Bacteria have been around for 3.5 billion years, so they have learnt the techniques of survival.

The discovery of antibiotics 70 years ago was just one more challenge to them. And they are set to surmount it.

In 1969 a government committee gave a warning of the number of strains of bacteria acquiring resistance to one or more drugs, and the ability of resistant strains to transmit resistant genes to other bacteria.

The committee said salmonella was becoming resistant to several drugs, and it warned that people could die.

People at risk

In the four years from 1992 there were 46 outbreaks of salmonella in the United Kingdom, most involving strains resistant to at least five antibiotics. Five people died.

The World Health Organisation says that resistant strains of four bacteria which affect humans have now been transmitted to people from animals - salmonella, campylobacter, enterococci, and E. coli.

And it says the bacteria may prove resistant, not just to the antibiotics used on animals, but to those used to fight serious illness in people.

More antibiotics are in fact used on animals than on humans. The WHO says more than half of global production is used on farm animals.

Pigs may receive up to ten separate drugs

A recent Soil Association report said about 1,225 tonnes of antibiotics were used annually in the UK - 38% for treating humans, 37% for farm animals, and 25% for pets and horses.

In the last 30 years the use of penicillin-type drugs in farm animals has increased by 600%, and of tetracyclines by 1,500%.

Antibiotics are used in farming in four main ways:

• To treat sick animals
• To protect individual healthy animals against a risk of infection
• To protect an entire herd or flock on a routine basis against the diseases of intensive farming
• As growth promoters, to make the animals grow up to 10% faster

The drugs are seldom used on beef cattle, though dairy cows have tubes of antibiotics inserted into the teats of their udders when they stop producing milk before calving again.

Fish farming also employs antibiotics, though most of what is used escapes into the environment without being ingested by the fish.

The main use of antibiotics in farming is in pigs and chickens.

Doubled growth rate

Laying birds - battery hens - are not normally given growth promoters, but may well receive therapeutic doses.

Intensive rearing can be stressful

Broiler chickens are given growth promoters and are also routinely treated with drugs for parasitic infections. They now reach their slaughter weight in six weeks, twice as fast as 30 years ago.

Even so, about 10% of the number killed annually in the UK - in other words, nearly 80 million birds - are contaminated with salmonella.

In 1996 a government report said 44% were contaminated with campylobacter.

The number of pigs reared annually in the UK for slaughter is about 14 million. They are given growth promoters and routine therapeutic doses to guard against the diseases of stressed and intensive rearing.

The Soil Association says pigs may receive up to 10 different antibiotics, by injection, in water and in feed.

Making a start

Peter Stevenson, of Compassion in World Farming, told BBC News Online he welcomed Grampian Country Food's decision to stop using growth promoters.

He hoped it would be a first step "to encourage real reform of the broiler industry".

"Farmers must not use antibiotics as a substitute for good husbandry. And this phase-out must be extended to cover pigs."