The attenuated vaccine strain S. choleraesuis C500 was obtained from Nanjing Biological Pharmaceutical Co., Ltd. Attenuated Salmonella typhimurium LB5010 was kindly provided by Prof. Aoquan Wang (Institute of Microbiology, Chinese Academy of Sciences). The plasmid pU6, which has the mouse U6 promoter (P U6 ), was constructed and maintained in our laboratory.