Conclusions
In conclusion, we have developed a new counter-selection system based on lysis gene E . In view of the generic lethality to Gram-negative bacteria, it is very promising to be a universal alternative for genomic seamless modification. Counter-selection cassette based on lysis geneE under two sets of expression regulation system, cI/pL and Arac/PBAD, were developed to adapt to different hosts. Seamless modification was conducted successfully and the highest selection stringency so far was obtained both in E. coli and in S. marcescens .