Scientists have identified "an entirely new super-toxic" strain of E.coli which has infected more than 1,600 people worldwide, including seven in the UK.
Chinese and German researchers have been working round the clock to identify the strain, which has struck people in 10 countries and killed at least 16.
A statement from the Beijing Genomics Institute, which has been working on sequencing the strain, said it contained several genes that were resistant to antibiotics.
Analysis shows the bacterium is an enterohaemorrhagic E.coli (EHEC) O104 strain, but is a "new serotype - not previously involved in any E.coli outbreaks".
More than 90% of the bacterium is the same as a virulent strain that causes serious diarrhoea, but the new strain has "also acquired specific sequences", the statement said. These are similar to those involved in haemorrhagic colitis and haemolytic uraemic syndrome (HUS) - a deadly complication of E.coli.
Earlier on Thursday, Hilde Kruse, a food safety expert at the World Health Organisation (WHO), said the strain was "unique" and "has never been isolated from patients before".
She said the new strain had "various characteristics that make it more virulent and toxin-producing", although it is not uncommon for bacteria to continually evolve and swap genes. The most severe E.coli cases are usually seen in children and the elderly, but all age groups are currently affected.
So far, seven people in the UK have been affected by the strain, including three Britons and four German nationals. According to the Health Protection Agency (HPA), all the UK cases caught the infection in Germany, where the outbreak began, with three now being treated for HUS.
It has issued a warning urging people travelling to Germany to avoid eating raw tomatoes, cucumbers and leafy salad, including lettuce, while experts try to find the source of the outbreak. Anyone returning from Germany with an illness, including bloody diarrhoea, is also urged to seek medical attention.
Nine patients in Germany have died of HUS and another six of EHEC. One person has also died in Sweden, bringing the total number of deaths to 16, according to WHO. Many more patients are in hospital, with several needing intensive care, including dialysis.