RP4 plasmid (also known as RK2, RP1, and the Birmingham plasmid) stands not only as a model of bacterial conjugation studied over the past 40 years, but also as one of the most conspicuous, broad-host range conjugative plasmids described in the literature. It mediates mating and plasmid transfer between a wide variety of Gram– donors/recipients (8) and is also capable of efficiently conjugating with Gram+, (9) yeast (10,11) and mammalian cells. (12)

Some conjugation systems, such as IncF, IncH, and IncI plasmids, transfer DNA efficiently in liquid media, while others, including the IncN, IncM, IncP, and IncW plasmids, achieve higher DNA transfer frequencies on solid media [28]. It is suspected that the ability to transfer DNA in different environmental conditions is related to variation in pilus formation, structure, and stability of cells during the conjugation process.