1. J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, 182 23 Prague 8, Czech Republic, Universität Ulm, Sektion für Röntgen- und Elektronenbeugung, D-89069 Ulm, Germany, and Institute of Microbiology, Academy of Sciences of the Czech Republic, 142 20 Prague 4, Czech Republic