1. Department of Medical Microbiology, Medical University of Sofia, Zdrave Street 2, 1431 Sofia, Bulgaria
2. University Department of Social Medicine and Healthcare Management, Medical University of Sofia, Sofia, Bulgaria
3. Departments of Gastroenterology and General Surgery, University Hospital St Ekaterina, Sofia, Bulgaria
4. Department of Gastroenterology, University Hospital St Ivan Rilski, Sofia, Bulgaria
5. Urgent Endoscopy Unit, Emergency Hospital Pirogov, Sofia, Bulgaria
6. Second Surgery Department, University Alexander Hospital, Sofia, Bulgaria