Bacteria as therapy
How do probiotics work? Until recently, it was thought their mode of action was simply to stick to the gut wall, preventing other bacteria from gaining a foothold.

But new laboratory evidence suggests that probiotics are much more proactive, producing substances such as acids, bacteriocins and hydrogen peroxide, which can kill harmful bacteria and interfere with their ability to produce toxins (Gut, 2003; 52: 827-33).

Other new evidence (from at least six different studies) shows that probiotics act directly on the immune system, boosting both T-helper and natural-killer (NK) cells, the system's major weapons. For example, a recent Korean study measured a 35-per-cent increase in NK cells in study participants given probiotics (J Med Food, 2006; 9: 321-7).

Perhaps ironically, a major use for probiotics is to combat the side-effects of their near-namesake-antibiotics. These powerful medicines don't discriminate between friend or foe, so the inevitable consequence of oral antibiotic therapy is the death of all gut bacteria, good and bad. Nevertheless, antibiotics may be less efficient at killing the bad species, as they can cause proliferation of one of the worst of them, Clostridium difficile. This stomach bug is fast becoming a major pest of hospitals, as it's involved in the development of superbug resistance, and has officially been dubbed a 'new epidemic' (Ann Intern Med, 2006; 145: 758-64).

However, probiotics have come to the rescue, with over 30 separate studies showing they can halve the incidence of diarrhea caused by C. difficile (Lancet Infect Dis, 2006; 6: 374-82)-not to mention diarrhea in general.

Interestingly, probiotics can cure constipation, too (Best Pract Res Clin Gastroenterol, 2006; 20: 575-87). With conventional drugs, these contrary effects would be considered impossible. But probiotics don't act like drugs. They are more like wise regulators, with a unique ability to restore gut bacteria to normal, whatever the state of the intestines.

Bowel disorders-and worse
Probiotics are now offering hope to people who have more serious bowel disorders such as irritable bowel syndrome, which afflicts about 3 per cent of the population. This can flare up for no apparent reason, and cause severe stomach pain, bloating, diarrhea and constipation.

One school of thought says it's due to food intolerance, but gut bacteria may also be involved, as probiotics have been seen to result in up to a 30-per-cent improvement in symptoms (J Clin Gastroenterol, 2006; 40: 264-9).

Crohn's disease (CD) and ulcerative colitis (UC), which are also known as 'inflammatory bowel disorders', may affect up to a quarter of a million Britons. Orthodox medicine considers these conditions to be essentially incurable. Yet, according to Dr Sandra Macfarlane of Dundee University: ". . . there is now indisputable evidence that a loss of beneficial bacteria is involved in the two conditions."

Although probiotics are a logical solution, only a few small-scale clinical trials have put them to the test. One, a pilot study at Bologna University, showed that a very high dose of a cocktail of different lactobacilli "induced remission" in two-thirds of CD patients. With UC, however, the results have been mixed. The explanation could be that these trials have tended to use single Lactobacillus species rather than a combination.