1. Department of Microbiology, Moyne Institute of Preventive Medicine, Trinity College, Dublin 2, Ireland
2. Nuffield Department of Clinical Laboratory Sciences, University of Oxford, John Radcliffe Hospital, Oxford OX3 9DU, UK
3. Department of Biochemistry, University of Pavia, 27100 Pavia, Italy
4. Department of Microbiology and Immunobiology, The Queen's University of Belfast, Grosvenor Road, Belfast BT12 6BN, UK