1. Laboratory of Animal Microbiology, Department of Microbial Biotechnology, Graduate School of Agricultural Science, Tohoku University, 1–1 Tsutsumidori, Amamiya-machi Aoba-ku, Sendai, Miyagi 981–8555, Japan
2. Electron Microscope room, Graduate School of Agricultural Science, Tohoku University, 1–1 Tsutsumidori, Amamiya-machi Aoba-ku, Sendai, Miyagi 981–8555, Japan
3. Department of Preventive Dentistry, School of Dentistry, Health Sciences University of Hokkaido, 1757 Kanazawa, Tobetsu, Ishikari-gun, Hokkaido 061–0293, Japan