The World Health Organization has said the E. coli bacteria responsible for a deadly outbreak that has left 18 dead and sickened hundreds in Europe is a new strain that has never been seen before. Preliminary genetic sequencing suggests the strain is a mutant form of two different E. coli bacteria, with aggressive genes that could explain why the Europe-wide outbreak appears to be so massive and dangerous. Hilde Kruse, a food safety expert at the WHO, said this is a unique strain that has never been isolated from patients before. She added that the new strain has various characteristics that make it more virulent and toxin-producing than the hundreds of E. coli strains that people naturally carry in their intestines. So far, the mutant E. coli strain has sickened more than 1,500 people, including 470 who have developed a rare kidney failure complication, and killed 18, including one overnight in Germany, the country hit hardest by the outbreak. Researchers have been unable to pinpoint the cause of the illness, which has hit at least nine European countries, and prompted Russia on Thursday to extend a ban on vegetables to the entire European Union. Within the EU, Sweden, United Kingdom, the Netherlands, Denmark and Spain have reported cases related to the ongoing outbreak.