New Virus Linked to Colorectal Cancer: Gut Microbiome Study Reveals Potential Biomarker

Colorectal cancer is a leading cause of cancer-related deaths in Western countries, and while age, diet, and lifestyle are known risk factors, the precise mechanisms that trigger the disease remain largely unclear. Increasingly, researchers are focusing on the gut microbiome – the complex community of bacteria, viruses, and other microorganisms living in the digestive tract – to understand its role in the development of this cancer.

A recently identified virus found within a common gut bacterium appears more frequently in patients with colorectal cancer, according to a study conducted by researchers in Denmark and published in the journal Communications Medicine. This discovery highlights a potential link between viruses that infect bacteria in the gut and the risk of developing the disease.

The research team from the University of Southern Denmark and Odense University Hospital investigated Bacteroides fragilis, a bacterium that has been repeatedly associated with colorectal cancer. However, B. Fragilis is also commonly found in healthy individuals, raising questions about its true role in the disease and why it’s linked to cancer only in certain cases. Researchers decided to examine whether differences existed *within* the bacterium itself.

In patients who later developed colorectal cancer, Bacteroides fragilis more frequently carried a specific virus. These viruses, known as bacteriophages, infect bacteria and live inside them, sometimes altering their behavior. The virus identified in the study appears to be a previously unknown type, according to the study authors.

The study demonstrates a statistical association between the presence of the virus and colorectal cancer, but it cannot determine whether the virus directly contributes to the development of the disease or is simply a marker of existing changes in the gut. As Flemming Damgaard from the Department of Clinical Microbiology at Odense University Hospital and the University of Southern Denmark explained, “It is not just the bacterium itself that seems interesting. It is the bacterium in interaction with the virus it carries.”

The research originated from data collected in a large population study in Denmark, encompassing approximately two million people. Researchers identified patients with severe bloodstream infections caused by Bacteroides fragilis, and a portion of these patients were subsequently diagnosed with colorectal cancer.

Analysis of the bacteria’s genetic material revealed a distinct pattern: bacteria from patients with cancer were more frequently infected with specific viruses.

Initial observations were based on a relatively small number of bacterial samples from Denmark. The hypothesis was then tested using larger, international datasets to validate the findings.

To confirm these results, researchers analyzed stool samples from 877 individuals with and without colorectal cancer from Europe, the United States, and Asia. Patients with colorectal cancer were approximately twice as likely to have traces of these viruses in their gut. This association was consistently reproduced, but the study does not prove that the virus *causes* the disease.

Up to 80 percent of the risk of developing colorectal cancer is linked to environmental factors, and gut microorganisms are considered among the most important. The vast diversity of bacteria in the gut has made it challenging to identify relevant differences between healthy individuals and those who develop the disease. The authors analyzed not only the bacteria but also the viruses within them to identify potential distinctions.

Researchers are currently investigating whether the virus alters the properties of the bacterium and whether this influences the gut environment. The reason for the virus’s presence and whether it plays an active role in the development of colorectal cancer remain unknown.

Currently, colorectal cancer screening often involves stool tests that detect very small amounts of blood invisible to the naked eye.

The authors suggest that, in the future, analyzing the presence of these viruses in stool samples could be added to screening protocols. Preliminary analyses showed that certain viral sequences could identify approximately 40 percent of cancer cases, while most healthy individuals did not exhibit these sequences.

These results are still in an early experimental stage, and further studies are needed before this information can be applied in clinical practice. The discovery opens a new avenue for research into the complex interplay between the gut microbiome and colorectal cancer, potentially leading to improved risk assessment, prevention, and treatment strategies.

September 5, 2025, a study published in Gut Microbes highlighted the ongoing efforts to identify reliable early diagnostic markers for colorectal cancer through gut microbiota analysis. Research published in February 2026 in Medical Xpress reported on the newly discovered virus linked to colorectal cancer, emphasizing the statistical association but acknowledging the need for further research to determine a causal relationship. A report from Frontiers also underscores the potential of microbiota as diagnostic biomarkers for early cancer detection and personalized therapeutic approaches.