Gut Bacteria and Cancer in Under-50s: Mayo Clinic Study

Mayo Clinic researchers analyzed stool samples from 1,364 cancer patients and found distinct gut bacteria patterns in younger adults with two types of tumor, colon cancer and breast cancer. The study was published on September 29, 2026 in the journal Cell (DOI 10.1016/j.cell.2026.09.009).

It concerns the gut microbiome, the trillions of microbes that live in the digestive tract. The study covered patients with different types of cancer, at different stages of disease and on different therapies. The aim was to see whether the microbiome tracks particular cancers and the side effects of treatment.

How was the study carried out?

The data come from a cohort called the Mayo Clinic Cancer Microbiome. Patients were recruited at the clinic’s sites in Arizona, Florida and Minnesota, and they come from 40 US states. Stool samples were taken before treatment began and linked to detailed clinical data on side effects and outcomes.

The researchers compared the cancer patients with 287 people without cancer to spot general differences in the microbiome, then compared the tumor types with one another in search of more specific signals. After accounting for other health conditions, they identified 341 bacterial species linked to five cancer groups.

What did they find in people 50 or younger?

The analysis of age-related patterns covered three tumors: colon, breast and brain. Patients with colon cancer who were 50 or younger had higher levels of lactate and more of the bacterium Veillonella parvula, which feeds on lactate. Tumors can produce large amounts of lactate, so the authors raise the question of a biological link, but they note that it is not known whether it plays a role in early-onset disease.

In early breast cancer, differences turned up in 64 bacterial species, along with lower levels of primary bile acids. Among those species is Clostridium scindens, a bacterium involved in the metabolism of bile acids and steroids. No comparable age-related differences were found in brain cancer.

The findings arrive at a time when cancer rates are rising among younger people. According to the American Cancer Society, the incidence of colon cancer among people aged 20 to 49 is rising by about 3 percent a year, and breast cancer among women under 50 by about 1.4 percent a year.

Five cancer groups and their bacteria

Of the five tumor groups, each had a different signature:

• neuroendocrine tumors: a broad loss of the common bacteria associated with gut health
• cancer of the liver and intrahepatic bile ducts: higher levels of several bacteria, among them Enterococcus faecalis
• esophageal cancer: higher levels of six species, including streptococci
• lymphoid leukemia, multiple myeloma and related plasma cell tumors: distinct links to bacteria

Bacteria tied to survival were found in cancers of the colon, liver and bile ducts, ovary and prostate, and in melanoma. In cancer of the liver and intrahepatic bile ducts, Bifidobacterium longum was linked to longer survival and Blautia A massiliensis to shorter survival.

Ruben Mars, a microbiome researcher at Mayo Clinic in Minnesota and a corresponding author, described what the result makes possible:

We can now narrow the search to those microbial changes that are most specific to individual cancers. Those are the signals we need to understand first if we want to determine whether the microbiome plays a causal role in cancer and ultimately develop targeted interventions.

In other words, the search narrows to the changes most specific to individual tumors, and only those need to be understood before the question of causation can be addressed.

Can the microbiome explain diarrhea after chemotherapy?

The team also examined whether the makeup of the microbiome before treatment says anything about diarrhea during chemotherapy, a side effect that makes treatment harder to tolerate. Among patients on 5-fluorouracil, a chemotherapy drug used for many types of cancer, those who later developed diarrhea had fewer bacterial genes able to break the drug down. Much of that function came from the bacterium Anaerostipes hadrus. The same signal was not linked to diarrhea in patients on carboplatin, another chemotherapy drug, so the finding applies only to 5-fluorouracil.

Purna Kashyap, director of the Mayo Clinic Microbiome Program and a corresponding author, added:

The microbiome is not the sole driver of cancer or treatment outcomes, but it is an underappreciated component that has not traditionally been considered in therapeutic approaches. This gives us a proof of concept that we can begin to understand why some patients experience a particular side effect and identify a target that could potentially be acted upon.

Kashyap does not present the microbiome as the only driver of disease or of treatment outcomes. He describes it as a neglected factor whose study may explain why some patients get a particular side effect.

What does this study not prove?

The findings for colon and breast cancer do not show that these microbial or metabolic differences cause disease in younger people, only that there are links worth investigating further. The samples were taken before treatment, so they cannot show whether the bacteria are a cause, a consequence or a companion of the tumor. The comparison group of 287 people without cancer is far smaller than the patient group, and in brain cancer the age-related differences are absent.

The next step, according to the authors, is to test whether the signals specific to individual tumors play a causal role and whether the prediction of treatment side effects holds up in larger groups of patients. The research was partly funded by the US National Institutes of Health, the Valhalla Foundation and the Maxine and Jack Zarrow Family Foundation.