Figure 1: Tryptophan degradation pathways. The image shows how different microbes are involved in several pathways of tryptophan degradation leading to the production of immunoreactive metabolites acting locally and systemically as agonists of PXR and AhR. IPA: indole-3-pyruvic acid; IAld: indole-3-aldheyde; ITE: 2-(1H-indol-3-ylcarbonyl)-4-thiazol carboxylic acid methyl ester; L-Kyn: L-Kynurenine; PXR: pregnane X receptor; AhR: aryl hydrocarbon receptor.