Plasmid Dissemination in Bacterial Populations

Certain bacterial species can achieve a state of natural competence for the uptake of naked plasmid DNA (transformation) (62), or can acquire DNA that has been packaged into a bacteriophage head and is injected into the host (transduction) (63). However, the conjugative transfer of DNA between donor and recipient cells is prob­ably the most common mechanism by which plasmids are disseminated in bacterial populations (64,65). A wide variety of phenotypes can be conferred by conjugative plasmids, including antibiotic resistance, bacteriocin production and immunity, and catabolic functions.

Conjugative plasmids have been identified in most major groups of Eubacteria, and more recently in Archaea (66). Furthermore, conjugative plasmid transfer is not lim­ited to closely related bacteria but has also been demonstrated between evolutionary-divergent Gram-negative and Gram-positive Eubacteria (67), and from Eubacteria to yeast (68). The T-DNA region of the Ti virulence plasmid of the Gram-negative bac­terium Agrobacterium tumefaciens is also transferred by a conjugation-like process to susceptible plant hosts, where it integrates in the plant genome and induces the forma­tion of crown gall tumors (69).