1. Department of Microbiology, Graduate School of Medical Sciences, Kumamoto University, Honjo 1-1-1, Kumamoto 860-8556, Japan; Kumamoto Health Science University, Izumicho 325, Kumamoto, Japan; and Faculty of Education and Division of Life Science and Engineering, Graduate School of Natural Sciences, Kumamoto University, Kurokami 2-40-1, Kumamoto 860-8555, Japan