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, the World Health Organization said Thursday. 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, the agency said.  Hilde Kruse, a food safety expert at the WHO, told The Associated Press that "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 many E. coli strains 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.