1. Institute of Medical Microbiology Immunology and Parasitology Medical Faculty Rheinische Friedrich-Wilhelms-Universität Bonn 53127 Bonn Germany
2. Present address: Centre for Microbiology and Environmental Systems Science Department for Microbiology and Ecosystem Science Division of Microbial Ecology University of Vienna Djerassiplatz 1 1030 Vienna Austria
3. Kekulé Institute of Organic Chemistry and Biochemistry Rheinische Friedrich-Wilhelms-Universität Bonn Gerhard-Domagk-Str. 1 53121 Bonn Germany