Conjugation is a mode of horizontal gene transfer in bacteria and some protists in which genetic material is transferred from a donor to a recipient cell through direct physical contact.
Explanation
Bacterial conjugation was first described in 1946 by Joshua Lederberg and Edward Tatum, who demonstrated that genetic traits could be exchanged between Escherichia coli cells. Unlike transformation, which involves uptake of free DNA, or transduction, which uses bacteriophages, conjugation requires contact between living cells. In Gram-negative bacteria such as E. coli, the donor cell harbors a conjugative plasmid, typically an F (fertility) plasmid, which encodes genes for pilus formation and DNA transfer. The pilus attaches to the surface of a recipient cell lacking the plasmid and contracts to bring the cells together. A single strand of the plasmid DNA is then nicked at the origin of transfer, transferred to the recipient via a type IV secretion system, and replicated in both cells to produce double-stranded DNA, resulting in two plasmid-bearing cells.
Conjugation can also involve chromosomal DNA transfer when the F plasmid integrates into the bacterial chromosome, creating an Hfr (high frequency recombination) strain. During mating, portions of the donor chromosome adjacent to the integration site are transferred and recombined into the recipient genome, introducing new alleles. Gram‑positive bacteria such as Enterococcus and Bacillus use pheromone‑responsive or surface proteins to mediate conjugation, often transferring antibiotic resistance genes. Conjugation is not limited to bacteria; ciliates like Paramecium exchange micronuclear material during sexual reproduction, although the mechanisms differ from bacterial plasmid transfer. Because conjugation spreads genes across species and genera, it has major implications for the evolution of pathogens and the dissemination of antibiotic resistance.
Mechanisms and Examples
In the F plasmid system of E. coli, F+ donor cells form a thin, flexible sex pilus that contacts F− recipients. Once mating pairs form, the relaxosome complex initiates a single‑strand nick at the origin of transfer, and the transferosome transports the DNA strand into the recipient. After replication, both cells possess the F plasmid and are capable of further conjugation. Hfr strains generated by F plasmid integration can transfer chromosomal loci sequentially; mapping the time of transfer allowed early genetic mapping of the E. coli chromosome.
Resistance plasmids in pathogens such as Salmonella, Shigella and Klebsiella often carry multiple drug‑resistance genes and conjugation machinery, facilitating rapid spread in hospitals and agriculture. In Gram‑positive bacteria, conjugation frequently occurs via aggregation substances; Enterococcus faecalis responds to peptide pheromones secreted by plasmid‑free recipients by expressing surface adhesins that promote cell clumping and plasmid transfer. Agrobacterium tumefaciens uses a conjugation‑like mechanism to transfer T‑DNA from its Ti plasmid into plant cells, causing crown gall disease and serving as a tool in plant genetic engineering. These examples illustrate the versatility of conjugative systems and their impact on microbial ecology.
Conjugation is a fundamental process of horizontal gene transfer that allows bacteria and some eukaryotes to exchange genetic material through direct contact. By mediating the spread of plasmids and chromosomal genes, conjugation contributes to genetic diversity, adaptation, and the emergence of traits such as antibiotic resistance. Understanding conjugative mechanisms is essential for controlling pathogen evolution and harnessing conjugation for biotechnology.
Related Terms: Horizontal gene transfer, F plasmid, Transformation, Transduction, Plasmid