An in vitro model, designated the Simulator of the Human Intestinal Microbial Ecosystem (SHIME), was used to study the effect of a soygerm powder rich in beta-glycosidic phytoestrogenic isoflavones on the fermentation pattern of the colon microbiota and to determine to what extent the latter metabolize the conjugated phytoestrogens. Initially, an inoculum prepared from human feces was introduced into the reactor vessels and stabilized over 3 wk using a culture medium. This stabilization period was followed by a 2-wk control period during which the microbiota were monitored. The microbiota were then subjected to a 2-wk treatment period by adding 2.5 g/d soygerm powder to the culture medium. The addition resulted into an overall increase of bacterial marker populations (Enterobacteriaceae:, coliforms, Lactobacillus: sp., Staphylococcus: sp. and Clostridium: sp.), with a significant increase of the Lactobacillus: sp. population. The short-chain fatty acid (SCFA) concentration increased approximately 30% during the supplementation period; this was due mainly to a significant increase of acetic and propionic acids. Gas analysis revealed that the methane concentration increased significantly. Ammonium and sulfide concentrations were not influenced by soygerm supplementation. Use of an electronic nose apparatus indicated that odor concentrations decreased significantly during the treatment period. The beta-glycosidic bonds of the phytoestrogenic isoflavones were cleaved under the conditions prevailing in the large intestine. The increased bacterial fermentation after addition of the soygerm powder was paralleled by substantial metabolism of the free isoflavones (genistein, daidzein and glycitein), resulting in recovery of only 12-17% of the supplemented isoflavones.