1. School of Biosciences, Hatherly Laboratories, University of Exeter, Prince of Wales Road, Exeter, U.K., EX4 4PS, Department of Fundamental Microbiology, Université de Lausanne, 1015 Lausanne, Switzerland, Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth, U.K., PL1 3DH, and School of Biology, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332