1. Laboratory of Microbiology, Wageningen University, Dreijenplein 10, 6703 HB Wageningen, the Netherlands
2. National Institute for Public Health and the Environment, Postbus 1, 3720 BA Bilthoven, the Netherlands
3. Department of Veterinary Biosciences, Helsinki University, P.O. Box 66, 00014 Helsinki, Finland
4. Department of Bacteriology and Immunology, Helsinki University, P.O. Box 21, 00014 Helsinki, Finland