1. Department of Oral Microbiology, School of Dentistry; Health Sciences University of Hokkaido; Hokkaido 061-0293 Japan
2. Department of Micbiology, School of Dentistry; Nagasaki University; Nagasaki 852-8588 Japan
3. Department of Food Science and Technology, Faculty of Bioindustry; Tokyo Univesity of Agriculture; Abashiri Hokkaido 099-2493 Japan
4. Department of Oral Bacteriology; Tsurumi University School of Dental Medecine; Tsurumi Kanagawa 2308501 Japan