1. The Maria Sklodowska-Curie National Research Institute of Oncology, Department of Microbiology, Warsaw, Poland;
2. The Maria Sklodowska-Curie National Research Institute of Oncology, Department of Gastroenterology, Warsaw, Poland;
3. The Center of Postgraduate Medical Education, Department of Pathomorphology, Warsaw, Poland;
4. The Maria Sklodowska-Curie National Research Institute of Oncology, Department of Pathology and Laboratory Medicine, Warsaw, Poland;
5. The Maria Sklodowska-Curie National Research Institute of Oncology, Department of Cancer Prevention, Warsaw, Poland;
6. Department of Transplantation Medicine, Oslo University Hospital, Oslo, Norway;
7. The Center of Postgraduate Medical Education, Department of Gastroenterology and Hepatology and Clinical Oncology, Warsaw, Poland