1. Division of Bacteriology, National Institute for Biological Standards and Control, Blanche Lane, South Mimms, Herts EN6 3QG, UK
2. Peter Medawar Building for Pathogen Research and Department of Zoology, University of Oxford, OX1 3SY, UK
3. Department of Biology, Pennsylvania State University, University Park, PA 16802, USA
4. Meningococcal Reference Unit, Health Protection Agency, Manchester Royal Infirmary, Oxford Road, Manchester M13 9WZ, UK