1. Stephan Angeloff Institute of Microbiology, Bulgarian Academy of Sciences, Akad. G. Bonchev St. bl. 26, 1113 Sofia, Bulgaria
2. Faculty of Biology, Sofia University “St. K. Ohridski”, 8 D. Tsankov Blvd., 1164 Sofia, Bulgaria
3. Institute of Polymers, Bulgarian Academy of Sciences, Akad. G. Bonchev St. bl. 103-A, 1113 Sofia, Bulgaria