By Michelle Roberts
BBC News health reporter
Experts have called for tougher controls on products claiming to contain 'friendly' bacteria purported to promote good health.
Numerous probiotics are available
Eating live bacteria can benefit people with certain illnesses, clinical trials have shown.
But what about people who are already healthy?
Should everybody be dashing down to the local supermarket to buy a drink or yoghurt packed full of 'friendly bacteria' in the chance they may get sick if they do not?
The use of foods containing bacteria - probiotics - dates back thousands of years.
It is reported that people in Biblical times drank sour milk to combat problems in the gut.
In recent years, a number of food products have been hitting the supermarket shelves and bombarding our TV screens.
The annual probiotic market is now worth an estimated £135 million in the UK alone.
There are two ways a probiotic can be classified.
Dr Simon Cutting, a reader in molecular microbiology at the School of Biological Sciences, Royal Holloway, University of London, explained: "One is a novel food - that's when you buy things from the supermarket. The other type is what is classified as a drug.
"When you have a drug, that means you have specific claims which have to be supported by clinical trials. You can't just say it will make you better - you have got to say specifically what it is.
"With novel foods you don't. So novel food is how probiotics are being sold at the moment in the UK."
He is working with a pharmaceutical company to develop a probiotic that can be classified as a drug.
Dr Cutting said: "The idea is by constantly topping up the bacteria in your body you're maintaining well-being."
Having a balance of "friendly bacteria" is thought to stop harmful bacteria taking a hold and causing disease, much like having a garden filled with good plants rather than weeds.
To do this, probiotic bacterial strains must be alive and be deliverable to the gut.
That is they must survive gastric and bile acid and digestive enzymes during the gastrointestinal transit until they reach the gut.
Dr Cutting said trying to prove the effects of probiotics in health scientifically was extremely complex.
"If you were to perform a clinical trial how can you be sure that the same number reach the small intestine for every person?
Some bacteria can be harmful to the gut
"At every step it becomes more and more difficult. And of course with a novel food you don't have a clinical trial, so it's really done on hearsay."
He doubted whether the novel food probiotics sold in the UK could work.
"In England, all they are selling at the moment are lactobacillus products.
"When you take a dose of lactobacillus, it's not clear how many of those can survive digestion.
"But in Japan, for example, they are all based on spore-forming bacteria.
"I have a lot of faith in the novel foods based on bacillus products simply because I understand that the spore can survive the stomach so when you take a dose it gets through the stomach into the small intestine."
Professor Jeremy Hamilton-Miller, from the Royal Free and University College Medical School, London, said: "Not all probiotics achieve the same results.
"Despite claims on the label, some contain too small a quantity of bacteria to be beneficial, some contain probiotic cultures that do not survive the gastrointestinal passage, whilst others contain strains of bacteria that are not shown to benefit health."
But the professor, who works with Seven Seas, which produces Multibionta, said many of the probiotic foods sold in the UK did promote good health.
Professor Collette Short, director of science at Yakult, said there were studies showing that probiotics boost health.
"There are scientific papers looking at people who are healthy and looking at different parameters, like mild conditions. Constipation would be one example.
"The other area you have is in healthy children going to nursery school where they are in an environment that they are more exposed to bacteria and potentially picking up infections.
"There are several studies that show that children on probiotics were better able to cope and had less use of antibiotics than other children."
THE 'NORMAL' GUT BACTERIA
She said it was difficult to prove probiotics' effects in healthy people because there were no obvious biomarkers to measure any improvements.
Professor George MacFarlane, professor of bacteriology at the University of Dundee, said: "To be honest, I'm not entirely sure there is any great benefit in young healthy people. On the other hand they may not do any harm."
He warned that some products being marketed as probiotics had no demonstrated probiotic properties whatsoever.
"Some actually contain bacteria that are harmful. People have to be very careful if they are buying these things over the internet or even from a health food store."
But he said this was not a problem with products from the big companies that had extensively tested their products over many years.
"Most of the things that are available now are well proven to be safe," Professor MacFarlane said.
"One of the biggest problems with all the work on probiotics is that they are funded by companies that have their own products, which means they are asking the questions that companies want answered," he said.
"The other thing is that different probiotics do different things.
"For example, if you take a probiotic that is proinflammatory, so it stimulates your immune system, it could actually do damage if you had something like ulcerative colitis.
"Ideally you would want to take something that reduces inflammatory processes. Many lactobacilli tend to be proinflammatory," he said.
Wendy Johnson, food scientist and member of the Institute of Food Science and Technology said: "I think probiotic organisms have a place to play in promoting health but providing the evidence is difficult."