1. Faculty of Pharmacy, Medical University of Sofia, 1000 Sofia, Bulgaria
2. The Stephan Angeloff Institute of Microbiology, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
3. Department of Microbiology, Laboratory of Molecular Virology, Hellenic Institute Pasteur, Athens 11521, Greece
4. Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria