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. Author manuscript; available in PMC: 2026 Feb 1.
Published in final edited form as: Nat Rev Microbiol. 2024 Oct 7;23(2):106–121. doi: 10.1038/s41579-024-01103-4

Fig. 5. Microbial tryptophan-indole metabolism influences colonic tumorigenesis.

Fig. 5.

a) Microorganisms can convert tryptophan into other indoles via different enzymatic reactions. The enzymes involved in tryptophan transformations are indicated in the coloured legend. b) Tryptophan conversion is dysregulated in tumorigenic environments. The overexpression of indoleamine 2,3-dioxygenase (IDO1) and the depletion of tryptophan by microbial metabolism contribute to increased ʟ-kynurenine levels and constitutive aryl hydrocarbon receptor (AhR) activation. Overproduction of interleukin (IL)-17 and IL-22, as well as the activation of regulatory T cells (Treg), drive immune tolerance and hyperproliferation. c) Morganella morganii produces a family of genotoxins, termed indolimines. The functional imine group forms through a proposed condensation of primary amines and the aldehyde of indole-3-aldehyde (IAld). DNA damage induced by indolimines has been shown to exacerbate colonic tumour burden. IAM, indole-3-acetamide; I3A, indole-3-acetaldehyde; I3P, indole-3-pyruvate; ILA, indole-3-lactic acid; I3AA, indole-3-acetic acid; IA, indoleacrylic acid; IPA, 3-Indolepropionic acid.