1. MRC Centre for Molecular Bacteriology and Infection, Department of Life Sciences, Faculty of Natural Sciences, Imperial College London, London SW7 2AZ, U.K.
2. MRC Centre for Molecular Bacteriology and Infection, Department of Medicine, Section of Microbiology, Imperial College London, London W12 0NN, U.K.
3. Department of Immunology and Infection, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, U.K.