By Elodie Vaz | Published on March 6, 2026 | 3 min readColorectal
cancer is among the most common cancers in Western countries and represents a
major cause of cancer-related mortality. While age, diet, and lifestyle are
well-established risk factors, the biological mechanisms that trigger the
disease remain insufficiently understood in most cases. Up to 80% of the risk
is thought to be linked to environmental factors, among which the intestinal
microbiome—comprising bacteria, viruses, and other microorganisms—is
increasingly considered a key element in etiological hypotheses.
A very
common… but suspicious bacterium
Among the
bacteria frequently associated with the disease, Bacteroides fragilis
holds a particular position. It is often detected in patients with colorectal
cancer but is also present in the vast majority of healthy individuals.
“It is
paradoxical to observe that the same bacterium is consistently associated with
colorectal cancer while also being an integral part of the intestinal
microbiota in healthy people,” explained Flemming Damgaard, MD, PhD in clinical
microbiology at Odense University Hospital and the University of Southern
Denmark, and the study’s lead author, in a press release.
What If
the real culprit were a hidden virus?
Researchers
from the University of Southern Denmark and Odense University Hospital
hypothesized that intra-species differences could explain this association.
Their objective was to determine whether a specific genetic
element—particularly viral—present within Bacteroides fragilis might be
correlated with colorectal cancer. The results were published in Nature
Communications Medicine on February 7.
The
research began in Denmark using a large population-based study including
approximately two million citizens. Researchers identified patients who had
experienced severe bloodstream infections caused by Bacteroides fragilis.
Among them, a subgroup was diagnosed with colorectal cancer several weeks
later.
When
viruses and bacteria work together
“We
discovered a virus that had not been described before and that appears to be
closely linked to the bacteria found in patients with colorectal cancer,”
explained Flemming Damgaard.
Initial
observations, based on a limited number of Danish samples, allowed the
researchers to formulate a hypothesis that was subsequently tested using
independent international datasets.
External
validation included 877 individuals with or without colorectal cancer from
Europe, the United States, and Asia, using fecal samples.
A future
tool to identify at-risk individuals?
Analyses
showed that patients with colorectal cancer were about twice as likely to carry
traces of this bacteriophage in their intestinal microbiota.
“It was
important for us to examine whether this association could be reproduced using
completely independent data—and it was,” said Flemming Damgaard.
This
represents a robust statistical association observed across several countries,
although it does not establish causality. “We still do not know whether the
virus contributes to the cause of the disease or whether it simply indicates
that something else has changed in the intestine,” he noted.
Researchers
emphasize that “it is not only the bacterium itself that appears to be
important. It is the bacterium interacting with the virus it carries.” The
identified bacteriophage may represent a previously undescribed viral type.
Preliminary
analyses suggest that a set of viral sequences could identify approximately 40%
of colorectal cancer cases, while most healthy individuals do not carry them.
This study
proposes a new interpretation of the link between the microbiome and colorectal
cancer. Beyond the mere presence of bacteria, the interaction between bacteria
and viruses could represent a key determinant. The complexity and diversity of
the microbiome make the identification of discriminating factors particularly
challenging.
“The number
and diversity of intestinal bacteria are enormous. Previously, identifying them
was like searching for a needle in a haystack. We therefore investigated
whether something inside the bacteria—the viruses—could explain the
difference,” explained Flemming Damgaard.
At this
stage, the clinical implications remain exploratory. “In the short term, we can
investigate whether this virus could be used to identify individuals at
increased risk,” he said. In the longer term, detecting specific bacteriophages
in stool samples could complement current screening strategies.
Ongoing
research will aim to determine whether this virus actively contributes to
carcinogenesis or simply represents a biomarker of an altered intestinal
ecosystem. The challenge is substantial: transforming a microbiological
correlation into a mechanism-based understanding—or even into a predictive
tool.
Read next: Colorectal cancer: early screening changes the game and enables earlier action
About the Author – Elodie Vaz
Health journalist, CFPJ graduate (2023).
Élodie explores the marks diseases leave on bodies and, more broadly, on human life. A registered nurse since 2010, she spent twelve years at patients’ bedsides before exchanging her stethoscope for a notebook. She now investigates the links between environment and health, convinced that the vitality of life cannot be reduced to that of humans alone.
