Several alternative strategies have been developed to minimize the risk that plasmid-free cells overtake the culture. One of the most promising of those strategies relies on the use of postsegretional killing genes (so called poison / antidote genes or selection modules) that induce host killing upon plasmid loss.
Delphi Genetics has designed a novel and highly effective stabilization system, called Staby®, based on the use of selection modules naturally found in plasmids, bacterial chromosomes and bacteriophages. A selection module (like the ccd system used by Delphi Genetics) is typically organized as an operon and composed of two genes: the ccdB selection gene codes for a small stable protein which is toxic for E. coli, whereas the ccdA antidote gene codes for a small unstable protein that neutralizes the toxic protein both transcriptionaly and via protein-protein interactions.
This unique system allows for the perfect stabilization of the plasmid without the use of antibiotics