Function of plasmids

The best known of the E. coli plasmids is the F’ plasmid which confers the ability of a cell to transfer this plasmid by conjugation. The genes that govern transfer are known as tra genes, and take up about one-third of the F¢ plasmid’s 99 159 bp. These genes code for proteins with a variety of functions including enablement of the movement of DNA and the rolling circle replication associated with it. The F¢ plasmid confers a very specific property on E. coli, but some plasmids carry genes that confer heavy metal resistance, while others can carry entire metabolic pathways to deal with unusual carbon sources. The plasmids of most concern in medical microbiology encode toxins, virulence factors or antibiotics. For example, enteropathogenic E. coli colonize the small intestine and produce toxins that cause diarrhea and hemolysis in humans. Both the protein that enables attachment to the intestine wall and the toxins themselves are plasmid encoded.

The current model for plasmid ecology is that a great diversity of genes is held extrachromosomally in any indigenous bacterial population. There is constant exchange of genetic material, but the majority of cells do not retain the plasmids acquired after a few cell divisions. When the population is stressed, only those bacteria carrying the plasmid-borne genes essential to deal with that stress survive and quickly divide to outnumber plasmid- free organisms. There is some transfer between members of the same species, but promiscuous plasmids may be transferred between cells of different species. This horizontal gene transfer has been noted in the spread of antibiotic and mercury resistance.