1. Division of Animal Experimentation, School of Medicine; Sapporo Medical University; Sapporo Hokkaido 060 Japan
2. Department of Preventive Dentistry, School of Dentistry; Higashi-Nippon-Gakuen University; Ishikari-Tobetsu Hokkaido 061-02 Japan
3. Department of Microbiology, School of Medicine; Sapporo Medical University; Sapporo Hokkaido 060 Japan
4. Department of Microbiology, Faculty of Medicine; Okayama University; Okayama Okayama 700 Japan