Evolution in bacterial-plasmid populations of Escherichia coli.

Abstract

This thesis describes and analyzes evolutionary changes occurring in bacterial-plasmid populations. The reproductive fitness of plasmid-containing strains isolated from two independent long-term chemostat populations grown in a glucose-limited environment increased dramatically over a period of approximately 800 generations. Genetic changes associated with the increase in reproductive fitness were shown to be associated with both the bacterial chromosome and the plasmid. In addition, some of the genetic changes in the bacterial chromosome and plasmid interact and thus may be considered to be co-evolutionary. The structural basis for some of these observed adaptive genetic changes in the evolved plasmid-containing strains were analyzed. Specifically in two independent long term populations, DNA derived from plasmid, and including the transposable element, TN3, was shown to be integrated between 12$\sp\prime$ and 14$\sp\prime$ in the bacterial chromosome. The integration of the transposable element resulted in an increase in the reproductive fitness of the bacterial chromosome. In one long-term population, a spontaneous deletion of about 2.2 Kb in the plasmid, encompassing the tetracycline resistance gene was identified. The change associated with the deletion in the plasmid resulted in a net increase in fitness. That is, the deleterious effect of the plasmid with the deletion, was smaller than that of the plasmid without the deletion. The role of adaptive genetic changes in the microbial genomes is discussed.