Conjugation

Conjugation is a mechanism whereby a bacterium can transfer genetic material to an adjacent bacterium. The genetic transfer requires contact between the two bacteria. This contact is mediated by the bacterial appendage called a pilus.

Conjugation allows bacteria to increase their genetic diversity. Thus, an advantageous genetic trait present in a bacterium is capable of transfer to other bacteria. Without conjugation, the normal bacterial division process does not allow for the sharing of genetic information and, except for mutations that occur, does not allow for the development of genetic diversity.

A pilus is a hollow tube constructed of a particular protein. One end is anchored to the surface of a bacterium. The other end is capable of binding to specific proteins on the surface of another bacterium. A pilus can then act as a portal from the cytoplasm of one bacterium to the cytoplasm of the other bacterium. How the underlying membrane layers form channels to the bacterial cytoplasm is still unclear, although channel formation may involve what is termed a mating pair formation (mpf) apparatus on the bacterial surface.

Nonetheless, once a channel has been formed, transfer of deoxyribonucleic acid (DNA) from one bacterium (the donor) to the other bacterium (the recipient) can occur.

Conjugation requires a set of F (fertility) genes. Transfer of DNA from the genome of a bacterium can occur if the F set of genes is integrated in the bacterial chromosome. These F genes enter the pilus and literally drag the trailing genome along behind. Often the pilus will break before the transfer of the complete genome can occur. Thus, genes that are located in the vicinity of the F genes will tend to be successfully transferred in conjugation more often than genes located far away from the F genes. This process was originally discovered in Escherichia coli. Strains that exhibit a higher than usual tendency to transfer genomic DNA are known as High Frequency of Recombination (Hfr) strains.

Conjugation also involves transfer of DNA that is located on a plasmid. A plasmid that contains the F genes is called the F episome or F plasmid. Other genes on the episome will be transferred very efficiently, since the entire episome can typically be transferred before conjugation is terminated by pilus breakage. If one of the genes codes for a disease causing factor or antibiotic resistance determinant, then episomal conjugation can be a powerful means of spreading the genetic trait through a bacterial population. Indeed, conjugation is the principle means by which bacterial antibiotic resistance is spread.

Finally, conjugation can involve the transfer of only a plasmid containing the F genes. This type of conjugation is also an efficient means of spreading genetic information to other bacteria. In this case, as more bacteria acquire the F genes, the proportion of the population that is capable of genetic transfer via conjugation increases.

Joshua Lederberg discovered the process of conjugation in 1945. He experimented with so-called nutritional mutants (bacteria that required the addition of a specific nutrient to the growth medium). By incubating the nutritional mutants in the presence of bacteria that did not require the nutrient to be added, Lederberg demonstrated that the mutation could be eliminated. Subsequently, another bacteriologist, William Hayes, demonstrated that the acquisition of genetic information occurred in a one-way manner (e.g., information was passing from one bacterium into another), and that the basis for the information transfer was genetic (i.e., mutants were isolated in which the transfer did not occur).

Another landmark experiment in microbiology also centered on conjugation. This experiment is known as the interrupted mating experiment (or blender experiment, since a common kitchen blender was used). Donor and recipient bacteria were mixed together and left to allow conjugation to begin. Then, at various times, the population was vigorously blended. This sheared off the pili that were connected the conjugating bacteria, interrupting the mating process. By analyzing the recipient bacteria for the presence of known genes that has been transferred, the speed of conjugation could be measured.

Conjugation has been exploited in the biotechnology era to permit the transfer of desired genetic information. A target gene can be inserted into the donor bacterial DNA near the F genes. Or, an F plasmid can be constructed in the laboratory and then inserted into a bacterial strain that will function as the donor. When conjugation occurs, bacteria in the recipient population will acquire the target gene.

See also Evolution and evolutionary mechanisms; Laboratory techniques in microbiology