Bacteria strains and plasmids ‘
All bacteria strains used in this paper are listed in Table 1. E. coli Strains MG1655 are generous gifts from Professor Sheng Yang.E. coli Strains harboring kil counter-selection system was constructed previously in our lab. S. marcescens strain GY1 was isolated previously in our lab. Plasmids pKD46 and pSim6 are generous gifts from Doctor Donald L. Court. Plasmid pKDsg-ack was purchased from Addgene (www.addgene.org, 62654).