4 +/- 3.6 mmol/day (mean +/- SEM). Acetic acid (10-19 mmol/day) and propionic acid (5-30 mmol/day) were produced in the largest amounts, whereas butyric acid was produced only in small amounts or not at all. Ammonia was produced in each culture in amounts that varied from day to day and between cultures. The osmolality and volatile fatty acid production increased when the carbohydrate input was doubled and decreased when the cultures were deprived of carbohydrate. Thus it is possible to maintain actively fermenting viable cultures in vitro for prolonged periods; such cultures respond to changing conditions and may be useful in investigating the metabolic function of the colon.