Virus detection in intestinal germ samples
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It is already known that our intestinal flora also contains billions of viruses. However, these viruses do not attack us, but rather specialize on bacteria as hosts. Such a bacteriophage infestation can change the behavior and physiology of an intestinal germ – and could thus make it a potential cause of colon cancer.
Damgaard and his team initially examined whether this was also the case with Bacteroides fragilis in 583 patients who had experienced a severe infection with this bacterium. Some of these patients developed colon cancer a few weeks later. The researchers analyzed bacterial samples that were taken from the test subjects during their infection. They also looked for traces of viruses in the DNA of the isolates.
Infestation with previously unknown bacteriophages
The team found what they were looking for: the patients who developed colon cancer after their infection showed genetic traces of two unknown bacteriophages, named FU and ODE. “These belong to two different, previously unknown families of the Caudoviricete virus class,” report Damgaard and his colleagues. Both bacteriophages were significantly more common in the Bacteroides fragilis samples than in the test subjects who did not have cancer.
“This gave us a concrete starting point that we could then test in larger data sets,” explains Damgaard. “It was important to us to check whether the connection could be reproduced in completely independent data.” To do this, he and his team analyzed genomic data from stool samples from 877 people with and without colon cancer from Germany, France, Austria, Japan, China and the USA.
In fact, the two unknown viruses were also found in these samples – again primarily in colon cancer patients. “Patients with colon cancer had detectable amounts of the bacteriophage FU or both bacteriophages twice as often. They also had the bacteriophage ODE 1.7 times more often than the control group,” report the researchers. “Our study is the first to identify a colon cancer-specific interaction of a bacterium with viruses.”
Correlation or causation?
However, this is still a correlation – a striking coincidence of colon cancer and virus-infected intestinal bacteria. “We don’t yet know whether the virus plays a causal role or is just a sign that something else has changed in the intestine,” says Damgaard.
However, there is some evidence of a causal connection. Laboratory experiments have already shown that such a virus infection changes the activity of more than 100 genes in Bacteriodes. “This so-called lysogenic conversion changes the physiology of the infected bacterium. In the context of colon cancer, these phenotypic changes could also change the disease-causing potential of the bacteria,” Damgaard and his team speculate.
Further experiments are underway
The scientists are conducting further laboratory tests to determine whether this is the case. “We are cultivating Bacteroides fragilis infected with the viruses in an artificial intestinal model to investigate how intestinal tissue, virus and bacteria interact with each other,” report the researchers. Experiments are also planned with mice whose intestinal flora will be specifically mixed with the bacteriophages.
If the connection is confirmed, the newly identified viruses could also help identify people at increased risk of colon cancer. In initial tests, Damgaard and his colleagues have already managed to use these virus sequences to identify around 40 percent of colon cancer cases using stool samples. (Communications Medicine, 2026; doi: 10.1038/s43856-026-01403-1)
Quelle: University of Southern Denmark, Communications Medicine
February 23, 2026 – Nadja Podbregar
