Mysterious Virus Hiding Inside Common Gut Bacterium Linked to Colorectal Cancer

Experts estimate that around 80 percent of the risk of developing colorectal cancer comes from our environment, steering researchers to investigate the role microorganisms play in the disease.

While searching for microbial triggers in the human microbiome, researchers from the University of Southern Denmark and Odense University Hospital stumbled upon a virus that had so far evaded detection and that frequently appears in patients with colorectal cancer.

The virus hides inside a bacterium commonly found in our guts, whether or not we have the disease. Flemming Damgaard from the Department of Clinical Microbiology at Odense University Hospital and the University of Southern Denmark, and lead author of the study published in Nature Communications Medicine, said in a press statement that further studying these types of viruses may help “assess the risk of colorectal cancer and potentially improve prevention and treatment.”

A Virus That Hides Inside Bacteria

Studying the microbiome of colorectal cancer patients has become a growing field, aimed at understanding microbes’ role in the disease and improving early diagnosis. One bacterium, Bacteroides fragilis, has repeatedly been linked to colorectal cancer. However, it is also common in healthy people.

“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,” said Damgaard.

When analyzing bacterial samples from patients who later developed colorectal cancer, the researchers discovered a virus living inside B. fragilis. These bacteriophages are viruses that infect bacteria, using them to replicate and sometimes altering their behavior. Interestingly, this particular virus appears to belong to a previously unidentified type.

“It is not just the bacterium itself that seems interesting. It is the bacterium in interaction with the virus it carries,” explained Damgaard in the release.

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Virus Found in Guts of Cancer Patients

The discovery began with a distinctive pattern in samples from a large Danish population study. Researchers focused on patients who had experienced bloodstream infections caused by B. fragilis; some of them were later diagnosed with colorectal cancer.

“It was in our Danish material that we first detected a signal,” said Damgaard in the release. “That gave us a concrete hypothesis, which we were then able to investigate in larger datasets.”

To test whether the pattern extended beyond Denmark, the team analyzed stool samples from 877 adults in Europe, the U.S., and Asia — some with colorectal cancer and some without.

Again, patients with colorectal cancer were about twice as likely to have traces of the virus in their gut. According to the press release, this indicates a strong statistical association across several populations.

A Promising Tool for Colorectal Cancer Screening

At this early stage, it is not clear whether the virus actually contributes to the disease. “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,” added Damgaard.

Still, exploring the virus’s potential role fits into a broader effort to understand how environmental factors — including microbes — influence colorectal cancer risk. The gut microbiome’s complexity has long made it difficult to know where to look.

“Previously, it has been like looking for a needle in a haystack. Instead, we have investigated whether something inside the bacteria — namely, viruses — might help explain the difference,” said Damgaard in the release.

The researchers hope that future stool tests, which currently screen primarily for blood, might also look for bacteriophages associated with colorectal cancer. Exploratory tests that searched for specific viral traces already identified approximately 40 percent of cancer cases. However, more research is needed before such testing could become part of routine clinical practice.

This article is not offering medical advice and should be used for informational purposes only.

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Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• This article references information from a study published in Nature Communications Medicine: Distinct prophage infections in colorectal cancer-associated Bacteroides fragilis