1. Department of Applied Chemistry and Microbiology, P.O. Box 27, (Latokartanonkaari 11), FIN-00014 University of Helsinki, Finland.
2. Corresponding author: Department of Applied Chemistry and Microbiology, P.O. Box 27 (Latokartanonkaari 11), FIN- 00014 University of Helsinki, Finland. Fax: + 358 9 191 58222. E-mail: vieno.piironen@helsinki.fi
3. Present address National Technology Agency, P.O. Box 69, FIN-00101 Helsinki, Finland.