Bacterial Pathogenesis and Functional Genomics Group, Sir William Dunn School of Pathology, University of Oxford, South Parks Road, Oxford. OX1 3RE, 1
Author for correspondence: Nigel J. Saunders. Tel: (+44) 1865 275521. Fax: (+44) 1865 275515. e-mail: saunders{at}molbiol.ox.ac.uk