Gut bacteria may be able to evolve over time to live outside of the intestine, a new study finds – and that might make them more dangerous, possibly bringing chronic inflammation and related health risks to other organs in the body.

There's plenty of research explaining the positive and negative effects that gut microbes can have on our health, but scientists still don't understand much about how these various biological mechanisms and chain reactions work.

Some health issues are put down to what's known as a 'leaky gut', where the intestine is more permeable than it should be, releasing bacteria outside of the digestive tract. However, there are lots of unanswered questions about the condition – including whether it's a symptom or a cause of inflammation problems.

"One mystery has been how potentially pathogenic bacteria can exist in healthy people for decades with no apparent health consequences," says immunobiologist Noah Palm, from Yale University.

In an attempt to solve the mystery, Palm and his colleagues introduced the potentially pathogenic bacterium Enterococcus gallinarum – a species found in around 6 percent of gut microbiomes in humans – into germ-free mice with no gut microbes of their own. Researchers then monitored the mice for three months.

Researchers observed the bacteria evolving into two distinct types. One was similar to the original strain, while another featured small DNA mutations so it could live in the intestine's mucosal lining – and survive in the lymph nodes and liver after escaping the intestine.

More worrying, the mutated bacteria can apparently remain hidden in organs and escape the attention of the immune system. In the experiments on mice, the presence of mutant microbes could lead to inflammatory responses, including those linked to autoimmune diseases (which in humans include type 1 diabetes and multiple sclerosis).

"We believe that this evolutionary process starts over in each new host due to the preferential transmission of non-pathogenic strains between individuals," says Palm.

In other words, within a human host, a niche may open up within the intestines as some non-harmful strains move on to other people, giving other bacteria in the gut room to evolve.

This phenomenon is known as 'within-host evolution' and is a possible explanation for why bacterial species living in our intestines can adapt over time, the scientists say. Environmental factors, including diet, have some influence on this.

The more diverse a bacterial community in the gut, the less space any one species has to grow, reducing the chances of unhealthy variants – those that can potentially escape – developing. Anything that affects that diversity, such as diet, could then help explain the risk of inflammatory conditions that sometimes have 'leaky gut' as a symptom, says Michael Palm, the study's senior author.

All of this means that if we can understand more about this bacterial evolution, we might be able to develop and introduce preventative therapies for these health problems – perhaps by targeting specific microbes before they can escape.

"These bacteria are essentially pre-adapted to exist in organs outside the intestine," says Palm.

The research has been published in Nature.