The Maintenance of Plasmid-containing Organisms in Populations of Escherichia Coli

Populations of Escherichia coli containing a small non-conjugative plasmid were grown in carbon-limited continuous culture. For all plasmids tested the presence of the plasmid lowered the growth rate of the host bacterium, and the proportion of plasmid-containing organisms in the total population declined initially. However, periodically, adaptive changes occurred in plasmid-containing organisms which increased their growth rate. This resulted in oscillations in the proportion of plasmid-containing organisms, and the delayed loss of the plasmid from the population.