1. Department of Microbiology, Biomedical Center, Uppsala University, Box 581, S‐75123 Uppsala, Sweden.,
2. Institute of Cell and Molecular Biology, University of Edinburgh, Kings Buildings, Mayfield Road, Edinburgh EH9 3JR, UK.,
3. Department of Microbiology, SLU (Swedish University of Agricultural Sciences), Box 7025, S‐75007 Uppsala, Sweden.