(AP) -- Scientists on Thursday blamed Europe's worst recorded food-poisoning .

(AP) -- Scientists on Thursday blamed Europe's worst recorded food-poisoning outbreak on a "super-toxic" strain of E. coli bacteria that may be brand new.

But while suspicion has fallen on raw tomatoes, cucumbers and lettuce as the source of the germ, researchers have been unable to pinpoint the food responsible for the frightening illness, which has killed at least 18 people, sickened more than 1,600 and spread to least 10 European countries.

An alarmingly large number of victims - about 500 - have developed kidney complications that can be deadly.

Chinese and German scientists analyzed the DNA of the E. coli bacteria and determined that the outbreak was caused by "an entirely new, super-toxic" strain that contains several antibiotic-resistant genes, according to a statement from the Shenzhen, China-based laboratory BGI. It said the strain appeared to be a combination of two types of E. coli.

"This is a unique strain that has never been isolated from patients before," Hilde Kruse, a food safety expert at the World Health Organization, told The Associated Press. The new strain has "various characteristics that make it more virulent and toxin-producing" than the many E. coli strains people naturally carry in their intestines.

However, Dr. Robert Tauxe, a foodborne-disease expert at the U.S. Centers for Disease Control and Prevention, questioned whether the strain is truly new, saying it had previously caused a single case in Korea in the 1990s. He said genetic fingerprints may vary from specimen to specimen, but that is not necessarily enough to constitute a new strain.

read more: