In order to remove invasive plasmids from Shigella flexneri 2a 2457T and Shigella sonnei S7 based on the principle of plasmid incompatibility Ori and inc genes were amplified by PCR from S. flexneri 2a invasive plasmids, and then they were cloned into plasmid pMD18 T, resulting recombinant plasmids pMDori and pMDinc After S flexneri 2a 2457T and S sonnei S7 were transformed with pMDori or pMDinc respectively Invasive plasmids were removed from S flexneri 2a 2457T and S sonnei S7