The reduction of bile pigments by faecal and intestinal bacteria

1. The action of intestinal and faecal bacteria on bile pigments was studied for elucidation of the mode of intestinal conversion of bilirubin to urobilinogen. 2. Biliverdin, free and conjugated bilirubin, mesobilirubin, mesobiliviolin, natural and "synthetic" i-urobilins, (+)-urobilin and (−)-stercobilin were incubated with the bacteria. 3. Mixed organisms from human faeces had no effect on biliverdin but reduced free and conjugated bilirubin, natural (+)- and i-urobilin and (−)-stercobilin to (−)-stercobilinogen but mesobilirubin to a mixture of i- and (+)-urobilins. They reduced mesobiliviolin to the chromogen of a non-urobilinoid pigment of unknown structure. 4. Natural G62 Clostridia reduced free and conjugated bilirubin to stercobilinogen, this action being synergised by Escherichia coli. Total sonicate of these organisms reduced free bilirubin but not conjugated bilirubin to stercobilinogen. The particulate fraction of the sonicate was ineffective; the supernatant converted free bilirubin to a non-urobilinoid chromogen but had no effect on conjugated bilirubin or mesobilirubin. 5. These results are discussed with relevance to complexity of the urobilins and the effect of vinylic unsaturation on bile pigment reduction.