Effect of fibre on bile acid metabolism by human faecal bacteria in batch and continuous culture.

The effect of various fibres on bile acid metabolism by human faecal bacteria was studied in batch and continuous culture models of the large intestine. The metabolism of the primary bile acids--chenodeoxycholic and cholic-was entirely oxidative in aerobic conditions, and the major metabolites were the 7-oxo-derivatives of the parent bile acids. Addition of both undigested and digested wheat bran and pea fibre to aerobic batch cultures reduced metabolism in a dose-dependent manner with undigested fibre being far more effective. In anaerobic conditions, the metabolism of primary bile acids was predominantly reductive: the major metabolites were the 7 alpha-dehydroxylated products of the parent bile acids--namely lithocholic acid (from chenodeoxycholic acid) and deoxycholic acid (from cholic acid), respectively. Addition of undigested crude fibre to batch cultures reduced 7 alpha-dehydroxylation in a dose-dependent manner; addition of digested crude fibre to anaerobic continuous cultures had no significant effect on bile acid metabolism. The soluble fibre lactulose needed to be added to reduce pH sufficiently (> or = 5.5), to prevent 7 alpha-dehydroxylation of primary bile acids. Reduction of 7 alpha-dehydroxylation was also associated with a substantial decrease in Bacteroides spp and a concomitant increase in Lactobacillus spp in the cultures. The failure of digested wheat bran to reduce bile acid metabolism in continuous culture is attributed mainly to the physical constraints of the systems which can handle no more than 4-6% particulate material due to blockage. It is concluded that anaerobic continuous culture systems with human faecal bacteria have some value as models of the human large intestinal environment, but further modifications are required before the effect of particulate fibres on the metabolism of bile acids can be fully addressed.