The cohort behind the study is notable for its real-world design. Rather than enrolling a narrow, highly selected patient population, the researchers recruited people with cancer at Mayo Clinic sites in Arizona, Florida and Minnesota, ultimately capturing patients from 40 states. Because stool samples were gathered before treatment started, the researchers could examine the gut microbiome in a state relatively unaltered by chemotherapy, radiation or surgery, and then connect those baseline microbial profiles to what happened to patients afterward. This prospective structure allowed the team to ask two separate questions: whether the microbiome differs systematically across cancers, and whether microbes present before therapy are associated with how patients tolerate and respond to treatment.

To establish broad differences, the investigators first compared the patients with cancer against 287 people without the disease, identifying general microbiome distinctions between the two groups. They then moved to finer-grained comparisons across cancer types. After statistically accounting for other health conditions that could confound the results, the team pinpointed 341 bacterial species whose abundance was associated with five cancer groups. This two-step approach matters technically: by filtering out signals attributable to comorbidities, the researchers aimed to isolate microbial changes more specifically tied to the malignancies themselves rather than to general ill health.

The cancer-specific signals were striking in their variety. Patients with neuroendocrine tumors showed a broad loss of common health-associated gut bacteria, a pattern suggesting an erosion of the microbial community typically maintained in a healthy digestive tract. Patients with liver cancer and cancers of the intrahepatic bile ducts had elevated levels of several bacteria, including Enterococcus faecalis. Esophageal cancer was associated with higher levels of six species, among them bacteria of the genus Streptococcus. Distinct microbial links also emerged in lymphoid leukemia and in multiple myeloma and related plasma cell cancers, extending the microbiome-cancer association beyond solid tumors into hematologic malignancies.

“We can now narrow the search to those microbial changes that are most specific to individual cancers,” said Ruben Mars, Ph.D., a microbiome researcher at Mayo Clinic in Minnesota and co-corresponding author of the study. “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.” The distinction Mars draws is an important one for the field. Associational studies can identify which microbes are present in which conditions, but establishing causation requires mechanistic work in models and, eventually, interventional trials. By cataloging the most cancer-specific signals, the study provides a prioritized map for that next phase of research.

Among the most timely findings are the age-related patterns. Colorectal and breast cancer rates are rising among younger adults: according to the American Cancer Society, colorectal cancer incidence is increasing about 3 percent per year among adults ages 20 to 49, while breast cancer incidence is rising 1.4 percent per year among women younger than 50. Against that backdrop, the Mayo Clinic team found that adults aged 50 or younger with colorectal or breast cancer carried microbiome patterns distinct from those of older patients with the same diseases. In early-onset colorectal cancer, patients had higher lactate levels and greater abundance of Veillonella parvula, a gut bacterium that feeds on lactate. The connection raises an intriguing biological question, because tumors can produce high levels of lactate, which V. parvula can use to grow. Whether that relationship contributes to early-onset disease is not yet known, but the overlap between tumor metabolism and microbial metabolism offers a concrete hypothesis to test.

Early-onset breast cancer showed an even broader microbial signature, with changes across 64 bacterial species and lower levels of primary bile acids. One of the differing species was Clostridium scindens, a bacterium known for its involvement in bile acid and steroid metabolism, a biochemical capacity that could plausibly intersect with hormone-related cancer biology. Notably, no comparable age-related microbiome differences appeared in brain cancer, the third cancer examined in the early-onset analysis, suggesting that the age-associated signals in colorectal and breast cancer are not a generic feature of being young with cancer. The researchers are careful to stress that these findings identify links that warrant further study; they do not establish that the microbial or metabolic differences cause early-onset disease.

The study also connected the pretreatment gut microbiome to patient outcomes. The researchers identified gut bacteria associated with survival in colorectal cancer, liver and intrahepatic bile duct cancer, ovarian cancer, prostate cancer and melanoma. In liver and intrahepatic bile duct cancer specifically, higher levels of Bifidobacterium longum were associated with longer survival, while Blautia A massiliensis was associated with shorter survival. These associations do not prove that the bacteria influence survival, but they demonstrate that microbial profiles measured before treatment carry information that tracks with long-term outcomes across multiple malignancies, a prerequisite for any future prognostic application.

Perhaps the most immediately actionable finding concerns chemotherapy side effects. The team examined whether the gut microbiome before treatment was associated with diarrhea during chemotherapy, a side effect that can force dose reductions or treatment interruptions. Among patients receiving 5-fluorouracil, or 5-FU, a widely used chemotherapy drug, those who later developed diarrhea had lower levels of bacterial genes capable of breaking down the drug. Much of that drug-metabolizing capacity came from Anaerostipes hadrus, a common gut bacterium. Critically, the same microbial signal was not linked to diarrhea in patients receiving carboplatin, a different chemotherapy agent, indicating the association was specific to 5-FU rather than a general marker of gastrointestinal sensitivity. That specificity is what makes the finding scientifically compelling: it suggests a mechanistic pathway in which microbes that metabolize a drug may influence its toxicity.

