The Origin of Plasmid DNA Transfer During Bacterial Conjugation

Abstract

Bacterial conjugation is the process of gene transfer requiring cell-to-cell contact. Such mating systems are widespread in bacteria and have been described in many gram-positive and gram-negative organisms (1–3). However, detailed studies on the mechanism of DNA transfer during conjugation have been carried out only in gram-negative bacteria (4). The conjugation systems studied in gram-negatives are encoded by plasmids, and mediate the transfer of plasmid and, occasionally, chromosomal DNA from the donor to the recipient cell. Although transformation and transduction of plasmids occur, conjugation appears to be the major mechanism of plasmid DNA transfer in nature. In addition, conjugation is the only means of gene-tic exchange between different groups of bacteria, since transformation and transduction are generally limited to exchange between closely related members of the same genus or species. The promiscuous gene transfer mediated by broad host-range plasmids means that the gene pool available to bacteria is immensely larger than if genetic exchange was restricted by species barriers. Therefore, broad host-range conjugation enables bacteria to draw upon a very large genetic reservoir when subjected to evolutionary pressures. The most dramatic clinical example of this exceptional adaptability is the development and spread of antibiotic resistance in pathogenic bacteria.