1. Institute of Molecular Biosciences; University of Graz; 8010 Graz Austria
2. Robert Koch-Institute; Nordufer 20 13353 Berlin Germany
3. Faculty of Biology, Microbiology; Albert-Ludwigs-University Freiburg; 79104 Freiburg Germany
4. Division of Infectious Diseases; University Medical Centre Freiburg; 79106 Freiburg Germany