As marketing campaigns go, it's one of the most seductive: we all have nice, friendly bacteria in our digestive systems, but stress, illness and bad diets kill them off and allow the evil bacteria to thrive and make us ill.
So, the theory goes, by topping up our levels of friendly bacteria with a daily yoghurt drink or probiotics supplement, we can keep the baddies at bay.
Probiotics in Britain are sold as food rather than drugs, enabling manufacturers to claim general health benefits without having done rigorous clinical trials.
Most products - like the yogurt drinks Yakult and Actimel, and the tablet Multibionta - contain just one or two of the 400 bacteria species: Lactobacillus and Bifidobacteria.
Some products don't even contain that: scientists at the Royal Free Hospital in London tested 39 products against six criteria - including whether the bacteria were able to reach the gut in sufficient numbers or whether they were killed off by stomach acids on the way.
Only one, Multibionta, satisfied all six. Another 13 were satisfactory, and others didn't work at all.
Many doctors are sniffy about probiotics, pointing out that when you take a dose of lactobacillus, 99% are killed by the acid in your stomach before they even reach your gut.
However, people have been eating live yoghurt as a digestive aid since biblical times. The father of probiotics, Russian scientist Elie Metchnikoff, was intrigued by the fact that so many Bulgarian peasants lived to be 100. In 1907 he concluded it was because they ate so much yoghurt and began to do so himself, confidently expecting to live to be 150. He managed 70.
Friendly bacteria: what are they?
- We have 100 trillion (100,000,000,000,000) bacteria in our bodies, mainly in the digestive system.
- They're so small that several hundred could fit on the full-stop at the end of this sentence.
- Around 400 different species live in the gut, mainly to ward off pathogens - harmful bacteria.
- They also help the digestive process and produce vitamins and minerals.
- There's growing evidence that our probiotic population - the "good" bacteria - declines with age; there are even probiotics supplements targeted at the over-50s.
Which diseases do probiotics help?
Despite the shortage of proper, in-depth studies, there is some evidence that probiotics can help people with gut disorders and inflammatory diseases.
Work at Dundee University with ulcerative colitis patients revealed that they had very low levels of probiotics. They were given a specially formulated strain of bacteria, and almost all noticed an effect.
However, in a round-up of trials the Drug and Therapeutics Bulletin could only conclude that they were inconclusive. Researchers looked at ulcerative colitis, Crohn's disease, Clostridium difficile infection (a common side effect of antibiotics, especially in the elderly) and Helicobacter pylori infection, and found there was little evidence in favour of probiotics.
Although it's very early days, scientists in Israel in 2004 found that rats given probiotics experienced fewer arthritis symptoms, and a Japanese study discovered that patients taking Lactobacillus casei (the probiotic in some yoghurt drinks) were less likely to develop colon cancer. Further trials are continuing.
Another promising area is in stopping the diarrhoea often caused by antibiotics, especially in older patients. Antibiotics kill 10% of our "friendly" bacteria as well as the baddies, rendering us vulnerable to other infections. In most of South East Asia, probiotics are prescribed in tandem with antibiotics.
"I've travelled a lot in Asia where people routinely take probiotics", says Dr Simon Cutting, from Royal Holloway, University of London, "and I speak to people over here with colitis and irritable bowel syndrome that use these supplements every day and swear by them. But if I went to one of the research councils for a grant, they would say it was snake oil and wouldn't be interested."
Dr Cutting is convinced that the future of probiotics is as a drug, not a supplement. He is working with a pharmaceutical company, Sanofi, on the first probiotic drug Enterogermina, which is licensed in Italy (where 100 million doses are sold a year) but not yet in Britain.
"I never would have believed it myself until I did a trial on chickens and yes, probiotics absolutely do prevent infections. We've just got an awful lot more to find out."
Which probiotic products actually work?
In a trial, Professor Jeremy Hamilton-Miller tested 39 probiotics products against six criteria, which required them to:
- Be safe for human consumption
- Be alive and able to survive gastric juices
- Have clinically proven health benefits demonstrated by scientific trials
- Have their contents clearly defined
- Be clearly labelled
- Be 'shelf stable'
Only one, the tablet Multibionta, satisfied all six. Thirteen other products rated satisfactory:
- Drinks Actimel, Yakult and ProViva (no longer available)
- Powder supplements G&G Prodophilus, Nutri/Natren SuperDophilus and Nutriscene Acidophilus Supreme
- Capsules Larkhall Natural Flow, Biocare Bioacidophilus, Healthcraft Acidophilus Extra, Lifespan Acidophilus, Culturelle, Blackmore Acidophilus Bifidus and Quest Non-dairy Acidophilus Plus.