Researchers from the University of Southern Denmark have identified a new virus in a common gut bacterium. The virus is found significantly more often in patients with colorectal cancer.
Colorectal cancer is one of the most common types of cancer in the Western world and a leading cause of cancer-related deaths. Age, diet and lifestyle are known risk factors. However, in most cases we still lack a precise understanding of what triggers the disease. In recent years, researchers have increasingly turned their attention to the ecosystem of the gut – the vast community of bacteria, viruses and other microorganisms that live there.
Now, researchers from the University of Southern Denmark and Odense University Hospital have discovered a previously undescribed virus in a common gut bacterium. The virus appears more frequently in patients with colorectal cancer - pitured above is colorectal adenocarcinoma.
For several years, one particular bacterium has been associated with colorectal cancer: Bacteroides fragilis. However, this bacterium is also present in the vast majority of healthy individuals. The researchers therefore decided to examine whether differences might exist within the bacterium itself.
The researchers identified patients who had experienced a serious bloodstream infection caused by B. fragilis. A smaller group of these patients were diagnosed with colorectal cancer within a few weeks. The team analysed the bacterium’s genetic material in patients with and without cancer and identified a distinctive pattern: bacteria from cancer patients were more often infected with specific viruses. The initial finding was based on a relatively small number of bacterial samples from Danish patients. However, it revealed a pattern that the researchers were later able to examine in larger international datasets.
To determine whether the pattern also applied outside Denmark, the researchers analysed stool samples from 877 individuals with and without colorectal cancer from Europe, the United States and Asia. They found that patients with colorectal cancer were approximately twice as likely to have traces of these viruses in their gut. The results point to a robust statistical association across several countries. However, they do not establish whether the virus causes the disease.
Up to 80% of the risk of developing colorectal cancer is linked to environmental factors. The microorganisms in the gut are believed to be among the most important. The gut contains thousands of bacterial species and even more genetic variations. This complexity has made it difficult to identify precisely what distinguishes healthy individuals from those who develop disease. If the virus alters the properties of the bacterium, it could potentially change the gut environment. This is one of the questions the researchers are now exploring.
Currently, colorectal cancer is detected partly through stool tests that screen for hidden blood. The researchers suggest that it may one day be possible to test stool samples for the viruses they have identified. In preliminary analyses, selected viral sequences were able to identify around 40% of cancer cases, while most healthy individuals did not carry them. The researchers emphasise that the findings are still at an early and experimental stage. Further studies are needed before the method could have implications for clinical practice.
- Damgaard F, Jespersen MG, Møller JK, et al. Distinct prophage infections in colorectal cancer-associated Bacteroides fragilis. Commun Med (Lond). Published online February 7, 2026. doi:10.1038/s43856-026-01403-1