“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,” said Purna Kashyap, M.B.B.S., co-corresponding author, director of the Mayo Clinic Microbiome Program and Bernard and Edith Waterman Director of the Microbiomics Program within the Mayo Clinic Center for Individualized Medicine. “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.” The researchers’ next steps follow directly from that logic: they plan to investigate whether the cancer-specific microbial signals play a causal role in disease and to validate the microbiome’s potential to predict treatment side effects in larger patient groups. The work was supported in part by the National Institutes of Health and by philanthropic gifts to the Mayo Clinic Microbiome Program and Center for Individualized Medicine. If the causal and predictive questions are answered affirmatively, a routine stool sample collected before treatment could one day help clinicians anticipate toxicity, tailor therapy and, perhaps, intervene on the gut ecosystem itself.

Subject of Research: Gut microbiome signatures associated with cancer types, early-onset cancers and chemotherapy side effects

Article Title: Distinct gut microbiome patterns found in 5 cancer groups and in early-onset cancers

Article References: Distinct gut microbiome patterns found in 5 cancer groups and in early-onset cancers. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: gut microbiome, cancer, early-onset colorectal cancer, breast cancer, Veillonella parvula, 5-fluorouracil, chemotherapy diarrhea, Bifidobacterium longum, bile acids, neuroendocrine tumors, liver cancer, Mayo Clinic

The research, led by Jiangchuan Zhu, Yiwen Qiu, and colleagues at Longhua Hospital Shanghai University of Traditional Chinese Medicine, together with collaborators in Nanjing and Hefei, took an unusually comprehensive approach. Rather than relying on a single experimental technique, the team integrated three complementary methodologies: metagenomic sequencing of patient stool samples to identify differential bacterial taxa, direct bacteria-cell co-culture experiments to test functional effects on pancreatic cancer cell lines, and two-sample Mendelian randomization analysis using large-scale genome-wide association study data to probe potential genetic associations between Veillonella abundance and pancreatic cancer risk. This triangulation of observational, functional, and genetic evidence is what distinguishes the work from many earlier microbiome-cancer association studies that stopped at correlation.

The investigation began with metagenomic sequencing, a technique that sequences all genetic material in a sample rather than targeting specific known organisms, allowing researchers to profile the entire microbial community without cultivation bias. Comparing the intestinal microbiota of patients with advanced pancreatic cancer against healthy adult volunteers, the team found that the abundance of Veillonella, a genus of Gram-negative anaerobic cocci that is a normal inhabitant of the oral cavity and gut, was significantly increased in the cancer patients. Non-metric multidimensional scaling and other community-level analyses confirmed that the overall microbial landscape differed measurably between the two groups, with Veillonella standing out as one of the most notable differential taxa.

To determine whether this enrichment was merely a byproduct of the disease environment or something with biological consequence, the researchers selected representative bacterial taxa for direct co-culture experiments with two well-characterized pancreatic cancer cell lines, PANC-1 and BxPC-3. Using a multiplicity-of-infection-controlled co-incubation protocol, they exposed the cancer cells to live Veillonella parvula and then measured a battery of malignant phenotypes. Cell viability was assessed with the CCK-8 colorimetric assay, long-term proliferative capacity with colony formation assays, and motility with both wound healing scratch assays and Transwell migration chambers. The results were consistent across these readouts: Veillonella parvula significantly enhanced both the proliferation and the migration of pancreatic cancer cells in vitro, suggesting the bacterium is not a passive passenger but a potential active participant in tumor progression.

Seeking the molecular underpinnings of these phenotypic changes, the team performed transcriptomic sequencing on the co-cultured cancer cells and compared their gene expression profiles with those of untreated controls. The analysis identified a substantial set of differentially expressed genes, and pathway enrichment analysis using Gene Ontology and Kyoto Encyclopedia of Genes and Genomes frameworks revealed significant enrichment of genes belonging to the PI3K/AKT signaling pathway. This pathway, which transmits signals from growth factor receptors to promote cell survival, proliferation, and motility, is one of the most frequently dysregulated cascades in pancreatic ductal adenocarcinoma. Its transcriptional activation following bacterial exposure points to a plausible mechanistic route by which Veillonella could push cancer cells toward more aggressive behavior, although the authors are careful to note that this enrichment indicates a regulatory link that warrants further functional dissection rather than a fully proven causal mechanism.

