The effect of bile acids on intestinal microflora

A variety of strains of Bacteroides fragilis, Clostridium perfringens, Lactobacillus species, and Enterococcus species were tested in vitro against human whole bile, conjugated bile acid, and deconjugated bile acid solutions. The test system revealed that all four groups of intestinal bacteria were sensitive to unconjugated bile acids; however, the sensitivity varied with the strains tested. Thin-layer chromatography studies revealed that human whole bile and conjugated bile acid solutions had no inhibitory effect unless bacteria released unconjugated bile acids. The dihydroxy bile acids, deoxycholic and chenodeoxycholic, were more inhibitory, causing inhibition at 1 and 2 mm concentrations, than the trihydroxy bile acid, cholic, which did not cause inhibition until a 5 mm concentration was added. Experiments revealed that increasing concentrations of the bile acids produced increasing inhibition. When a fixed amount of bile acid was added, increasing amounts of organisms overcame the inhibitory effect. Initial experiments revealed that the effect may or may not be additive, depending on the strain of bacteria and the bile acids tested. Combinations of two or three organisms in single free bile acid solutions revealed no significant difference than when one organism was tested. The multiple organisms exhibited the same dose-response phenomenon as noted with individual organisms. The data support the theory that unconjugated bile acids are inhibitory to bacteria and may play a ce:role in controlling in vivo intestinal bacterial populations. However, the exact mechanism of their inhibition and the in vivo significance still remains to be clarified.