A VIRUS that kills the food-poisoning bacterium E. coli O157:H7 has been discovered in sheep. The virus could help eliminate the bacterium in farm animals, greatly reducing the chance of human infections.
O157:H7, a toxic strain of the normally harmless gut bug E. coli, is a major cause of food poisoning. Three-quarters of cases can be traced directly to livestock, which harbour the bug without becoming ill. Meat can be contaminated when the animals are slaughtered, and manure can also be a source of infections.
So Andrew Brabban at Evergreen State College in Washington state and his team wanted to test different antibiotics to find those which would eliminate the bugs from farm animals. First, they had to infect sheep with E. coli. But they hit an unexpected problem: the bacteria just kept disappearing from the animals. The team re-infected the sheep three times, and every time the bacteria mysteriously vanished.
It turned out that the sheep harbour a bacteria-killing virus, or bacteriophage, that infects certain E. coli strains. When the team tested the phage against the food-poisoning bug in the lab, they found it kills 16 out of 18 toxic strains. "That includes all the big ones you've ever heard about," says Brabban, such as the strain responsible for an outbreak at Jack in the Box fast-food outlets in the US in 1993, which left four people dead. But the phage, christened CEV1, only kills 8 out of 73 harmless E. coli strains.
Brabban now hopes to use the phage to wipe out O157:H7 in herds and flocks. In a small trial in sheep, the phage reduced numbers of the toxic bacterium by 99 per cent in just two days, he told a meeting of the Society for General Microbiology in Edinburgh earlier this month.
And using bacteriophages has all sorts of advantages. Phages are far more discriminating than antibiotics, so the natural microbial flora of animals' guts should not be affected. Also, while antibiotics are expensive and must be given to every animal, infecting just one animal with the CEV1 phage is likely to be enough to pass the phage to a whole herd or flock- and the numbers of the phage will rise exponentially as long as there are host bacteria left to infect.
What's more, the phage seems to persist in animals, suggesting it continues to replicate in a harmless E. coli strain after all the O157:H7 bacteria have been destroyed. Finally, while bacteria can develop resistance as they do to antibiotics, the phage can out-evolve them.
Brabban thinks that giving the phage to animals is more practical than using it to treat people. For instance, killing E. coli 0157:H7 releases large quantities of its toxin, which can make a patient's condition worse. And animal treatment would not have to meet the strict safety standards for human therapies, one reason why phage still are not used in the West (New Scientist, 5 April, p 36). However, the team will need to show that the phage will not have an adverse effect on human gut flora if they are passed to people via food.
New Scientist issue: 26 April 2003.
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Author: James Randerson, Edinburgh
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