1. Federal Environmental Agency (Umweltbundesamt), Department II 2.4. Microbiology and Parasitology, Corrensplatz 1, Berlin, D-14195, Germany
2. Technical University of Munich, Institute of Water Quality Control and Waste Management, Am Coulombwall, Garching, D-85748, Germany, E-mail: M.Lebuhn@bv.tu-muenchen.de