TNFRSF9 and colitis: Thirdly, ACD and IAAH, two core bacterial enzymes involved in the synthesis of IPA and IAA, and the bacteria known to encode these enzymes (e.g., Roseburia, Faecalibacterium, and Clostridium) [5], were enriched in response to administration of ILA or ILA-producing L. reuteri. Fourthly, a mutant strain of L. reuteri deficient in ArAT, a major bacterial enzyme responsible for the conversion of tryptophan to ILA [11], lost its ability to produce IPA and IAA or protect mice against DSS-induced colitis.