1. Department of Animal Sciences, Katholieke Universiteit Leuven, Kasteelpark Arenberg 21 (Laboratory of Gene Technology) and 30 (Laboratory of Physiology and Immunology of Domestic Animals), 3001 Leuven, Belgium
2. Department of Molecular Biotechnology, Ghent University, Coupure Links 653, 9000 Ghent, Belgium
3. Department of Clinical Microbiology, University Hospital, S-751 85 Uppsala, Sweden
4. Department of Microbiology, University of Georgia, Athens, GA 30602, USA