1. Department of Oral Microbiology, St Bartholomew's and the Royal London School of Medicine and Dentistry, London E1 2AD, UK
2. Department of Medical Microbiology, St Bartholomew's and the Royal London School of Medicine and Dentistry, London E1 2AD, UK
3. Joint Microbiological Research Unit, King's College School of Medicine and Dentistry, London SE5 9RW, UK