Colorectal Cancer and Gut Microbiome

Researchers from Odense University Hospital and the University of Southern Denmark report in Communications Medicine that a previously undocumented virus living inside the common gut bacterium Bacteroides fragilis is found about twice as often in people with colorectal cancer as in those without the disease. The virus is a bacteriophage—infecting bacteria rather than human cells—and was detected by sequencing subtle genetic differences in B. fragilis strains, helping explain why a bacterium present in many healthy people seems more problematic in some cancer patients. In an analysis of 877 stool samples from Europe, the U.S. and Asia, colorectal cancer patients were roughly two times more likely to carry traces of this phage, although the authors stress that the study shows association only and does not prove the virus causes tumors. The team is now running lab and animal experiments to see whether the phage changes bacterial behavior in ways that might promote cancer, work that could eventually inform more precise screening or microbiome‑based risk markers. The findings add to a growing body of evidence that components of the gut microbiome — including viruses as well as bacteria — may influence cancer development, an area drawing increasing attention from oncologists and U.S. research funders as colorectal cancer rates rise in younger adults.