Yale study: How gut microbes can evolve and become dangerous

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A group of Yale researchers have found insights into how gut microbes can evolve and become dangerous, according to the university.

Looking at this issue arose because, while gut microbes have been linked to good health, they also have been linked to “promotion of diseases such as autoimmune disorders, inflammatory bowel diseases, metabolic syndrome, and even neuropsychiatric disorders,” according to the university.

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The university noted that one explanation has been the “leaky gut” hypothesis — “in which potentially damaging bacteria are said to escape the intestine, triggering a chronic inflammatory response that can contribute to a variety of diseases.”

“But one mystery has been how potentially pathogenic bacteria can exist in healthy people for decades with no apparent health consequences,” Noah Palm, assistant professor of immunobiology at Yale said in a statement about a new study published July 13 in the journal Nature.

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Yi Yang, a graduate student in Palm’s lab, is lead author of the paper, according to the university.

In the study, researchers describe “how gut bacteria can evolve over time, becoming more pathogenic by gaining the ability to migrate across the gut barrier and persist in organs outside of the intestine, thereby driving chronic inflammation and associated pathologies,” the university statement said.

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The team studied the “genetics and behavior of a species of potentially pathogenic bacteria which they introduced into” a species of mice that does not have its own gut microbes, according to the university statement. “Over time, they found that these microbes diverged into two distinct populations: one that behaved similarly to the ancestral strain, and another that acquired tiny DNA mutations that allowed them to live in the mucosal linings of the intestine and persist in the lymph nodes and liver after escaping the gut.”

The researchers found colonies of relocated bacteria “over time can eventually trigger inflammatory pathologies such as autoimmune diseases. This phenomenon may at least partially explain why some people with potentially pathogenic bacteria never get sick, but why the risk of illness increases with age,” the university said in a statement about the study.

The phenomenon referred to as “within-host evolution” explains “why individual bacterial species living in our intestines are able to adapt and evolve over the course of our lifetimes,” according to the researchers.

Further, according to the university, the research suggests “environmental factors” influence within-host evolution.

“For instance, people who consume a healthy diet tend to develop diverse bacterial communities in their guts,” the university statement said. “This means that many different microbes must compete for space and resources, limiting the population size of any individual species and thus depressing the chances that potentially unhealthy variants will emerge and escape the gut.”

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“These bacteria are essentially pre-adapted to exist in organs outside the intestine,” Palm said in the statement. “We believe that this evolutionary process starts over in each new host due to the preferential transmission of non-pathogenic strains between individuals.”

Understanding how within-host evolution shapes “bacterial behavior in the gut,” according to Palm, may “eventually reveal novel therapeutic interventions that can restrict or redirect this process to prevent the development of diverse diseases associated with ‘leaky gut.’”