ENHANCED PLASMID MAINTENANCE IN A CSTR UPON SQUARE-WAVE OSCILLATIONS IN THE DILUTION RATE

The effects of forced square-wave perturbations in the dilution rate on plasmid maintenance and gene expression of a population of Escherichia coli K12 strain carrying the vector plasmid pBR322 grown in a chemostat with a non-selective medium were studied. It was observed that in the control experiments, where the dilution rates were kept constant, the percentage of plasmid-containing cells decreased after a period of time. Eventually, the culture was displaced by the plasmid-free cells. However, when the cells were exposed to forced oscillations in the dilution rate, the reactor culture was able to maintain a mixed population of plasmid-free and plasmid-containing cells for a longer period of time. The above observation seems to be independent of the source of the host cells. This is, the same results were obtained when the plasmid-free cells were generated from the culture itself due to defective partitioning of the plasmids or introduced externally.