1. Unit of Gastrointestinal Infections; Statens Serum Institut; Artillerivej 5 DK-2300 Copenhagen S Denmark
2. Laboratory for Foodborne Zoonoses, Public Health Agency of Canada, and Department of Laboratory Medicine and Pathology; University of Alberta; Edmonton Alberta T6G 2G3 Canada
3. Department of Medical Microbiology and Immunology; University of Alberta; Edmonton Alberta T6G 2H7 Canada