Some of the bacteria in your gut may behave less like a private ecosystem than a shared one. A study published in May 2026 in Nature reports that certain gut bacterial populations spread between people, and in some cases across continents, within only a few decades.
Several of the same populations were more common in people with inflammatory bowel disease, colorectal cancer or type 2 diabetes. Researchers led by the University of Vienna analyzed nearly 17,000 gut bacterial genomes from people on multiple continents.
Using an approach called reverse ecology, the team showed that many familiar bacterial species actually contain hidden subpopulations, each shaped by different pressures inside the human gut.
Hidden lineages inside familiar gut microbes
Most microbiome studies group bacteria by species. That shorthand can miss meaningful differences between strains living in the same body. The Vienna team looked instead for selective sweeps, where one strain outcompetes its neighbors and leaves a distinctive fingerprint in the genome.
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"If you don't just count species but take evolutionary adaptation into account, you can identify the biologically relevant units in the microbiome much more accurately," says lead author Xiaoqian Annie Yu, in a statement from the University of Vienna.
Of 77 commensal sweep clusters the team identified, 54 spanned multiple continents. And 26 of those had appeared within roughly the past century.
Bacteroides uniformis, a common gut microbe, showed hidden lineages that were more frequent in older adults and in people with colorectal cancer, Crohn's disease, ulcerative colitis and type 2 diabetes, even though the species as a whole showed no such pattern.
Transmission, not just diet
The results suggest the microbiome is shaped by more than diet, medication and lifestyle. "Well-adapted strains can spread internationally and occupy new ecological niches," says study leader Martin F. Polz.
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Person-to-person transmission appears to be an important force. Strain acquisition happens most often between related individuals, but also, at lower rates, between unrelated people, the researchers report.
This does not translate into a new diet or supplement. It may change how scientists diagnose and treat microbiome-linked disease, by targeting specific bacterial populations rather than whole species.
That precision could sharpen the search for microbiome biomarkers, especially for cancers like colorectal cancer, where US screening now begins at 45.
Symptoms that may point to inflammatory bowel disease or colorectal problems, such as persistent changes in bowel habits, unexplained weight loss or blood in the stool, warrant a conversation with a doctor rather than a home microbiome test.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.