1. Department of Biosciences; Università degli Studi di Milano; Via Celoria 26 20133 Milan Italy
2. Department of Environmental Microbiology; EAWAG; Swiss Federal Institute of Aquatic Science and Technology; Ueberlandstrasse 133 8600 Dübendorf Switzerland
3. Institute of Biogeochemistry and Pollutant Dynamics (IBP); ETH; Swiss Federal Institute for Technology; Universitätsstrasse 16 8092 Zürich Switzerland
4. Laboratoire Adaptation et Pathogénie des Micro-organismes; Institut Jean Roget; Domaine de La Merci; Université Joseph Fourier; BP 170 38042 Grenoble cedex 9 France