Conjugation

Conjugation is a form of horizontal gene transfer that primarily occurs in bacteria and some archaea, promoting genetic diversity and adaptation. Bacteria can acquire resistance genes through conjugative plasmids, allowing them to survive antibiotic treatments that would otherwise be lethal. This process involves direct contact between cells through specialized structures such as the sex pilus and is mediated by conjugative plasmids, including the F (fertility) factor.

Conjugation requires different mating types. The donor cell, termed F⁺, harbors the conjugative plasmid, while the recipient cell, designated F⁻, lacks it. The F stands for fertility, and the plasmid is called the F factor, as in E. coli, which is a key element containing genes responsible for pilus formation and the transfer of genetic material. The sex pilus, encoded by the F factor, establishes a bridge between the donor and recipient cells, initiating the transfer process.

During conjugation, the sex pilus initiates contact between the F⁺ donor and F⁻ recipient. After pilus retraction, a mating bridge forms, allowing a single strand of plasmid DNA to transfer from the donor to the recipient. The transferred strand is then replicated within both cells, resulting in two F⁺ cells, each containing a complete plasmid. This transfer is non-reciprocal, meaning genetic material flows unidirectionally from donor to recipient.

Sometimes, the F factor integrates into the bacterial chromosome; the donor cell is termed a high-frequency recombinant or Hfr strain. Hfr strains facilitate the transfer of chromosomal genes along with plasmid genes during conjugation. However, the transfer is often incomplete due to the time-dependent nature of the process, leaving the recipient as an F-cell with newly acquired chromosomal genes.

F' plasmids arise due to errors when the F factor excises from the chromosome, carrying adjacent chromosomal genes. During conjugation between an F' donor and an F⁻ recipient, the transferred plasmid introduces both chromosomal and plasmid genes, rendering the recipient partially diploid for certain genetic loci. This mechanism enhances genetic variation and can contribute to the spread of advantageous traits, such as antibiotic resistance.