Nurses work in the intensive care unit where patients with the enterohaemorrhagic E. coli (EHEC)are treated in the university clinic in the northern German city of Luebeck on June 7, 2011. The source of the killer bacteria remained elusive after Germany announced that initial tests on suspected organic sprouts had proved negative and the outbreak's death toll grew to 23. Some 2,000 people have become ill across Europe from the enterohaemorrhagic E. coli (EHEC) outbreak which has affected 12 countries. AFP PHOTO / MARKUS SCHOLZ GERMANY OUT