1. Department of Biomolecular Sciences, UMIST, Manchester M60 1QD,1
2. Wellcome Trust Centre for the Epidemiology of Infectious Disease, Department of Zoology, University of Oxford, Oxford OX1 3PS,2United Kingdom
3. Division of Bacteriology, National Institute for Biological Standards and Control, South Mimms, Potters Bar, Hertfordshire EN6 3QG,3 and