Taking probiotic supplements is of no benefit to a healthy person and can harm people with compromised immune systems, a leading microbiologist has warned.

Michael Wilson, Professor of Microbiology at University College London, said that the promotion of daily probiotics was devoid of robust scientific evidence that they improved health in any way. He added that while topping up on “good bacteria” might sound sensible for rebalancing or enhancing conditions in the human gut, it was based on “a lot of shaky understanding”.

The use of foods containing bacteria dates back thousands of years, with reports of the drinking of sour milk to combat problems in the gut in Biblical times.

However in recent years, probiotics have been promoted as daily supplements for healthy living, with the market for products such as yoghurts and capsules now estimated to be worth up to £200m annually in the UK.

Prof Wilson said that while clinical trials have shown that eating live bacteria can help sufferers of certain illnesses such as antibiotic-associated diarrhoea, there was no evidence of benefits for healthy people. He called for much greater investment in rigorous, double blind trials of supplements that presently were being sold without proper assessment of their safety.

“There are certain instances when probiotics are useful but the problem is there’s no regulation,” he said. “They are regarded as food supplements not medicinal products - anyone can get a suspension of bacteria and market it as a probiotic. With medicinal treatments, the pharmaceutical industry makes sure the things they produce are safe.

There’s none of that with probiotics and the point is we just don’t know.

“It’s all well and good saying that certain bacteria are good for you, but we don’t know about all the other species in the gut and how they all interact. We are basing a lot of probiotic understanding on shaky ground. You need to know you are using appropriate strains for appropriate conditions in appropriate people and we just don’t know those things.”

Prof Wilson, an expert in the indigenous microbiology of humans who spoke today at The Times Cheltenham Science Festival, said that there were at least 2,000 species of bacteria - probably a massive underestimate - and millions upon millions in each group within the human body.

He said that there was some “instinctive sense” in thinking that manipulating the gut flora - or microbiota - might help with adverse events. But for people with compromised immune systems, increasing the bacterial load could risk problems such as septicaemia blood poisoning if there was a defect in the barrier in the gut separating bacteria from sterile tissue.

“No bacterium is totally innocuous. If you are healthy there is probably no harm in taking probiotics, but there is also no benefit. But to increase the bacterial burden if you are immuno-compromised is asking for trouble.”Prof Wilson added that the possibility of problems linked to probiotics would not be picked up because doctors rarely considered them as a cause.

“It doesn’t get attributed to probiotics, because the first thing doctors ask is not are you taking probiotics. You have got to be very careful. Even with the best will in the world, even if GPs did have the training, the fact is we know so little about microbiota.”

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