1. School of Biochemistry, University of Bristol, University Walk, Bristol BS8 1TD, UK
2. School of Cellular and Molecular Medicine, University of Bristol, University Walk, Bristol BS8 1TD, UK
3. Microbial Sciences Division, Faculty of Health and Medical Sciences, University of Surrey, Guildford, Surrey GU2 7XH, UK
4. Department of Bacteriology, Veterinary Laboratories Agency, Weybridge, Woodham Lane, New Haw, Addlestone, Surrey KT15 3NB, UK
5. School of Veterinary Sciences, University of Bristol, Langford, Bristol BS40 5DU, UK