Perhaps the most novel component of the study is its use of Mendelian randomization, a statistical technique that exploits naturally occurring genetic variation as a form of randomization to test whether an exposure is plausibly causally related to an outcome. Because genetic variants are fixed at conception, they are largely immune to the reverse causation and confounding that plague conventional observational studies. The researchers drew on summary statistics from large-scale genome-wide association studies, including data from the Dutch Microbiome Project linking genetic variants to gut microbiome composition, and from GWAS of pancreatic cancer risk. Using inverse variance weighted analysis as the primary method, supplemented by sensitivity analyses including MR-PRESSO to detect and correct for pleiotropic outliers, they found genetic evidence supporting a potential association between genetically predicted Veillonella abundance and pancreatic cancer risk, with an odds ratio of 1.39 and a 95 percent confidence interval of 1.05 to 1.83, corresponding to a P value of 0.02.

The authors are appropriately candid about the limitations of this genetic analysis. In their conclusion, they acknowledge that residual pleiotropy, the phenomenon in which genetic variants influence the outcome through pathways unrelated to the exposure, and weak instrument bias cannot be fully excluded. These caveats matter, because Mendelian randomization results in the microbiome field have sometimes failed to replicate, and an odds ratio near the boundary of statistical significance should be interpreted as suggestive rather than definitive. Nevertheless, the convergence of three independent lines of evidence, patient metagenomics, in vitro functional assays, and population-scale genetic association, makes a stronger cumulative case than any single method could provide on its own.

The findings also fit into a broader and rapidly evolving literature on the microbiome in pancreatic cancer. Previous work has documented intratumoral bacteria within pancreatic ductal adenocarcinoma specimens, shown that microbial translocation from the gut can influence the tumor immune microenvironment, and demonstrated in mouse models that antibiotic depletion of the microbiome can alter tumor growth and chemotherapy response. Lipopolysaccharide, a structural component of Gram-negative bacterial outer membranes that the study authors reference, is known to activate Toll-like receptor 4 signaling, which can intersect with inflammatory cascades and, in some contexts, potentiate oncogenic pathways. Veillonella’s metabolic profile, including its ability to consume lactate and convert it into short-chain fatty acids, offers additional plausible routes by which it could reshape the metabolic environment of the gut and, potentially, of the tumor itself.

From a clinical standpoint, the study raises the tantalizing but still distant prospect of microbiome-targeted interventions in pancreatic cancer. If Veillonella enrichment does contribute to disease progression in patients, then monitoring its abundance could conceivably serve as a biomarker, and modulating it through diet, probiotics, antibiotics, or fecal microbiota strategies could become part of a prevention or treatment toolkit. However, the authors and the field at large caution that in vitro co-culture findings do not automatically translate to the complex in vivo tumor microenvironment, and that the direction of causality in human patients remains difficult to establish with certainty. It is entirely possible that advanced pancreatic cancer alters gut ecology in ways that favor Veillonella, rather than the reverse, even in light of the genetic evidence.

What the study undeniably delivers is a rigorous, multi-pronged template for interrogating microbiome-cancer relationships, and a specific, testable hypothesis about a bacterial genus that most people carry harmlessly but which may behave differently in the context of pancreatic malignancy. The work, funded by the National Natural Science Foundation of China and conducted under ethical approval from Longhua Hospital with written informed consent from all participants, was published open access on 1 October 2026. As the authors state, their study provides, for the first time, genetic evidence supporting a potential causal link between Veillonella and pancreatic cancer, while acknowledging the inherent uncertainties of the method. The next steps will likely involve animal models, mechanistic dissection of the PI3K/AKT connection, and replication in independent patient cohorts, all of which will determine whether this common gut resident becomes a genuine therapeutic target or remains an intriguing correlation in one of medicine’s most stubborn diseases.

Subject of Research: The role of the gut bacterium Veillonella in pancreatic cancer progression

Article Title: Veillonella promotes pancreatic cancer progression as revealed by integrated functional, Mendelian randomization, and transcriptomic analyses

Article References: Zhu, J., Qiu, Y., Ren, M., Sun, X., Zhao, R., Ma, F., & Cao, N. (2026). Veillonella promotes pancreatic cancer progression as revealed by integrated functional, Mendelian randomization, and transcriptomic analyses. BMC Cancer. https://doi.org/10.1186/s12885-026-17050-7

Image Credits: AI Generated

DOI: 10.1186/s12885-026-17050-7

Keywords: pancreatic cancer, Veillonella, gut microbiota, metagenomics, Mendelian randomization, PI3K/AKT signaling, tumor microbiome, Veillonella parvula, cell proliferation, cancer migration, GWAS, BMC Cancer