Scientists have discovered a "friendly" virus in a herd of sheep that can kill E. coli, a lethal strain of food-poisoning which is easily passed from farm animals to humans.

The virus, CEV1, is a bacteriophage, or "bacteria eater", and was found by chance when scientists were studying new antibiotics. Bacteriophages infect bacteria naturally.

Andrew Brabban, a microbiologist at Evergreen State College in Washington state, wanted to test different antibiotics on sheep infected with the highly toxic E. coli O157:H2.

This strain of E. coli infects a high proportion of cattle and is responsible for thousands of cases of food-poisoning each year, some of which are lethal.

However, Dr Brabban and his colleagues found it was almost impossible to infect sheep with the strain of E. coli because it kept disappearing of its own accord before the antibiotics could be administered.

The team tried to re-infect the animals three times but on each occasion the bacteria mysteriously vanished.

When the scientists managed to extract the bacteriophage, they found that it killed 16 out of 18 toxic strains of E. coli. "That includes all the big ones you've ever heard about," Dr Brabban told New Scientist magazine. Furthermore, CEV1 only kills eight out of 73 harmless strains of E. coli, which normally live in the gut and are essential for good health.

Dr Brabban said he hoped to use the bacteriophage to wipe out the O157:H2 strain of E. coli from domestic livestock, from which it can pass to people via contaminated meat or faeces.

Bacteriophages are preferable in treating cattle and sheep and could offer many advantages over conventional antibiotics. They are more specific and less likely to kill useful bacteria and also are passed easily from one infected individual to another.

A further advantage of using the CEV1 virus is that it continues to replicate in harmless strains of E. coli long after the dangerous strains have been removed, which might prevent re-infection. But scientists have yet to show that the virus will not have adverse affects on the harmless E. coli bacteria.

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