1. Department of Medical Microbiology, University Medical Center Utrecht, 3584 CX Utrecht, The Netherlands
2. Laboratory of Gene Technology, Department of Biosystems, Katholieke Universiteit Leuven, B-3000 Leuven, Belgium
3. Centre of Microbial and Plants Genetics, Department of Microbial and Molecular Systems, Katholieke Universiteit Leuven, B-3000 Leuven, Belgium