In nature, a great deal of DNA exchange occurs by transfer of certain plasmids, called conjugative plasmids, from donor cells to recipient cells. It is possible in this way to transfer a gene that can complement a defective genomic copy of the same gene in the recipient. The best studied conjugative plasmid is the F factor of E. coli. The membranes of the two cells fuse over a small region. The F plasmid is copied by rolling circle replication and delivered into the recipient.
Conjugation is extremely efficient.
If a conjugative plasmid confers a selective advantage, an entire population of bacteria will rapidly acquire it. Conjugative transfer is possible between between harmless bacteria and pathogenic bacteria, often of different genera. Moreover, many conjugative plasmids carry multiple antibiotic resistance genes. Hospitals offer an environment where selection for such conjugative transfers is most likely to occur, since many patients are infected with pathogens, and administration of antibiotics is routine. Bacterial pathogens that acquire conjugative plasmids conferring antibiotic resistances are very difficult to eliminate and can be lethal.