1. Laboratory of Enteric Infections, Pathogenic Cocci and Diphtheria, Department of Microbiology, National Center of Infectious and Parasitic Diseases (NCIPD), Sofia, Bulgaria
2. Department of Epidemiology, Faculty of Medicine, Medical University of Sofia, Bulgaria
3. Clinic of Pediatric Infectious Diseases, Hospital for Infectious and Parasitic Diseases ‘Prof. Iv. Kirov’, Medical University of Sofia, Sofia, Bulgaria