Single-celled organisms called archaea aren’t generally thought to cause human disease, but one species has been implicated in colorectal cancer
By Chris Simms
3 July 2025
Evidence is mounting that a microbe called Methanobrevibacter smithii is involved in colorectal cancer
KATERYNA KON/SCIENCE PHOTO LIBRARY/Alamy
Mysterious ancient microorganisms that are neither bacteria nor viruses seem to play a role in colorectal cancer. This supports the idea that such microbes, which were thought to be harmless, could actually be involved in human disease.
There are three domains of life. The first comprises single-celled bacteria. The second is made up of organisms known as eukaryotes that are often multicellular, including all animals and plants. These have larger cells packed with internal structures, including a nucleus stuffed with DNA.
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Archaea make up the third domain. These are single-celled organisms that were originally thought to be bacteria because they lack a nucleus, but were found to share some similarities with eukaryotes. In fact, it is now thought that the first eukaryotes may have been archaea that engulfed free-living bacteria.
Although the trillions of bacteria and viruses lurking within our gut have been linked to many conditions – including cancer, diabetes, obesity and heart disease – the archaea that live there aren’t normally examined for such associations.
“Most scientists who are working on the human microbiome overlook archaea and totally filter out these organisms,” says Roxy Mohammadzadeh at the Medical University of Graz in Austria. Nonetheless, high numbers of archaea have been seen to occur alongside conditions including colorectal cancer, Parkinson’s disease and infections of the gums and urinary tract.