BILE-ACID METABOLISM BY COLONIC BACTERIA IN CONTINUOUS-CULTURE - EFFECTS OF STARCH AND PH

Secondary bile acids (BA) have been shown to be involved as a promotin g agent in the adenoma-carcinoma sequence of colorectal cancer. In pre vious studies, fermentation of starch has been shown to inhibit the de gradation of primary to secondary BA by the colonic microflora. This s tudy was designed to investigate BA metabolism in continuous cultures of mixed fecal bacteria to get further insights into the mechanisms of this inhibition. Fermentation Vessels were fed with media containing cholic (0.6 g/l) and chenodeoxycholic acid (0.3 g/l). Cultures were ei ther starch-free or enriched with starch (10 g/1). pH was controlled a nd adjusted to 7.0 or 6.0. Total culture duration was 28 days and conc entrations of BA, short-chain fatty acids (SCFA), and starch were meas ured periodically. At pH 6, significantly more primary BA remained in the media and less secondary BA were produced. Total BA concentrations were lower at pH 7. SCFA concentrations were higher in the vessels su pplemented with starch. Starch was completely fermented and not presen t in significant amounts in any fermentation vial after the first week . These data indicate that bacterial breakdown of primary to secondary BA is inhibited when starch is simultaneously fermented. This effect can be explained by the reduction of pH resulting from SCFA production . Considering these findings, resistant starch which escapes assimilat ion in the small bowel may be a protective factor against colorectal c ancer.