Microbial conversion of bilirubin to urobilins in vitro and in vivo.

No urobilins are formed from bilirubin in germ-free rats. To isolate and investigate the strains of intestinal microorganisms responsible for this transformation, a suitable test medium was adopted. The strength of the medium and a rather high initial pH were found to be of importance. In this medium, suspensions of rat faeces and a single strain, Cl. ramosum (G62), converted bilirubin to urobilins. Cultivations of Cl. ramosum (G62) together with E. coli significantly enhanced the conversion, whereas addition of 4 other bacterial strains was without the influence. The highest in vitro formation of the urobilins was about 10% of the bilirubin present. When the 6 strains investigated in vitro were established in EXG rats, the in vivo conversion of bilirubin to urobilins was found to be about 15%, compared to 70% in CONV rats.