Preliminary genetic sequencing suggests the strain is a mutant form of two E. coli bacteria, with lethal genes that could explain why the Europe-wide outbreak appears to be so dangerous, the agency said.
A food safety expert at the WHO, Hilde Kruse, said it was ''a unique strain that has never been isolated from patients before''. The new strain had ''various characteristics that make it more virulent and toxin-producing''.
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So far, the mutant E. coli strain has sickened more than 1500 people, including 470 who have developed a rare kidney failure complication. Researchers have been unable to pinpoint the cause of the outbreak in at least nine European countries.
Nearly all the sick people either live in Germany or recently travelled there. Two people who were sickened are now in the US, and both had recently travelled to Hamburg, where many of the infections occurred.
The newest cases were reported yesterday in Britain by the Health Protection Agency, with seven people infected with the bacteria. Three of those infected were British nationals who had recently travelled to Germany and four were German nationals, it said.
