Researchers are raising the alarm over a common gut bacteria that may be behind the mystery surge in colon cancer cases.
The microbe, Bacteroides fragilis, lives in virtually everyone's gut and is generally considered to be harmless.
However, researchers in Denmark discovered the bacterium has a previously unknown virus embedded within it, and this virus is significantly more common in cancer patients.
In a study analyzing the gut bacteria of more than 800 people, half of whom had colon cancer, researchers found that, among cancer patients, the bacteria was twice as likely to contain a viral infection.
They believe this infection may alter the way the bacteria behave and trigger a change that could raise the risk of colon cancer.
Dr Fleming Damgaard, a molecular biologist who led the research, and colleagues, wrote: 'These findings suggest a partnership between bacteria and their viruses that may shape disease.'
'If confirmed, this... may support earlier detection of colorectal cancer and guide new ways to treat and prevent this disease.'
Colon cancer is the third most common cancer globally, behind 1.9 million cases, and the second biggest cause of cancer-linked death, linked to 900,000 fatalities.
In the US, there are about 160,000 colon cancer cases and 55,000 deaths every year.
But officials and scientists have raised the alarm over the cancer in recent years after it began to surge among young people. Latest studies show cases have risen about three percent per year among people under 50 since 2018.
Scientists say that all cellular life, including bacteria, can be infected with a virus. Bacteria specifically are infected by a type of virus called a bacteriophage, which does not infect human cells.
In some cases, these viruses are also able to embed themselves in a bacteria and become a prophage, or hitchhiker within a bacteria.
This can change the bacteria's behavior and potentially cause disease. For example, cholera is caused when a common gut bacteria is infected by a bacteriophage that causes it to release toxins, while diphtheria is triggered by the same mechanism.
In their study, Danish researchers analyzed bacteria samples from 48 patients with bloodstream infections, half of whom had colon cancer.
They found that, in colon cancer patients, B. fragilis was infected with two previously unknown prophages. These were largely absent in patients who did not have cancer.
Further analysis showed the prophages did not have any genes that were known to trigger behavior that could raise cancer risk. The scientists said, however, that little is known about prophages and more research is needed.
In the second part of their study, the team then screened fecal samples from 877 people from Europe, the US and Asia, half of whom had colon cancer.
In similar results, they found that patients with colon cancer were twice as likely to have detectable levels of prophages in B. fragilis compared to those who did not have the disease.
The scientists warned that their study had only detected an association and did not prove that the viruses were causing the cancer.
They said much more research was needed, including into the biological mechanism that may be triggered by the infections that could raise the risk of cancer.
Limitations of the study also included that some 'healthy' individuals had not been confirmed to be cancer-free. The paper was also released as a preprint, meaning it had not been reviewed by other experts.
The researchers' paper is, however, the latest to suggest that the rise in early-onset colon cancer cases may be linked to shifts in the gut microbiome.
Many studies have also blamed rising levels of obesity, poor diets and environmental factors for the shift.
Conditions such as inflammatory bowel disease can also cause inflammation and people with this condition are up to three times more likely to develop colon cancer than the general population.
Since 2020, early-onset colon cancer cases have made up about 12 percent of all colon cancer diagnoses in the US, alarming officials.