Question
Conjugation
Answer
Conjugation is a mechanism of horizontal gene transfer in which genetic material (typically a plasmid, e.g., the F plasmid or an R plasmid) passes from a donor to a recipient bacterial cell through direct cell-to-cell contact, mediated by a specialized surface appendage called the sex pilus (F pilus).
Mechanism: a donor cell carrying the fertility (F) plasmid (F⁺ cell) extends its sex pilus to make contact with and draw a recipient (F⁻) cell close; a conjugation bridge/pore forms between the two cells; a single strand of the plasmid DNA is nicked at the origin of transfer and transferred through the bridge into the recipient, where it is replicated to double-stranded form — the recipient is thereby converted into an F⁺ cell (in simple F-plasmid transfer, the bacterial chromosome itself is not transferred).
Hfr conjugation: when the F plasmid integrates into the bacterial chromosome (forming an Hfr — high frequency of recombination — strain), conjugation can transfer part of the donor’s chromosomal DNA along with it, allowing chromosomal gene transfer and recombination in the recipient. Since transfer of the entire chromosome takes considerably longer than the connection typically persists, only genes near the origin of transfer are usually transmitted, with the probability of transfer decreasing for genes farther along the chromosome — a property historically exploited to map bacterial chromosomes by timed interruption of mating.
Clinical significance: conjugation is the principal mechanism by which R (resistance) plasmids, carrying multiple antibiotic-resistance genes, spread rapidly between bacteria — even across different genera — driving the emergence and dissemination of multidrug-resistant organisms, a major concern in hospital-acquired infections. Because a single conjugative plasmid can carry resistance genes to several unrelated antibiotic classes simultaneously, a single conjugation event can confer multidrug resistance on a previously susceptible recipient bacterium.