Only 5 of 206 Gut Bacteria Strains Produce the Anti-Inflammatory Molecule IAA

A molecule derived from dietary tryptophan, indole-3-acetic acid (IAA), has attracted growing attention for its reported ability to dampen gut inflammation and protect intestinal tissue. Yet a new study published in Gut Microbes by DeFeo and colleagues suggests that the capacity to produce IAA in meaningful quantities is far rarer among gut bacteria than previously assumed, and that the organisms capable of it rely on an unusual anaerobic enzyme system to do so. The findings are preliminary but point toward a specific metabolic niche that could eventually be targeted therapeutically.

IAA Production Is Confined to Five Strains Out of 206 Screened

The research team screened 206 human gut bacterial isolates under carbohydrate-limited conditions supplemented with 1 mM tryptophan, measuring IAA output by LC-MS after 48 hours. Only five strains exceeded the study's quantitation limit of 21 micromolar. All five belonged to the Bacillota phylum, and the highest producers were two strains of Blautia hydrogenotrophica, which cleared more than 1 millimolar IAA, and Intestinibacter bartlettii DSM 16795, which reached approximately 340 micromolar. Two Clostridioides difficile isolates also made the cut, though at lower concentrations in the 30 to 80 micromolar range. The numbers underline just how selective this capacity is: robust IAA output appears to be a metabolic specialty confined to a handful of gut acetogens rather than a broadly distributed trait. Gut microbiota-derived indole-3-acetic acid has long been linked to intestinal dysbiosis when levels drop, which makes identifying the specific producers a meaningful step forward.

Why It Matters for Inflammatory Disease and Gut Homeostasis

IAA is not just a metabolic curiosity. Research has documented IAA's multifaceted protective role in human diseases, including its ability to modulate oxidative stress, reduce intestinal inflammation, and influence lipid metabolism in conditions like nonalcoholic fatty liver disease. Separate work has shown that IAA inhibits colorectal tumorigenesis by inducing IL-35 expression in immunosuppressive cells and suppressing colitis-related cancer development. Beyond the colon, IAA and mucin sulfation in gut homeostasis are now mechanistically linked through the AHR-Papss2-Slc35b3 pathway, suggesting the molecule helps maintain the protective mucus layer of the intestine. Conditions associated with low IAA, including inflammatory bowel disease, multiple sclerosis, and metabolic syndrome, motivate understanding exactly which organisms are responsible for production and how their activity is regulated.

How the Team Traced the Biochemistry in Producer Strains

The credibility of the finding rests on a combination of genomic analysis, heterologous enzyme assays, and isotope tracing. After identifying the five producer strains, the researchers searched their genomes for expanded repertoires of 2-oxoacid:ferredoxin oxidoreductases, known as OFORs. These enzymes, which function under strict anaerobic conditions, were found to span activities across pyruvate, branched-chain, and aromatic 2-oxoacids. The key proposed step is the OFOR-catalyzed conversion of indolepyruvate, itself derived from tryptophan, to indoleacetyl-CoA, a putative intermediate en route to IAA. The team used position-specific carbon-13 tracing to confirm that CO2 released during amino acid oxidation is reassimilated into acetate through the Wood-Ljungdahl pathway, a hallmark of acetogens that allows them to maintain redox balance without a fermenting partner strain. Gnotobiotic mice mono-colonized with Intestinibacter bartlettii confirmed that these metabolic processes occur in a living host, not just in culture. Mass spectrometry-based metabolomics for gut metabolites enabled high-confidence identification of the full array of small molecules produced, revealing that amino acid oxidation in these acetogens generates abundant branched-chain and aromatic organic acids alongside IAA.

Open Questions About the Indoleacetyl-CoA Intermediate and Pathway Completeness

The study is careful to flag that the exact anaerobic pathway converting indolepyruvate all the way to secreted IAA has not been fully resolved. The authors describe indoleacetyl-CoA as a "putative intermediate," meaning the enzymatic steps downstream of the OFOR reaction still require direct biochemical validation. It is also worth noting that limitations in indole research more broadly include gaps in understanding how multiple indole compounds interact in combination and how well findings from gnotobiotic mouse models translate to the complex human gut environment. The five-strain screen, while methodical, sampled a finite library; other high-producing organisms may exist among the many bacterial taxa not yet cultured or included in isolate collections.

