The word comes from 'pro', which means for, and 'bio', which means life. So probiotic is just a general word that means 'for life'. The term originally slipped into common use through health food and supplement manufacturers, who used it to describe products that may enhance your health.

More recently, it has been used to describe supplements of live bacteria - the World Health Organisation defines probiotics as "live micro-organisms which, when administered in adequate amounts, confer a health benefit".

Bacteria help the body

We often think of bacteria as harmful organisms, but while many do cause disease, some are beneficial to the body. We all carry millions of bacteria in and on the surface of our body (there are more bacteria in just one person's intestines than there are human beings in the world), and they play a variety of important roles. Perhaps their most vital job is fending off harmful organisms (known as pathogens).

Some diseases, stress and medicines, such as antibiotics, can wipe out the body's supply of friendly, or 'good', bacteria. If these can't be replaced, then problems may develop.

Many women have experienced how healthy bacteria protect us. Antibiotics, taken perhaps for a bad throat or cystitis, kill not just the harmful (or 'bad') bacteria but also the beneficial ones that live in the vagina and keep this delicate area healthy. Once these are gone, other organisms can take over - for example, the yeast Candida can then increase in number, causing the itchy discharge of thrush.

Friendly bacteria guard the gut

'Good' bacteria in the gastrointestinal tract are part of the body's immune system (the gut contains more immune cells than anywhere else in the body and may be thought of as the body’s largest immune organ).

These good bacteria help to prevent bad bacteria from growing in the bowel or entering the body through the wall of the intestines to cause infection. So there's an increased risk of infection when the gut bacteria are lost - for example, after a strong enema to clear out your intestines before surgery, or a preoperative course of antibiotics.

Recent studies have linked the loss of the gut bacteria to high rates of infection after operations such as bowel surgery or transplants.

Good bacteria also play another important role, breaking down food we eat to provide a source of energy essential for the cells that line the intestines. If you lose these friendly bacteria, the cells lining the intestines temporarily die off, which can cause diarrhoea and difficulties absorbing food from the intestines.

And yet another benefit is that good bacteria help to make vitamins needed by the body, and keep the digestive system working as it should.

Probiotics restore healthy bacteria

If you take a probiotic, you can repopulate your gut with healthy bacteria. But there's some controversy about which types of bacteria are the best to take. Lactobacilli plantarum, for example, might be good because they're found on many fresh foods, such as the surfaces of unwashed vegetables. You might want to try making yourself a large fresh vegetable smoothie every day.

But to get enough probiotics this way you might need to consume a very large amount of vegetables daily. These days it's simpler to look in supermarkets and health food shops for products that contain live bacteria. These foods, such as 'live' yoghurts and similar dairy products, usually contain a variety of types of lactobacilli or bacteria called bifidobacteria.

Some experts believe that bifidobacteria are more important for our health than lactobacilli, but lactobacilli are more easy to use in manufactured foods.

Whatever probiotics you choose, you'll need to take them regularly to get the benefit. One live yoghurt won’t be enough to repopulate your intestines with healthy bacteria.

Prebiotics may be more effective

Some experts believe that, for the best results, you need to eat the right sort of foods for healthy bacteria to feed on. These foods are known as prebiotics, and consist of nondigestible food fibres and complex sugars that specifically stimulate the growth of good bacteria in the bowel.

Breast milk, for example, is rich in prebiotics, which may help to explain why breast-fed babies are healthier and less vulnerable to infections.

In fact it may be more effective to take prebiotics that boost growth of the good bacteria you already have in your gut, rather than take supplements of live bacteria that may be destroyed by the acidity of the stomach as soon as you swallow them.

Prebiotics are found naturally in small amounts in foods such as wheat, oats, bananas, asparagus, leeks, garlic and onions. But to get an adequate daily dose, you may want to look for foods in the supermarket that have been enriched with prebiotics, or even consider prebiotic supplements.

Whether you take probiotics or prebiotics, or both, the latest research shows they may have widespread health benefits including the treatment and prevention of acute diarrhoea and antibiotic-induced diarrhoea, Clostridium difficile colitis, urinary tract infections, respiratory infections in children, dental caries, irritable bowel syndrome, inflammatory bowel disease, allergic disease, rheumatoid arthritis and, possibly, the prevention of bowel cancer.

This article was last medically reviewed by Dr Trisha Macnair in June 2008