Plasmid maintenance and recombinant cell fitness explored in bacterial colonies

During growth of recombinant bacteria, irregular plasmid partitioning gener ates non-productive, plasmid-free cells whose proportion usually increases in the culture. For Escherichia coli producing engineered beta -galactosida ses, we have shown a coincidence between plasmid stability and the extensio n of white/blue areas within individual colonies on X-gal plates. In this c ontext, a good correlation between plasmid permanence in colonies and param eters accurately describing the dynamics of plasmid-free cell population in liquid cultures has been observed. Moreover, the impact of lacZ gene engin eering and the metabolic burden imposed by the encoded proteins has been ev aluated through plasmid stability by simple image analysis, revealing an en hanced plasmid loss rate as the cells enter into the stationary phase that is modulated by the expression of particular recombinant genes.