New Virus Inside Gut Bacteria May Explain Link Between Common Microbe and Colorectal Cancer Risk

A newly discovered virus hiding inside a common gut bacterium may help explain why a microbe found in both healthy individuals and cancer patients is linked to colorectal cancer. Researchers from the University of Southern Denmark and Odense University Hospital identified a previously unknown bacteriophage — a virus that infects bacteria — within strains of Bacteroides fragilis that are more frequently present in people with colorectal cancer. This finding suggests that the interaction between gut bacteria and the viruses they carry could play a key role in disease risk and may one day lead to earlier screening methods.

The study, published in Communications Medicine, analyzed gut bacteria from cancer patients in a large Danish population study. Researchers found that in individuals with colorectal cancer, B. Fragilis often carried a specific bacteriophage not previously described. While this bacterium is commonly found in the gut of healthy people, its association with cancer has long puzzled scientists. The discovery of this hidden virus offers a potential explanation for why the same microbe appears in both healthy and diseased states.

It has been a paradox that we repeatedly find the same bacterium in connection with colorectal cancer, while at the same time We see a completely normal part of the gut in healthy people.

Flemming Damgaard, microbiologist at Odense University Hospital and the University of Southern Denmark

The research team used genetic sequencing to catalog bacteria and viruses within bacteria from patient samples. They initially detected the virus in a smaller group and later confirmed the findings in a larger cohort of 877 people, both with and without colorectal cancer. In those with the disease, B. Fragilis was significantly more likely to harbor the newly identified bacteriophage.

Bacteriophages are viruses that infect bacterial cells, using them to replicate and spread. Although they are natural components of the gut microbiome, their role in human disease is not fully understood. This study suggests that certain phages may influence bacterial behavior in ways that contribute to cancer development, possibly by altering how the bacteria interact with the intestinal lining or immune system.

Colorectal cancer is among the most common cancers in Western countries and a leading cause of cancer-related deaths. While factors such as age, diet, and lifestyle are known to influence risk, the exact biological triggers remain unclear. In recent years, scientists have increasingly focused on the gut microbiome — the complex community of bacteria, viruses, and other microorganisms in the digestive system — as a potential key to understanding disease onset.

The researchers emphasize that their findings do not establish causation but reveal a strong association worth further investigation. Future studies will need to determine whether the virus directly contributes to cancer development or serves as a marker of an altered gut environment that increases risk. If confirmed, detecting this virus could become part of early screening strategies aimed at identifying individuals at higher risk before tumors form.

According to the research team, the virus represents a previously unidentified type of bacteriophage, highlighting how much remains unknown about the viral components of the gut microbiome. By examining not just bacteria but also the viruses they carry, scientists may uncover new layers of influence on health and disease.

The study was conducted by researchers from institutions in Denmark and Australia and builds on prior work linking Bacteroides fragilis to colorectal cancer. While the bacterium alone does not explain cancer risk, its combination with this specific virus may provide a more precise biomarker for disease susceptibility. The authors suggest that targeting the bacterium-phage relationship could open new avenues for prevention, diagnosis, or treatment.

As scientists continue to explore the gut microbiome’s role in health, discoveries like this underscore the importance of looking beyond individual microorganisms to understand how their interactions — including those with viruses — shape disease outcomes. For now, the findings offer a promising lead in solving a long-standing mystery about how a common gut bacterium can be tied to cancer.