ABSTRACT

Plasmids are circular pieces of DNA found almost ubiquitously in bacteria. Plasmids often carry genes which are favourable for the survival of bacteria in adverse environments, where selection pressure would increase the likelihood of their transfer, for example catabolic genes and antibiotic and heavy metal resistance genes (Stotzky and Babich, 1986; Trevors et al., 1987; Sayler et al., 1990). Conjugation is the direct transfer of genetic information, in the form of plasmids, between bacterial cells and is dependent on cell-to-cell contact. Self-transmissible (conjugative) plasmids can be readily isolated from bacteria in most environments (Sizemore and Colwell, 1977; Stotzky and Babich, 1986; Hermansson et al., 1987; Trevors et al., 1987; Fredrickson et al., 1988; Genthner et al., 1988; Fry and Day, 1990; Sayler et al., 1990; Dahlberg et al., 1998a; 1998b). Conjugal transfer of plasmids occurs between closely related bacterial strains as well as between diverse genera (for reviews see Stotzky and Babich, 1986; Trevors et al., 1987; Trevors and Oddie, 1986; Ippen-Ihler, 1989; Mazodier and Davies, 1991; Veal et al., 1992). Cells into which plasmids have self-transferred (i.e. conjugated) are called transconjugants. There is evidence of conjugation occurring among bacteria in such diverse environments as the gastrointestinal tract of warm blooded animals, the human urinary and respiratory tracts, wounds, on plant surfaces, and in soil, water and sewage (Stotzky and Babich, 1986). As a result, plasmid transfer may contribute greatly to gene transfer in natural environments and to the adaptability of microorganisms to environmental stress.