Disease-related gut bacteria may reside in "secret" lineages.

A list of species names is included with every microbiome investigation. A third increases during inflammation, one decreases in older persons, and one increases in cancer patients. The unit that counts is the species name. That has been the working assumption, at least.

That assumption is called into question by a fresh analysis. A single species of gut bacteria may contain multiple separate lineages leading various lifestyles. Cancer, diabetes, and other diseases are genuinely linked to those lineages rather than the species label.

When populations of microorganisms are concealed

The conventional method of counting gut bacteria involves grouping all sufficiently comparable organisms into a species and then determining whether the species increases or decreases in illness. The problem is that bacteria with the same species name can have quite varied adaptations, live in various parts of the gut, and react differently to different selection forces.

To identify those hidden subgroups, Xiaoqian Annie Yu and her colleagues at the University of Vienna employed a technique known as reverse ecology. Instead of beginning with a list of identified species, the technique works backward from genomes.

"You can identify the biologically relevant units in the microbiome much more accurately if you take evolutionary adaptation into account instead of just counting species," Yu stated.

A clue in the shape of a broom

One bacterial cell can outcompete its neighbours and produce a cluster of almost identical progeny when it acquires a beneficial mutation. Researchers refer to this pattern—a genetic fingerprint—as a "genome-wide selective sweep." That pattern appears as a broom on a family tree of bacterial genomes, with a long, thin handle that leads to a head of densely bunched branches.

To identify those brooms across over 16,000 isolated genomes, the team developed a software workflow. Each candidate was then verified by the researchers using 1,477 gut-microbiome surveys from individuals across various nations.

Sweeps that are specific to the gut

Prior to this work, sweeps such as this have largely been reported in infections. The common bacteria that make up a healthy gut microbiome had not been mapped. The group verified 124 sweeps in 25 families and 66 bacterial groupings.

The majority of the impacted lineages were common gut inhabitants like Bacteroides, Clostridium, and Lachnospira rather than pathogens. Sweeps were found in both often and infrequently gene-swapping bacteria. Even though regular gene exchange should have precluded whole-genome takeovers, selection appears to have been strong enough to support them.

Earth will be circled for decades.

The mutations that each sweep's members have accumulated since splitting off can be used to determine how old the sweep is. Twenty-six of the clusters were less than a century old, while others were hundreds or thousands of years old. At that moment, members of the younger sweep clusters had already dispersed over continents.

The classic example of microorganisms spreading around the world is pathogens. The intestinal microorganisms that live there are not. It was usually believed that they moved slowly, primarily through intimate personal contact and families.

Bacteria spread among unrelated individuals.

Gut strains move across unrelated individuals more frequently than chance permits, according to a 2023 study. These travellers now have a more distinct identity according to the new results. Three distant communities—the Baka and Beti in Cameroon and the Matses in Peru—that have no direct interaction with one another were found to have a single Bacteroides uniformis sweep cluster.

It is between 100 and 1,000 years old. The best explanation is a lengthy history of transmission through intermediate groups, followed by extinction in those intermediates as lifestyles changed.

Illness exists in branches.

The most convincing evidence came from health data. For sweep clusters associated with colorectal cancer, Crohn's disease, ulcerative colitis, type 2 diabetes, or advanced age, the scientists examined 6,783 metagenomes.

178 sweep clusters were linked to at least one of those characteristics, according to the researchers, and many species that sheltered sweep clusters pulled in opposing directions. Through various lineages, Bacteroides uniformis shown connections to all five conditions. At the species level, none. These similar apparent nulls were found in a previous study on gut bacteria and diabetes; the signal was always concealed in the lineages.

Immune dodges and capsules

The response consistently focused on chemicals around the bacterial surface when asked which genes were specific to each sweep. Particularly in Bacteroides, sweep-specific genes showed a disproportionate amount of capsular polysaccharides, which are the sugary coatings surrounding each cell.

A bacterium interacts with the human immune system through those coatings. Previous research has demonstrated that they aid some Bacteroides species in controlling inflammation and avoiding immune attacks—the same structural arms race that infections depend on.

Although these sweep clusters are associated with disease conditions, the study is unable to determine whether they cause the conditions or merely flourish in the conditions' settings. These age estimations must be regarded as approximations due to their large error margins.

What transpires after this

Up until now, microbiome investigations that identify a species as protective or dangerous have only reported averages. One sweep cluster that harms someone and another that protects them can be found in the same Latin name.

According to our research, gut microorganisms are also more dynamic than previously believed. According to Martin F. Polz, the study's principal author, "well-adapted strains can spread internationally and occupy new ecological niches."

In search of microbiome biomarkers, physicians can enquire not only about species but also about which sweep cluster is present. Instead of focusing on entire species, probiotics may target certain lineages. Once a curiosity of pathogens, the spread of such lineages now influences how medicine views population health and illness.