1. Department of Microbiology; Hiroshima University School of Dentistry; Hiroshima Hiroshima 734 Japan
2. Department of Removable Prosthodontics; Hiroshima University School of Dentistry; Hiroshima Hiroshima 734 Japan
3. Department of Microbiology; Tokushima University School of Dentistry; Tokushima Tokushima 770 Japan