Gut Bacterium Alistipes Shahii Linked to Anxiety and Depression in IBS

If you have ever noticed that a flare of stomach pain seems to arrive with a wave of anxiety, a new study offers a concrete biological explanation. Researchers traced the anxiety and depression common in irritable bowel syndrome (IBS) back to a single missing gut bacterium, a chemical it normally produces, and a specific patch of brain tissue that reads that chemical's signal. IBS affects an estimated 7% to 21% of people worldwide, and the mood symptoms that often ride along with it may be traceable, at least in part, to what is missing from the gut rather than being a separate problem.

Key takeaways

• Loss of the gut bacterium Alistipes shahii lowers levels of indole, a bacterial metabolite, and this shortfall tracks with anxiety and depression in both IBS patients and mouse models.
• In mice, restoring A. shahii or boosting indole directly reversed anxiety- and depression-like behavior by changing activity in a mood-related hippocampal circuit.
• The human data comes from a small, correlational cohort; every causal experiment, including tests of the drug diosmin, was done in mice, not people.

Alistipes Shahii Loss Tracks With Anxiety in IBS

Researchers led by Wang and colleagues, writing in Cell Metabolism, found that people and mice with IBS carry far less of the gut bacterium Alistipes shahii than healthy controls, and that this loss tracks directly with anxiety and depression symptoms. The team ran metagenomic sequencing on stool from human IBS patients and 16S rRNA gene sequencing on IBS mice, and both methods turned up the same signal: a significant drop in A. shahii in IBS groups compared with healthy controls. The less A. shahii a person or mouse had, the worse their anxiety and depressive symptoms tended to be, an inverse relationship the researchers describe as significant.

Why IBS Comes Bundled With Anxiety and Depression

IBS presents with abdominal pain, abnormal bowel movements, and altered gut microbiota composition, and it is frequently accompanied by anxiety and depression, a pattern clinicians have long observed but struggled to explain mechanistically. This fits into the neurobiology linking IBS to anxiety and depression more broadly, and connects to the broader gut-brain axis research in IBS that has looked at microbiota composition and mood. In the human arm of this new study, IBS patients scored higher for depression and anxiety symptoms than healthy controls. The same pattern showed up in mouse models of IBS built using a high-fat diet combined with ice-water gavage, and in germ-free mice colonized with fecal microbiota transplanted from IBS patients. That the anxiety and depression-like behaviors transferred along with the transplanted microbiota, not just with the physical gut symptoms, is what pointed the researchers toward the microbiome itself as a driver.

How Researchers Traced Bacteria to a Brain Circuit

To measure the behaviors directly, the team used three standard rodent assays: the open field test, which gauges anxiety by how much time an animal spends in the exposed center of an arena, and the tail suspension and forced swim tests, which gauge depression-like behavior by how quickly an animal stops struggling and goes immobile. IBS mice spent less time in the center during the open field test and stayed immobile longer in the tail suspension and forced swim tests than healthy controls. The researchers then depleted the gut bacteria of IBS mice with antibiotics and recolonized some of them with A. shahii alone. Mice that got the bacterium back showed less anxiety and depression-like behavior than antibiotic-treated mice that did not, direct evidence that restoring this one microbe changes behavior rather than merely correlating with it.

What This Discovery Means If Your Mood Tracks Your Gut

For someone whose anxiety or low mood seems to track their gut symptoms, this study offers a mechanistic explanation rather than a treatment. A missing bacterium lowers a specific molecule, indole (a compound gut bacteria make from the amino acid tryptophan), and that shortfall changes how a mood-regulating region of the hippocampus fires. Untargeted metabolomics showed indole was reduced in the feces, serum, and colon tissue of IBS model mice, and in the feces and blood of IBS patients, with lower indole tracking worse emotional symptoms. The enzyme responsible for making indole, tryptophanase (TnAse), and the gene that encodes it, tnaA, were both reduced in human IBS patients and IBS mice, reflecting the loss of TnAse-producing bacteria like A. shahii. Indole acts through AhR (aryl hydrocarbon receptor, a protein that turns genes on or off in response to certain chemical signals), which is expressed in a hippocampal region called the ventral dentate gyrus. That fits AhR's broader role as a host-microbiota signaling receptor documented elsewhere in the gut-brain literature. Nothing here has been tested as a treatment in people, so this is not a case for a specific probiotic or supplement, but it does suggest gut and mood symptoms in IBS may share one biological root rather than being two unrelated problems.

