The transfer of chromosomal genes through bacterial conjugation in Escherichia coli

Evolution in bacteria occurs through the combined effects of spontaneous mutations and horizontal gene transfer (HGT). Several mechanisms can lead to HGT: (i) transformation, the uptake of DNA from the environment; (ii) transduction, the transfer of DNA carried by a bacteriophage into another bacterium during infection; and (iii) conjugation, bacterial mating mediated by a conjugative plasmid like the F-factor. HGT through conjugation can lead to the transfer of resistance and virulence genes, which often reside on conjugative plasmids. Conjugation can occur both within a species or between different species. The F-factor plasmid may sometimes integrate into the chromosome by recombination if there is homology between IS elements on the plasmid and the chromosome. Cells with an integrated F-factor can transfer chromosomal DNA with high efficiency and are called Hfr-cells. There are two clinical pathogens (Klebsiella pneumoniae ST258 and Escherichia coli ST1193) that are highly successful (pathogenicity-wise) and thought to originate through the transfer of chromosomal DNA via conjugation creating unique strains with hybrid chromosomes. Our question was how frequently the transfer of chromosomal DNA occurs when we use clinical isolates of E. coli with a plasmid and mate them with an E. coli recipient. We hypothesized that any conjugative plasmid might integrate into the chromosome, thus creatingHfr-cells with the potential for transfer of chromosomal DNA to create hybrid strains. Our prediction was that some clinical isolates should be able to transfer chromosomal DNA to another bacterial strain. By plating conjugation mixtures on selective medium where neither donor nor recipient could grow, we were able to isolate 15 possible transconjugants with hybridgenotypes occurring at a frequency of ~10-10