1. Microbiology and Tumor Biology Center (MTC), Karolinska Institutet, 17177 Stockholm, Sweden 2,3Department of Cell Biology and Immunology2 and Department of Microbiology3, Gesellschaft für Biotechnologische Forschung, 38124 Braunschweig, Germany 4Department of Microbiology, University of Guelph, Canada N1G2W1