1. Department of Clinical Pharmacology, St. Bartholomew's and the Royal London School of Medicine and Dentistry, Charterhouse Square, London EC1M 6BQ, U.K.
2. The Ernest Cooke Clinical Microvascular Unit, St. Bartholomew's Hospital, London EC1A 7BE, U.K.
3. Department of Medical Microbiology, St. Bartholomew's Hospital, London EC1A 7BE, U.K.