1. Public Health Laboratory Sciences and Toxicology; West China School of Public Health; Sichuan University; No.17 People's South Road Chengdu 6100041 P. R. China
2. Institute for Chemical Carcinogenesis; Guangzhou Medical University; Xinzao Panyu District Guangzhou 511439 P. R. China
3. Department of Microbiology; Third Military Medical University; Chongqing 400038 P. R. China