Microbe of the Month: A cure in your beer: Bernard Dixon looks at how brewer's yeast is used to treat severe diarrhoea

Administering one microbe to a patient in order to fight disease caused by another is an idea of beguiling simplicity. Unfortunately, despite several initially promising attempts, that theoretical strategy has never been translated into practical success.

Now, as described in the Lancet, work at St Charles and St Mary's hospitals, both in London, shows that the microbe which makes beer can be used to cure a dangerous and previously difficult-to-treat condition.

The infection in question is a severe form of diarrhoea, caused by the bacterium Clostridium difficile, which sometimes leads to potentially lethal colitis (inflammation of the colon).

Doctors have been exploiting the power of microbes for several decades. Bacteria and fungi provide the penicillin streptomycin and other antibiotics that have advanced our capacity to treat diseases such as lobar pneumonia and tuberculosis. But this revolution, based on the realisation that some microbes produce substances (antibiotics) that are lethal to others, has not led to the successful inoculation of microbes themselves into the body as a therapeutic measure.

The infection treated by David Schellenberg and his colleagues at St Charles and St Mary's hospitals is known as C. difficile diarrhoea. Although first recognised many years ago, often following surgery on the intestinal canal, this type of diarrhoea (accompanied by cramps and other symptoms) is unusual in having become more common with the advent of antibiotics.

C. difficile (so named because it is difficult to grow in the laboratory) can thrive when antibiotic therapy alters the balance of bacteria growing in the intestine. It may then begin to produce the poisons that damage the intestinal wall in colitis.

Although C. difficile diarrhoea does not always precipitate lethal colitis, the condition itself may be extremely severe. In addition, while it responds to drugs such as vancomycin and metronidazole, up to a fifth of individuals treated in this way later relapse.

Schellenberg and his colleagues decided to send brewer's yeast to war against C. difficile in light of reports a few years ago that another yeast, Saccharomyces boulardii, can be used to prevent diarrhoea associated with antibiotic therapy and to treat recurrent colitis caused by C. difficile. Unfortunately, administration of S. boulardii has a serious side-effect, in that the living yeast cells can pass into the bloodstream.

They are especially likely to do so in patients whose immune systems are deficient - for example, as a consequence of Aids or as a result of treatment with drugs to prevent the rejection of a transplanted organ.

The results reported in the Lancet indicate that brewer's yeast (also known as Saccharomyces cerevisiae) is a safe answer to the problem. It certainly worked for each of three individuals in whom conventional treatment had achieved only transient success.

One patient was a 79-year- old man who suffered profuse diarrhoea associated with C. difficile for two days after antibiotic therapy. The problem abated when he was given vancomycin. Thirteen days later it recurred and he was treated with metronidazole. Again the diarrhoea went away, but came back within another 14 days. Put on to brewer's yeast tablets, however, the man recovered within three days and he remained well eight weeks later.

Another patient was a man aged 69 whose diarrhoea returned after initially responding to vancomycin. Brewer's yeast tablets resolved the problem within three days, and he continued to be well six weeks afterwards. The third victim was an 87-year-old woman whose diarrhoea also failed to respond permanently to the two drugs. Within three days on yeast tablets, she was well again and remained so six weeks later.

It is still not clear how brewer's yeast brings about improvements of this sort. One possibility is that the yeast cells become attached to C. difficile, preventing its damaging actions. Or the yeast may work by altering the overall balance of microbes in the bowel. A third explanation is that it enhances the formation of protective antibodies. Whatever the explanation, there is considerable merit in a form of treatment that is not only highly effective but also easily administered and readily available. In addition, it is astonishingly cheap. NHS accountants will be overjoyed to learn that while the cost of a week's course of vancomycin is pounds 88.20, the bill for an equivalent supply of yeast tablets comes to

just 19p.