1. Department of Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands, Department of Internal Medicine, University of Groningen, Oostersingel 59, 9713 EZ Groningen, The Netherlands, and CLBE, Department of Biochemistry of Biomembranes, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands