Bacterial conjugation is a mechanism of horizontal gene transfer in which a donor cell transfers DNA directly to a recipient cell through cell–cell contact, typically mediated by a conjugative plasmid.
Explanation
During conjugation the donor cell, often carrying a fertility (F) plasmid or related integrative element, produces a thin sex pilus that attaches to a recipient cell lacking the plasmid. The pilus draws the cells together and forms a mating channel. The plasmid initiates a rolling circle replication in the donor, and a single-stranded copy of the DNA is transferred across to the recipient where it is converted into a double‑stranded form. When the plasmid integrates into the chromosome to create an Hfr (high frequency recombination) strain, chromosomal genes near the integration site can also be mobilized, resulting in recombination in the recipient. Some plasmids carry antibiotic resistance or virulence genes and can transfer between different species, contributing to the spread of multidrug resistance. Conjugation differs from transformation and transduction because it requires direct contact and usually transfers larger DNA segments. Gram-positive bacteria use surface adhesion rather than pili, and conjugative transposons like Tn916 can move between streptococci, enterococci and other genera.
Illustrative cases
Classic experiments with Escherichia coli demonstrated transfer of the F plasmid from F+ donors to F– recipients, converting them into donors. Crosses between Hfr and F– strains have been used to map bacterial chromosomes by timed mating. Conjugative R plasmids spread resistance to β‑lactams and other antibiotics among Enterobacteriaceae. The Tn916 family of conjugative transposons transfers tetracycline resistance between Enterococcus faecalis, Streptococcus pyogenes and Bacillus subtilis. Archaeal plasmids also transfer via conjugation-like mechanisms. In eukaryotes, the term conjugation is used to describe sexual exchange of nuclei between certain ciliates, but the underlying biology differs from bacterial plasmid transfer.
Conjugation plays a pivotal role in shaping bacterial genomes and accelerating the dissemination of adaptive traits. Understanding the molecular machinery of plasmid transfer is essential for controlling the spread of resistance genes and for harnessing conjugative elements in biotechnology.
Related Terms: Horizontal gene transfer, Plasmid, F pilus, Hfr strain, R plasmid