1. Centre for Molecular Microbiology and Infection, Department of Biological Sciences, Imperial College London, London SW7 2AZ, UK 2Microbiology Department, Royal Free Hospital, Pond Street, London NW3 2QG, UK 3Laboratory of Enteric Pathogens, Specialist and Reference Microbiology Division, Health Protection Agency, 61 Colindale Avenue, London NW9 5HT, UK