1. Department of Food and Environmental Sciences/Microbiology and Biotechnology, University of Helsinki, PO Box 65, Helsinki 00014, Finland
2. Department of Biological and Environmental Science/Centre of Excellence in Biological Interactions, University of Jyväskylä, PO Box 35, Jyväskylä 40014, Finland
3. Department of Biology, University of York, York YO10 5DD, UK