Good gut bacteria in probiotic yoghurts could evolve in the stomach and become harmful, scientists fear.
The bacteria are marketed as being beneficial for our digestive system and, in the process, improve our health.
But in a study on mice, bacteria in the probiotics ate the protective coating of their intestines – which can lead to irritable bowel syndrome.
They also showed a level of antibiotic resistance, a growing threat to humanity that is considered as severe as terrorism.
Scientists said the probiotics appeared to evolve the most in unhealthy stomachs, working against them rather than improving them.
Probiotics may be doing more harm than good, scientists at Washington University School of Medicine fear after finding they evolved in the guts of mice to create antibiotic resistance
Researchers from Washington University School of Medicine in St Louis, led Dr Gautam Dantas, studied a stain of bacteria called E. coli Nissle 1917 (EcN).
It has been used to combat diarrhoea for a long time – especially in children – after being discovered over 100 years ago.
The researchers used mice with different gut microbiomes. One had no gut bacteria at all, while the others had various healthy and unhealthy guts.
They were each given the probiotic and fed a different diet, either lab mouse chow, a more natural mouse diet, a Western diet high in fats and sugars, and a Western diet with more fibre.
After five weeks, the researchers took the bacteria from the mice’s guts and analysed the microbes’ DNA.
In the mice who were fed westernised foods, EcN had accumulated genetic mutations.
This helped them to gain a competitive edge over the other gut bacteria and stay for a longer amount of time.
Under some conditions, the probiotics ate away at the protective coating on the intestine, which could lead to irritable bowel syndrome.
The probiotic also made the gut more resistant to antibiotics.
In the journal Cell Host and Microbiome, the authors wrote: ‘By exposing the candidate probiotic E. coli Nissle (EcN) to the mouse gastrointestinal tract over several weeks, we uncovered the consequences of gut transit, inter-species competition, antibiotic pressure, and engineered genetic function.’
In the mice with healthy guts, there wasn’t much of a change.
Aura Ferreiro, a graduate student involved in the study, 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 suggest that one probiotic which may be beneficial to one person may be harmful to another, and that we are a step closer to understanding safer probiotics.
Dr Dantas, a pathologist and biomedical engineer, said: ‘If we’re going to use living things as medicines, we need to recognize 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 hours later.
‘There is no microbe out there that is immune to evolution. This isn’t a reason not to develop probiotic-based therapies, but it is a reason to make sure we understand how they change and under what conditions.’
Probiotics can be taken in foods such as yoghurt in the hope of restoring the right balance of flora in the gut.
There is evidence that it can help the symptoms of conditions such as irritable bowel syndrome, but the NHS said other health claims – such as those for eczema – aren’t so strong.