Population dynamics of a recombinant culture in a chemostat under prolonged cultivation

The dynamics of a chemostat culture of Escherichia coli K12 harboring plasmid pBR322 under prolonged cultivation with a nonselective complex medium were studied. The ability of the culture to form colonies on plates supplemented with different ampicillin concentrations was monitored. It was observed that almost all cells sampled were able to grow on a high concentration of ampicillin at the beginning of the experiment. However, a subpopulation which formed colonies on intermediate-concentration (500-1000 mg/L) plates, but not on a high-concentration (2000 mg/L) plate, was detected just before the appearance of the plasmid-free cells. As time progressed, the percentage of this subpopulation increased, reached a maximum, then decreased toward the end of experiment. At this time the culture was dominated by a subpopulation which could not form colonies on the 100 mg/L ampicillin plates. These results indicate that three major processes may occur in the chemostat: a gradual shift of the higher plasmid copy number population toward a relatively lower copy number population; the complete shedding of the plasmid due to faulty segregation of plasmids during cell division; and growth competition among the subpopulations. A previously derived model is extended to account for all subpopulations. The model agrees qualitatively with the experimental results.