1. Karolinska Institute, Stockholm SE-171 77, Sweden
2. Public Health Agency of Sweden, SE-171 82, Sweden
3. Institute of Molecular and Cell Biology, A*STAR (Agency for Science, Technology and Research), Singapore
4. Department of Microbiology, Yong Loo Lin School of Medicine, National University Health System (NUHS), National University of Singapore, Singapore
5. National Veterinary Institute, Uppsala SE-756 51, Sweden