1. Laboratory of Food Microbiology and Hygiene, Graduate School of Biosphere Science, Hiroshima University, Higashi-Hiroshima 739-8528, Japan 2Division of Biological Science, Hiroshima City Institute of Public Health, Hiroshima 733-8650, Japan 3Division of First Microbiology, Hiroshima Prefectural Institute of Public Health and Environment, Hiroshima 734-0007, Japan