1. Department of Biology, Biotechnical Faculty, University of Ljubljana, Večna pot 111, 1000 Ljubljana, Slovenia, Helmholtz Centre for Infectious Research, Mascheroder Weg 1, D-38124 Braunschweig, Germany, Institute for Medical Microbiology, University Giessen, Frankfurterstrasse 107, D-35392 Giessen, Germany, and Institute for Cell and Molecular Biosciences, University of Newcastle upon Tyne, Framlington Place, Newcastle upon Tyne NE2 4HH, U.K.