The effect of a plasmid on growth and survival of E. coli

We studied the growth characteristics of a pair of Escherichia coli strains, isogenic apart from the possession of a nonconjugative plasmid. There was no difference between the two strains when they were grown separately. In mixed culture, a second slow phase of growth that normally occurred following the end of rapid exponential growth, was absent from the plasmid-carrying strain. This resulted in a considerable decrease in the proportion of the cells that carried the plasmid after overnight incubation. The effect of different conditions of growth is reported. The plasmid-carrying strain survived extended incubation (150 days at 37 degrees C) as well as did the plasmid free strain separately. In a mixture, the proportion of plasmid-carrying cells declined rapidly, and none was detected after 100 days.