1. Centre for Biomedical Microbiology, BioCentrum-DTU; Technical University of Denmark; Building 301 2800 Lyngby Denmark
2. Department of Microbiology, Institute of Plant Biology; University of Zürich; CH-8008 Zürich Switzerland
3. Department of Gastrointestinal Infections, Statens Serumsinstitut; 2300 Copenhagen S Denmark