Your gut isn’t home to a few hundred bacterial species living peacefully side by side, it’s more like a sprawling city where, within a single species, different “neighborhoods” of bacteria have quietly evolved to specialize in very different lifestyles. A new study led by researchers at the University of Vienna and published in Nature shows that many familiar gut bacterial species are actually made up of several genetically distinct populations, each adapted to different conditions inside the human body, and some of these hidden populations show up more often in older age, inflammatory bowel disease, colorectal cancer, and type 2 diabetes.

The Problem With Counting Species

Microbiome research usually sorts bacteria into species or broad genetic clusters. That’s convenient, but it can blur the picture: a species linked to a disease in one study might be harmless or even protective in another. The reason may be that we’ve been lumping together bacteria that look similar on paper but have actually gone their separate evolutionary ways inside the gut.

To get a sharper view, the team used a bioinformatic approach rooted in “reverse ecology”, inferring how organisms have adapted to their environment by reading the genetic traces left behind, rather than observing them directly. They combed through thousands of bacterial isolates from the human gut plus large-scale metagenomic data spanning multiple countries, ages, and health states.

Selective Sweeps: Evolution’s Fingerprints

What they were hunting for were signs of genome-wide selective sweeps, events where one bacterium picks up a beneficial mutation and, over time, out-competes and displaces its close relatives. A sweep like this narrows genetic diversity in the short term, but it also leaves behind a population that’s unusually uniform, both in ancestry and in function, and therefore easy to spot as a distinct cluster in genomic data.

Applying this lens to the data, the researchers found that many well-known gut bacterial species actually split into several such lineages, each apparently thriving under different conditions. As lead author Xiaoqian Annie Yu (Centre for Microbiology and Environmental Systems Science, University of Vienna) put it, accounting for evolutionary adaptation, rather than just counting species, reveals the biologically meaningful units in the microbiome far more precisely. Some of these sub-populations turn out to be disproportionately common in specific diseases, a pattern that gets washed out when a species is treated as one single entity.

Gut Bacteria Can Go Global, Fast

Perhaps the most striking finding: some of these highly successful bacterial populations spread across continents within just a few decades. That kind of rapid, global dispersal has mostly been documented in pathogens before, seeing it in ordinary gut commensals suggests person-to-person transmission may shape the microbiome as much as diet, medication, or lifestyle do. As study leader Martin F. Polz notes, well-adapted strains appear able to travel internationally and colonize new ecological niches, making the gut microbiome considerably more dynamic than previously assumed.

Why It Matters

If disease risk tracks with specific evolutionary populations rather than entire species, future diagnostics could get much more precise, flagging the exact bacterial lineage that matters instead of an entire, mixed-bag species. Longer term, this could open the door to therapies that selectively boost beneficial populations or suppress problematic ones, rather than broadly targeting a species that includes both. The next step for the team is figuring out which genes actually distinguish these populations, and what those genes do.

• Many gut bacterial species contain several evolutionarily distinct sub-populations, not just one uniform group.
• Some of these populations are linked to ageing, colorectal cancer, inflammatory bowel disease, and type 2 diabetes.
• Successful lineages can spread worldwide within a matter of decades, a pattern once thought unique to pathogens.
• The approach could sharpen future microbiome diagnostics and enable more targeted therapies.

Original publication: Xiaoqian et al. (2026), “Genome-wide sweeps create ecological units in the human gut microbiome,” Nature. DOI: 10.1038/s41586-026-10476-w