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