What These Findings Could Mean for Microbiome-Based Therapies

Pinpointing the organisms and enzyme families behind IAA synthesis opens a more targeted path for intervention. If Blautia hydrogenotrophica and Intestinibacter bartlettii are confirmed as the primary IAA contributors in the human gut, researchers could design probiotic or dietary strategies aimed specifically at supporting their growth or activity. The OFOR enzyme family itself is a potential target: understanding its regulation by carbohydrate availability, for instance, could explain why IAA levels fluctuate with diet. The authors note that understanding tryptophan metabolism in the gut microbiota could inform future therapeutics, and given the therapeutic potential of IAA across lung disorders, cancer adjunct therapy, and neuroinflammation, the clinical case for that work is compelling. For now, the picture is early: the pathway needs biochemical confirmation, the human relevance of gnotobiotic mouse data requires careful extrapolation, and the clinical significance of strain-level differences in IAA output remains to be established. But the identification of a specific enzyme family, a narrow set of producer organisms, and a coherent metabolic logic gives future research a concrete foundation to build on.

Evidence rating

INVESTIGATIONAL — 78/100

Caveats:

• This is a mechanistic laboratory study combining in vitro screening with gnotobiotic mouse experiments — it does not test effects in humans.
• The proposed pathway intermediate (indoleacetyl-CoA) is described as putative and requires further validation.
• Anti-inflammatory effects of IAA on human health are referenced from prior literature but are not tested in this study.
• Funding was not disclosed by the authors.

Mechanistic study combining LC-MS screening of 206 human gut bacterial isolates, gnotobiotic mouse mono-colonization, heterologous enzyme assays, and position-specific 13C isotope tracing. Identifies specific producers and proposes a step-by-step enzymatic pathway, but does not demonstrate effects in humans and relies partly on a putative intermediate. Not yet independently replicated.

References

1. Mary E. DeFeo, Yuanyuan Liu, Zhiwei Zhou, Steven K. Higginbottom, Dylan Dodd (2026). Indole-3-acetic acid production is rare among gut bacteria and reflects OFOR-driven amino acid oxidation in acetogens. Gut Microbes. https://doi.org/10.1080/19490976.2026.2689610
2. Jun Shen, Lianjun Yang, Ke You, et al. (2022). Indole-3-Acetic Acid Alters Intestinal Microbiota and Alleviates Ankylosing Spondylitis in Mice. Frontiers in Immunology. https://doi.org/10.3389/fimmu.2022.762580
3. Nargis Shaheen, Jiaxing Miao, Boyu Xia, Yutong Zhao, Jing Zhao (2025). Multifaceted Role of Microbiota‐Derived Indole‐3‐Acetic Acid in Human Diseases and Its Potential Clinical Application. The FASEB Journal. https://doi.org/10.1096/fj.202500295r
4. Juanjuan Wang, Yang Hao, Yazheng Yang, Yuan Zhang, Chen Xu, Rongcun Yang (2025). Gut microbiota derived indole-3-acetic acid ameliorates precancerous inflammatory intestinal milieu to inhibit tumorigenesis through IL-35. Journal for ImmunoTherapy of Cancer. https://doi.org/10.1136/jitc-2024-011155
5. Mengfan Li, Hailong Cao (2025). IDDF2025-ABS-0368 Gut microbiota metabolite indole-3-acetic acid maintains intestinal epithelial homeostasis through mucin sulfation. Basic Hepatology. https://doi.org/10.1136/gutjnl-2025-iddf.39
6. Xuewei Ye, Haiyi Li, Komal Anjum, et al. (2022). Dual Role of Indoles Derived From Intestinal Microbiota on Human Health. Frontiers in Immunology. https://doi.org/10.3389/fimmu.2022.903526
7. Nargis Shaheen, Jiaxing Miao, Boyu Xia, Yutong Zhao, Jing Zhao (2025). Multifaceted Role of Microbiota‐Derived Indole‐3‐Acetic Acid in Human Diseases and Its Potential Clinical Application. The FASEB Journal. https://doi.org/10.1096/fj.202500295r