A previously unknown virus linked to bowel cancer has been discovered

Danish scientists have discovered that the common gut bacterium Bacteroides fragilis may harbor a mysterious virus linked to the development of colorectal cancer. The results of the breakthrough study have been published in the journal Communications Medicine (ComMed).

According to the researchers' estimates, up to 80% of colorectal cancer cases are caused by environmental factors. A key role in this is played by the gut microbiome — a highly complex community of bacteria, fungi, and viruses inhabiting the digestive tract.

Bacteroides fragilis had long been considered an ordinary and relatively harmless inhabitant of the gut. However, scientists repeatedly observed a curious pattern: in people with oncological diseases, the numbers of these microorganisms were notably higher than normal.

Initially, the researchers hypothesized that the "cancer-associated" bacteria might belong to a separate evolutionary lineage, but this hypothesis fell through. The team then shifted its attention to genetic elements that bacteria can acquire externally — for example, through infection by viruses.

It was then that two previously unknown prophages — viruses embedded directly in the bacterium's DNA — were discovered in the genomes of bacteria taken from cancer patients. In bacteria from the intestines of healthy individuals, such viruses were virtually absent.

To verify the results, the scientists analyzed stool samples from 877 people across Europe, the United States, and Asia. Among the participants, 434 had been diagnosed with colorectal cancer, while 443 had not. It turned out that in patients with bowel cancer, the likelihood of detecting these viral elements was more than twice as high.

The authors of the study emphasize that for now, the findings concern exclusively a statistical association. The question remains open — whether the viruses contribute to the development of cancer, or whether, conversely, the altered intestinal environment during the disease creates favorable conditions for such bacteria.

Nevertheless, the discovery may acquire serious practical significance. The researchers believe that viral "signatures" in the microbiome could become a tool for early diagnosis of bowel cancer. Preliminary analysis showed that testing for fragments of these viruses' genomes can detect approximately 40% of cases of the disease.