1. Department of Bacteriology The Jikei University School of Medicine 3–25‐8, Nishi‐Shimbashi, Minato‐ku Tokyo 105‐8461 Japan
2. Department of Infectious Disease and Control The Jikei University School of Medicine 3–25‐8, Nishi‐Shimbashi, Minato‐ku Tokyo 105‐8461 Japan