Scientists at the University of Southern Denmark and Odense University Hospital have identified a previously unknown virus inside the common gut bacterium Bacteroides fragilis that appears more frequently in people with colorectal cancer. The finding, detailed by lead researcher Flemming Damgaard, resolves a long-standing paradox since the bacterium is also present in healthy individuals. While the link is strong, the virus's role in causing cancer remains unproven.
Colorectal cancer ranks among the most common cancers in Western countries and a leading cause of cancer deaths. Flemming Damgaard, a medical doctor and PhD at Odense University Hospital and the University of Southern Denmark, noted the paradox: Bacteroides fragilis is found in most healthy people yet linked to the disease. 'It has been a paradox that we repeatedly find the same bacterium in connection with colorectal cancer, while at the same time it is a completely normal part of the gut in healthy people,' Damgaard said. The team discovered that in cancer patients, this bacterium often carries a specific bacteriophage, a virus that infects bacteria, which had not been described before. 'We have discovered a virus that has not previously been described and which appears to be closely linked to the bacteria we find in patients with colorectal cancer,' Damgaard explained. The research began with Danish patients experiencing bloodstream infections from Bacteroides fragilis, drawn from data on about two million people. Among those, some developed colorectal cancer shortly after. Comparing samples, the virus was more prevalent in cancer cases. To validate, the team analyzed stool from 877 people across Europe, the United States, and Asia. Cancer patients were roughly twice as likely to harbor these viruses. 'It was important for us to examine whether the association could be reproduced in completely independent data. And it could,' Damgaard said. Although the association holds statistically, causation is unclear. 'We do not yet know whether the virus is a contributing cause, or whether it is simply a sign that something else in the gut has changed,' Damgaard cautioned. Up to 80 percent of colorectal cancer risk ties to environmental factors like the gut microbiome. The discovery could pave the way for new screening tests detecting viral markers in stool, potentially identifying 40 percent of cases early. Damgaard's team pursues further studies, including artificial gut models, tumor analysis, and mouse experiments, funded by foundations like Novo Nordisk. The results appear in Communications Medicine.
