1. Department of Pathology, Institute of Clinical Medicine, University of Oslo , Oslo 0372 , Norway
2. Department of Pathology, Oslo University Hospital-Rikshospitalet , Oslo 0372 , Norway
3. Institute of Oral Biology, University of Oslo , Oslo 0372 , Norway
4. Department of Microbiology, University of Oslo, Rikshospitalet , Oslo 0372 , Norway