Overview
Indole is a microbial metabolite produced from tryptophan by bacterial tryptophanase (TnaA). It is one of the most abundant microbial signals in the gut (concentrations reach 250–1,100 µM in the colon) and the primary endogenous ligand for the aryl hydrocarbon receptor (AhR) — a transcription factor that regulates barrier integrity, immune tolerance, and xenobiotic metabolism. With 120 source mentions, indole derivatives are among the most discussed microbial metabolites in this wiki.
Key Indole Derivatives
| Metabolite | Producer | Receptor | Function |
|---|---|---|---|
| Indole | E. coli, Bacteroides, Clostridium (TnaA+) | AhR | Barrier integrity, IL-22 induction |
| Indole-3-aldehyde (3-IAld) | Lactobacillus | AhR | Tolerogenic immunity in MS zelante 2024 microbial ahr ligand 3iald tolerogenic ms |
| Indole-3-propionic acid (IPA) | Clostridium sporogenes | PXR, AhR | Neuroprotection, barrier strengthening |
| Indole-3-acetic acid (IAA) | Multiple genera | AhR | Anti-inflammatory |
| Indoxyl sulfate | Host liver sulfation of indole | Toxic at high levels | Uremic toxin in CKD |
Dual Role
- Beneficial (at physiological levels): AhR activation → IL-22 production → antimicrobial peptide secretion → barrier integrity → colonization resistance.
- Harmful (at excess levels or after hepatic modification): Indoxyl sulfate accumulates in CKD → cardiovascular and neurological toxicity.
Disease-Specific
- ASD: Faecalibacterium hominis-derived indole activates AhR signaling, rescuing social behavior in BTBR mice yu 2025 faecalibacterium hominis indole ahr asd btbr.
- MS: Microbial 3-IAld promotes tolerogenic dendritic cell responses zelante 2024 microbial ahr ligand 3iald tolerogenic ms.
- Chlamydia: BV-associated bacteria provide indole to C. trachomatis, rescuing IFN-γ-induced tryptophan starvation — the microbiome determines pathogen survival chen 2021 chlamydia vaginal microbiota tubal infertility.
Cross-References
- tryptophan — precursor amino acid
- ahr — primary receptor for indole derivatives
- kynurenine pathway — competing tryptophan catabolism route
- faecalibacterium prausnitzii — F. hominis indole production
- chlamydia trachomatis — indole rescues tryptophan starvation
- metabolites — broader microbial metabolite context