1. The Biotechnology Centre of Oslo, University of Oslo,1 and
2. Department of Microbiology, Institute of Pharmacy,2 Blindern, 0349 Oslo, and
3. Department of Bacteriology, National Institute of Public Health, Torshov, 0403 Oslo,3 Norway, and
4. Toxines et Pathogenie Bacteriennes, URA 2172 CNRS, Institut Pasteur, 75724 Paris Cedex 15, France4