Conjugation (microbiology)
Introduction
Conjugation in microbiology is a form of horizontal gene transfer that occurs between two living bacteria cells. This process involves the transfer of genetic material from a donor cell to a recipient cell through a direct physical connection known as a conjugation bridge or pilus. The genetic material transferred is often a plasmid - a small, circular piece of DNA that is separate from the chromosomal DNA.
Mechanism of Conjugation
The process of bacterial conjugation can be divided into several steps: contact between the donor and recipient cells, formation of the conjugation bridge, transfer of the plasmid DNA, and finally, separation of the conjugating cells.
Contact and Pilus Formation
The initial contact between the donor and recipient cells is mediated by a special type of pilus known as the sex pilus, which is produced by the donor cell. The sex pilus recognizes and binds to specific receptor molecules on the surface of the recipient cell, drawing the two cells together.
Transfer of Plasmid DNA
Once the conjugation bridge is established, the plasmid DNA in the donor cell is prepared for transfer. The plasmid DNA is nicked at a specific site by an enzyme called relaxase, and one strand of the plasmid DNA is then transferred to the recipient cell through the conjugation bridge. The transferred DNA strand is simultaneously replicated in both the donor and recipient cells, ensuring that both cells end up with a complete copy of the plasmid DNA.
Separation of Conjugating Cells
After the transfer of the plasmid DNA, the conjugation bridge between the donor and recipient cells is broken, and the two cells separate. Each cell now contains a copy of the transferred plasmid DNA, which can confer new traits to the recipient cell, such as antibiotic resistance or the ability to metabolize new substrates.
Types of Conjugation
There are two main types of conjugation in bacteria: F-plasmid conjugation and Hfr conjugation.
F-plasmid Conjugation
F-plasmid conjugation involves the transfer of an F (fertility) plasmid from a donor cell (F+) to a recipient cell (F-). The F plasmid contains genes that allow the bacterium to form a sex pilus and establish a conjugation bridge with a recipient cell. Once the F plasmid is transferred to the recipient cell, it becomes an F+ cell and can now act as a donor cell in future conjugation events.
Hfr Conjugation
Hfr (High frequency of recombination) conjugation involves the transfer of chromosomal DNA from a donor cell to a recipient cell. In Hfr conjugation, the F plasmid is integrated into the chromosomal DNA of the donor cell. During conjugation, a portion of the chromosomal DNA is transferred to the recipient cell along with the F plasmid. This can result in the transfer of large amounts of genetic material and can lead to significant genetic recombination in the recipient cell.
Significance of Conjugation
Conjugation plays a crucial role in bacterial evolution and adaptation. By transferring genetic material between cells, conjugation allows bacteria to rapidly acquire new traits and adapt to changing environments. This process is particularly important in the spread of antibiotic resistance among bacterial populations.