Biographical note of the first author: Li ZHU, female, born in 1973, Ph. D., majoring in the field of detection of microbiology. Correspondence should be addressed to Pro. Shou-Yi YU, School of Public Health and Tropical Medicine, Southern Medical University, Guangzhou 510515, Guangdong, China. Tel: 86-20-61648312. E-mail: zhuli89@126.com