1. Department of Pediatrics and Communicable Diseases, University of Michigan, 1150 W. Med. Ctr. Dr., MSRB III, Rm 8323, Ann Arbor, MI 48109-0646, USA
2. Laboratorium voor Microbiologie, Universiteit Gent, K. L. Ledeganckstraat 35, B-9000 Gent, Belgium