1. MRC Centre for Molecular Bacteriology and Infection (CMBI); Division of Cell and Molecular Biology; Imperial College London; London; SW7 2AZ; UK
2. Division of Molecular Biosciences; Imperial College London; London; SW7 2AZ; UK
3. Division of Molecular Microbiology; College of Life Sciences; University of Dundee; Dow Street; Dundee; DD1 5EH; UK