Probiotic “friendly” bacteria taken by millions of people to improve digestive health can go through a Jekyll and Hyde transformation in the body, scientists have warned.
A study suggests they can evolve into new forms that are less beneficial and may even be harmful.
Researchers in the US studied a strain of Escherichia coli (E. coli) widely sold as a supplement to combat diarrhoea.
They discovered that the bacteria’s DNA changed after living in the guts of mice for a few weeks.
Under some conditions, the bugs turned on their hosts and started eating the intestine’s protective coating.
Destruction of this tissue layer has been linked to irritable bowel syndrome.
The way the probiotic microbes evolved was influenced by diet and other bacteria in the gut.
Lead researcher Professor Gautam Dantas, from Washington University School of Medicine, said: “If we’re going to use living things as medicines, we need to recognise that they’re going to adapt, and that means that what you put in your body is not necessarily what’s going to be there even a couple of hours later.
“There is no microbe out there that is immune to evolution.”
The digestive tract holds a vast community of bacteria and fungi that help us process food, regulate inflammation and keep out dangerous bugs.
Probiotics are live bacteria and yeasts taken in the form of supplements or fermented foods such as yogurt to promote good health.
The Washington team studied a probiotic known as E. coli Nissle 1917. The strain was isolated more than a century ago from a First World War soldier who emerged unscathed from an epidemic of severe diarrhoea that made his comrades ill.
Mice with healthy “microbiomes” containing a diverse range of beneficial bacteria were not affected by changes to Nissle.
The same was not true of mice on fatty, sugary diets which developed unhealthy microbiomes.
Co-author Aura Ferreiro, also from Washington University, said: “In a healthy, high-diversity background we didn’t capture a lot of adaptation, maybe because this is the background that Nissle is used to.
“But you have to remember that quite often we wouldn’t be using probiotics in people with a healthy microbiome. We’d be using them in sick people who have a low-diversity, unhealthy microbiome. And that seems to be the condition when the probiotic is most likely to evolve.”
The findings, published in the journal Cell Host and Microbe, open up opportunities for personalised probiotic-based medicine, said the researchers.
Prof Dantas added: “Evolution is a given. Everything is going to evolve. We don’t need to be scared of it.
“We can use the principles of evolution to design a better therapeutic that is carefully tailored to the people who need it. This is an opportunity, not a problem.”