Your Gut Bacteria May Signal Which Cancer You Have, Mayo Study Finds

If you or someone you love is navigating a cancer diagnosis, you have probably already wondered whether the gut had anything to do with it, or whether it might affect how treatment goes. A new real-world study out of Mayo Clinic, published in the journal Cell, gives a large-scale look across several cancer types at once, and it finds that gut bacteria patterns are not one-size-fits-all: they differ by cancer type, they differ in early-onset disease, and in at least one case they line up with who develops a painful chemotherapy side effect. The full text of the paper was not obtained for this article, so what follows is drawn from its published abstract and a Mayo Clinic press release describing the work, not the full manuscript.

Key takeaways

• Researchers compared the gut bacteria of 1,364 cancer patients to 287 people without cancer and found 341 bacterial species tied to five distinct cancer types.
• Early-onset colorectal and breast cancers each showed their own bacterial signature, distinct from the general cancer-associated pattern.
• One gut microbe, Anaerostipes hadrus, was linked to protection from a common chemotherapy side effect – but this is an association from one cohort, not a validated test.
• The findings are associational, not causal, and come from a single health system; the full study text was not obtained for this article, which is based on the published abstract and a Mayo Clinic press release.

341 Gut Bacteria Species Linked to Five Cancer Types

Researchers at Mayo Clinic identified 341 bacterial species in the gut that were statistically associated with five cancer classes: neuroendocrine tumors, liver and intrahepatic bile duct cancers, esophageal cancer, lymphoid leukemia, and multiple myeloma (a blood cancer of plasma cells). To get there, the team built a statistical framework designed to filter out microbiome differences that were really just tracking other things, like a patient's other health conditions, age, or demographics, rather than the cancer itself. The species that survived that filtering are what the researchers describe as the most plausible cancer-linked signatures in the group. The gut microbiome's relationship to cancer, from tumor immunology to treatment response, is an active research focus across the federal cancer research portfolio, and this cohort adds a multi-cancer comparison to that broader effort.

Not every cancer type looked the same. Neuroendocrine tumors (rare tumors that start in hormone-producing cells) showed a broad loss of the bacteria usually associated with a healthy gut, while esophageal cancer patients instead showed elevated levels of six specific species, including Streptococcus.

Early-Onset Colorectal and Breast Cancers Show Their Own Signatures

Younger patients researching a new cancer diagnosis have particular reason to pay attention here, since the study specifically dug into early-onset disease, cancers diagnosed at unusually young ages, a category oncologists have been watching rise for years. In early-onset colorectal cancer, patients had higher levels of fecal lactate and more Veillonella parvula, a bacterium, compared with older colorectal patients. In early-onset breast cancer, the researchers saw changes across 64 bacterial species alongside lower levels of primary bile acids (compounds made from cholesterol that help digest fat) and of a bacterium called C. scindens. The study also looked for an age-related signature in early-onset brain cancer but did not find one, a negative result the researchers reported alongside the positive ones. Bile acids and colorectal cancer are already an established research thread; other peer-reviewed work has traced mechanistic links between gut bacteria, bile acids and colorectal cancer, which is part of why the bile-acid signal in early-onset breast cancer patients caught the researchers' attention here.

A Real-World Cohort of Nearly 1,650 People

The findings come from the Mayo Clinic Cancer Microbiome cohort, a real-world observational study, meaning patients were enrolled as they came through the clinic rather than assigned to a controlled experiment. Researchers collected stool samples from 1,364 cancer patients before treatment began and compared them with samples from 287 people without cancer, linking everything to detailed clinical records on side effects and outcomes. Patients were recruited from Mayo Clinic sites in Arizona, Florida, and Minnesota, drawing participants from 40 states, which gives the sample more geographic breadth than a typical single-site study, though it remains one health system's patient population.

What This Could Mean If You're Facing Treatment

For someone weighing chemotherapy, the most concrete finding here concerns drug side effects rather than diagnosis. Patients who had fewer bacterial genes capable of breaking down the chemotherapy drug 5-fluorouracil were more likely to later develop diarrhea from it, and the researchers specifically identified Anaerostipes hadrus, a gut bacterium carrying a gene for an enzyme called dihydropyrimidine dehydrogenase (DPD, which helps metabolize the drug), as associated with protection from that side effect. That fits a growing body of research on how the intestinal microbiota shapes chemotherapy-induced diarrhea more broadly, though this study is the first to point at this specific bacterium and drug pairing in a mixed-cancer cohort. Separately, Bifidobacterium longum, a bacterium already familiar from probiotic research, was associated with longer survival in patients with liver and bile duct cancers. None of this means a stool test can currently tell you your cancer risk or steer your chemotherapy regimen. As Dr. Purna Kashyap, one of the study's researchers, put it: "The microbiome is not the sole driver of cancer or treatment outcomes, but it is an underappreciated component." Anyone curious about their own gut health in the context of cancer treatment should raise it with their oncology team rather than acting on these findings alone.

What the Study Can't Tell Us Yet

This is an associational study, not a causal one: it shows which bacteria travel with which cancers and outcomes, not that any bacterium causes or prevents them. The researchers were explicit that the early-onset findings identify associations warranting further investigation, not a cause. The cohort also comes from a single health system, and the full peer-reviewed manuscript, published in the journal Cell, was not obtained for this article; this piece is based on the study's published abstract and a Mayo Clinic press release describing its findings, not on the full text.

A Foundational Resource for Future Biomarker Research

The researchers describe this cohort as a foundational resource, intended to support future work identifying cancer-specific microbiome signatures and predictive biomarkers. Dr. Ruben Mars, another researcher on the study, framed the value of separating cancer-specific signals from the noise this way: "We can now narrow the search to those microbial changes that are most specific to individual cancers." Validating any of these signatures, including the 5-fluorouracil diarrhea association, as an actual clinical tool would require follow-up studies the abstract does not describe a timeline for.

Common Questions

Can a stool test currently diagnose which cancer type someone has from their gut bacteria? No. The study found statistical associations between certain bacteria and cancer types in one research cohort; it did not produce or validate a diagnostic test.

What is Anaerostipes hadrus, and why does it matter for chemotherapy? It's a gut bacterium that the study found was associated with protection from diarrhea caused by the chemotherapy drug 5-fluorouracil, linked to bacterial genes that help break the drug down.

Does this mean gut bacteria cause cancer? No. The researchers describe the findings as associational, meaning the bacteria and the cancers were found together more often than expected, not that one causes the other.

Why did researchers focus on early-onset cancers separately? Early-onset colorectal and breast cancers showed their own distinct bacterial patterns, different from the broader cancer-associated signatures, which is why the study analyzed them as their own category.

References

1. Lu Yang, Vaidhvi Singh, Brent J. Gawey, et al. (2026). Microbiome signatures linked to cancer and treatment adverse events in a real-world cohort. Cell. https://doi.org/10.1016/j.cell.2026.09.009
2. Microbiome Research | EGRP/DCCPS/NCI/NIH. epi.grants.cancer.gov. https://epi.grants.cancer.gov/microbiome-research/
3. Toshiaki Takahashi, Ajay Goel (2026). The Gut Microbiome and Colorectal Cancer: From Association to Causation. Cancer Biome and Targeted Therapy. https://doi.org/10.66505/cbtt.v1i2.40
4. Wanrou Jiang, Yongjun Wu, Xiuyun He, et al. (2025). Important Role of Intestinal Microbiota in Chemotherapy-induced Diarrhea and Therapeutics. Journal of Cancer. https://doi.org/10.7150/jca.99421