The Limits of a Small, Mostly Animal Study

The human side of this research is a small cohort, and the human findings are correlational: an association between low A. shahii, low indole, and mood symptoms, not proof that fixing one fixes the other in people. Every causal experiment, the antibiotic depletion, the bacterial recolonization, and the direct manipulation of indole and of hippocampal brain activity, was done in mice, not in human trials. The researchers also tested diosmin, a drug already approved for other uses that activates AhR directly, and found it improved affective behavior in IBS mice without needing to restore the missing bacteria. But diosmin has not been tested for mood effects in human trials, and its safety profile for this kind of use in people still needs further examination before it can be considered anything close to an available treatment.

What Comes Next for Indole-Based Research

The researchers flag several open questions for follow-up work: whether other tryptophan-derived compounds, such as indole-3-carboxaldehyde and indole-3-propionic acid, act on AhR signaling on their own or alongside indole; whether the indole-AhR pathway affects brain regions and behaviors beyond the hippocampus and the anxiety and depression measured here, and whether it also shapes the physical, gut-level symptoms of IBS; and, most importantly, whether this same A. shahii-indole-AhR pathway identified in mice operates the same way in the human brain. That last point matters because other tryptophan-metabolizing gut bacteria shown to act on brain tissue via AhR suggest this signaling route is not unique to A. shahii, and confirming it in humans is the step that would move this from an animal mechanism to something with direct relevance to IBS patients.

Common Questions

What is Alistipes shahii? It is a bacterium normally found in the human gut that produces indole from dietary tryptophan. The study found people and mice with IBS have significantly less of it than healthy controls.

Does this mean probiotics could treat IBS-related anxiety? Not yet. The bacterial recolonization and indole-boosting experiments that reversed anxiety and depression-like behavior were done in mice and have not been tested as a treatment in human trials.

What is indole and why does it matter for mood? Indole is a chemical gut bacteria make from tryptophan, an amino acid from food. The study found it acts on a receptor called AhR in a mood-related part of the hippocampus, and low indole levels tracked with worse anxiety and depression symptoms in both mice and IBS patients.

Is diosmin a proven anxiety or depression treatment? No. Diosmin is an already-approved drug that activated the same brain receptor and improved mood-related behavior in IBS mice, but it has not been tested for mood effects in human trials and its safety profile for that use is still unexamined.

References

1. Minjeong Kim, Helen E. Vuong (2026). Indole-AhR signaling shapes emotional regulation. Cell Reports. https://doi.org/10.1016/j.celrep.2026.117747
2. The neurobiology of irritable bowel syndrome. Nature. https://www.nature.com/articles/s41380-023-01972-w
3. Intestinal microbiome-gut-brain axis and irritable bowel syndrome. NIH. https://pmc.ncbi.nlm.nih.gov/articles/PMC5860136/
4. Fangcong Dong, Gary H. Perdew (2020). The aryl hydrocarbon receptor as a mediator of host-microbiota interplay. Gut Microbes. https://doi.org/10.1080/19490976.2020.1859812
5. George Zhang Wei, Katherine A. Martin, Peter Yuli Xing, et al. (2021). Tryptophan-metabolizing gut microbes regulate adult neurogenesis via the aryl hydrocarbon receptor. Proceedings of the National Academy of Sciences. https://doi.org/10.1073/pnas.2021091118