Bacterial Conjugation

Bacterial conjugation is the transfer of DNA from one bacterial cell to another through direct contact. It is a form of horizontal gene transfer, meaning that genes move between cells that are not parent and offspring, and it is a main route by which antibiotic resistance spreads.

Joshua Lederberg and Edward Tatum found in 1946 that two strains of Escherichia coli could exchange genes. In the best-studied case, a donor cell carries the F ("fertility") plasmid, which encodes a thin tube called a pilus. The pilus connects to a recipient, an enzyme nicks one strand of the plasmid at its origin of transfer, and a single strand is fed through a channel, a type IV secretion system, into the recipient. Each cell then builds the complementary strand, so both end up with a full plasmid.

When the F plasmid integrates into the bacterial chromosome, the cell becomes an Hfr (high-frequency recombination) strain and can pull chromosomal genes into a recipient in a fixed order. The recipient's DNA can then recombine with the donor's, a fact that was used to map the order of genes on the E. coli chromosome by timing how long mating lasted. The process is a counterpart to the sexual exchange of genes in other organisms.

Plasmids that spread by conjugation often carry genes for antimicrobial resistance, so a resistant bacterium can pass its defenses to unrelated species, including Gram-negative bacteria in hospitals. Scientists also use conjugation to move genes between species in genetic engineering. Together with transformation and transduction by bacteriophage, it shapes how bacteria evolve.

• Plasmid — the DNA that is usually transferred
• Horizontal Gene Transfer — the broader process
• Antimicrobial Resistance — a problem it spreads