Researchers have identified the enzyme, now called bilirubin reductase (BilR), that converts bilirubin to urobilinogen, giving urine its distinctive yellow colour. The reduction of bilirubin to urobilinogen is performed solely by the gut microbiota; however, hitherto the gut microbial enzyme that reduces bilirubin to urobilinogen had not been discovered.

Using a fluorescence assay on various bacteria from the major phyla of the human gut microbiota and extracts from bacteria grown in media with bilirubin, gut microbial species that were putatively capable of bilirubin reduction were identified. Putative bilirubin reduction activity was observed in nine strains, including three species not before known to undertake bilirubin reduction: Clostridium symbiosum, Clostridium sp. M62/1 and Ruminococcus gnavus. All bilirubin reducers were found to be in the class Clostridia of the phylum Firmicutes.

研究人员发现了一种酶，现在称为胆红素还原酶（BilR），它可以将胆红素转化为尿胆原，使尿液呈现出独特的黄色。胆红素还原为尿胆素原仅由肠道微生物群完成；然而，迄今为止，尚未发现将胆红素还原为尿胆素原的肠道微生物酶。

通过对人类肠道微生物群主要门的各种细菌和在含有胆红素的培养基中生长的细菌的提取物进行荧光测定，鉴定出被认为能够降低胆红素的肠道微生物种类。在九个菌株中观察到推定的胆红素降低活性，其中包括三个以前未知的降低胆红素的物种：共生梭菌、梭状芽胞杆菌。 M62/1 和瘤胃球菌。所有胆红素降低剂均属于厚壁菌门梭菌纲。