Stability of ColE1-like and pBR322-like plasmids in Escherichia coli

The average copy number, the level of ampicillin resistance conferred by one plasmid, and the degree of plasmid multimerization were determined for several ColE1-like and pBR322-like plasmids. From the results obtained, the variance of the units of partition corresponding to each plasmid studied was calculated. Experimentally determined plasmid stability was compared with that calculated using the variance of the units of partition and the ratio between the generation times of plasmid-free and of plasmid-carrying cells, assuming that the units of partition are distributed randomly between daughter cells. Stability of the pBR322-like plasmids present mainly as monomers in the bacterial host was consistent with random partitioning, whereas pBR322-like plasmids, present mainly as dimers, and the ColE1-like plasmid showed greater stability than that predicted with random partitioning at cell division.