1. Department of Oral Bacteriology, Ohu University School of Dentistry, Tomitamachi, Koriyama 963-8611
2. Department of Preventive Dentistry, Kyushu University Faculty of Dental Science, Higashi-ku, Fukuoka 812-8582
3. Department of Preventive Dentistry, Kyushu Dental College, Kokurakita-ku, Kitakyushu 803-8580, Japan