Bacterial conjugation is the exchange of genetic material by direct contact. It is one of the methods of horizontal gene transfer.
Unlike transduction and transformation, conjugation involves cellular contact. It is controlled by the donor plasmid. This is called plasmid mediated conjugation.
Conjugation involves delivering a plasmid to a recipient cell. Plasmids tend to contain genes that increase survival in hostile environments.
These genes can confer antibiotic resistance or can allow the bacteria to metabolise different carbon sources.
All three types of horizontal gene transfer increase genetic variation through a population. Conjugation is the most effective type of transfer for this purpose.
This is because transduction and transformation involve the transfer of random sections of the bacterial genome.
Conjugation only involves the transfer of genes that tend to be highly advantageous for the organism receiving them.
Conjugation is controlled by plasmids. Only plasmids with specific genes can undergo conjugation. Conjugation occurs in several steps.
- The donor cell has protein filaments on the outside of the cell called sex pili. These sex pili (singular pilus) seek out a recipient cell and attach to it.
- The attachment of the sex pili causes the two cells to fuse, creating a cytoplasmic bridge called the conjugation tube.
- At this point, the plasmid DNA is replicated and passes through the conjugation tube, into the recipient cell.
This completes the transfer and the cells separate.
Not all plasmids can undergo horizontal gene transfer by conjugation. Only plasmids that carry genes to produce the required proteins can do this.
The best-characterised horizontally transmitted plasmid is the F (fertility factor) plasmid.
The F plasmid is a large plasmid found in E. coli. It carries genes to make sex pili and other structures for conjugation.
The R factor plasmid is another large plasmid that can undergo conjugation.
R factor plasmids typically carry genes for antibiotic resistance and so pose a threat to human health. The R plasmid can carry genes encoding for up to 8 antibiotic resistances.
Horizontal gene transfer is responsible for widespread antibiotic resistance.
M. tuberculosis, the bacterium that causes tuberculosis (TB), is becoming increasingly resistant to antibiotic drugs.
In 2003, the first totally drug resistant strain of TB was discovered. As of 2012, this strain has become widespread.
Of the three types of horizontal gene transfer, bacterial conjugation has been the most effective at spreading antibiotic resistance.
Horizontal gene transfer has also been very useful in the field of genetics. Bacterial plasmids allowed the first genes to be cloned.