Plants were the original candidates for genetic studies, which began in the late 1800’s. Studies with animals soon followed; bacteria did not become candidates for such study until the mid-1940’s, when adequate technology for handling bacteria developed. Bacteria have become extremely useful organisms for genetic studies since the early 1950’s.
Two major features of bacteria make them desirable subjects. First, bacterial cells typically divide every twenty minutes. Their rapid rate of reproduction allows a very large number of bacteria to be produced in a short time. This, in turn, provides the researcher with more opportunity to detect the "rare genetic events" of mutation or recombination.
Even more important, unlike all other organisms, bacteria have a single chromosome with a single set of genes. Thus, genetic modifications are more likely to result in immediately observable changes. In organisms that have multiple chromosomes, a change in a single gene may go undetected because its effect is masked by genes on other chromosomes.
All bacteria have a single circular chromosome, composed of deoxyribonucleic acid (DNA). The DNA is subdivided into specific message areas known as genes, and the chromosome carries from four thousand to five thousand individual genes. For many bacteria, this constitutes the entirety of its genetic information.
A number of bacteria, however, have additional DNA in the form of plasmids. A plasmid is a small additional circular piece of DNA, independent of the chromosome,which can hold an additional twenty to one hundred genes. Plasmid-containing cells often have several plasmids.
Many researchers have described the plasmid genes as nonessential to the normal activities of bacteria. Under certain circumstances, however, those genes might provide a survival advantage to the possessor. For example, genes for antibiotic resistance are often carried on a plasmid.
Normally, antibiotics are not present in the bacteria’s environment; such resistance genes would therefore be unnecessary. If the bacteria later were to come into contact with antibiotics, however, having antibiotic- resistant genes would be to their distinct advantage.
Two major types of plasmids exist: F plasmids, or fertility plasmids, and R plasmids, or resistance plasmids. Both types can carry resistance genes. Only the F plasmids, however, are able to control the formation of a special cytoplasmic tube known as the sex pilus. Cells with the F plasmids are known as F+, or donor cells. Cells without the F plasmids are called F-, or recipient cells.
The plasmid is a prerequisite to one type of genetic exchange, conjugation. During conjugation, the donor cell copies its plasmids and transfers them to a recipient cell to which it has attached itself by means of a sex pilus.
The recipient cell can now take advantage of whatever additional genes it has received. If, in the process, it received an F plasmid, it has also become a potential donor cell.Whenever bacterial cells undergo cell division, any plasmids they possess are typically passed on to their progeny.
Originally it was thought that conjugation could occur only between members of the same species, but that is not always true. For example, it is now known that some strains of the bacteria responsible for causing gonorrhea, Neisseria gonorrhoeae, have received antibiotic-resistant genes from unrelated species of bacteria.
There is one other type of donor cell, the Hfr+, or high-frequency recombinant, cell. Instead of the plasmid remaining independent of the cell’s chromosome, it inserts itself into the chromosome. When that plasmid gets ready to copy itself, the chromosomal genes are the first to be copied.
Unless the donor and recipient cells are able to maintain direct contact for a fairly long period of time, which almost never occurs, the recipient cell will not receive the plasmid. It will, however, receive numerous chromosomal genes from the donor. Those genes may later be incorporated into the chromosome of the recipient, causing gene replacement.
Not all species of bacteria participate in conjugation. Some rely on transduction as a means of receiving new genetic information. This is how Staphylococcus aureus has developed resistance to many antibiotics.
In generalized transduction, a bacteriophage infects a bacterial cell. Shortly after infection, the bacterial chromosome becomes fragmented, and viral components are produced. Later the viral components are assembled to form a complete virus particle.
Occasionally during this assembly process, a particle becomes contaminated with fragments of the bacterial chromosome or plasmids. After assembly is completed, the bacterial cell ruptures, allowing the escape of all virus particles.
Eventually these virus particles will invade other bacterial cells. Any cells that are invaded by contaminated bacteriophage particles are said to be transduced, because they have received DNA from another bacterium. The DNA received in this manner is strictly random.
Specialized transduction involves what is known as a latent bacteriophage. After the initial invasion of a bacterium, the bacteriophage inserts itself into a specific region of that cell’s chromosome. At some later time, the bacteriophage removes itself from the chromosome and accidentally takes a few bacterial genes located near its original insertion point.
When the bacterial cell finally begins making new bacteriophage components, it behaves as if those particular genes are part of the bacteriophage and replicates them as such. Therefore, all the newly formed bacteriophage particles will contain those bacterial genes. Transduction then occurs when these bacteriophage particles invade other bacterial cells.
Transformation also occurs in a wide variety of other bacteria. The process of transformation requires that a population of actively reproducing bacteria come into contact with DNA fragments, often from closely related dead bacteria. These DNA fragments are referred to as either naked or cell-free DNA.
A small portion of that DNA can be absorbed and utilized by the growing bacteria. These recipients can then take advantage of any usable genes that the fragments might contain, incorporating them into their chromosome in place of their own copies of these genes by the process of recombination.
Conjugation, transduction, and transformation are all mechanisms of genetic change within a bacterial population. These mechanisms allow a specific characteristic to be spread throughout the population within a few hours.
A wide number of bacterial genes have been found to be transferred by these methods, including genes that control a bacterium’s ability to cause disease, to produce toxins, and to develop resistance to antibiotics and other drugs as well as genes that control a number of other characteristics.
The purpose of these mechanisms, as far as the bacteria are concerned, is to enable the bacteria to adapt to changing environmental conditions so that their survival is ensured. Scientists, however, have found ways to adapt some of these mechanisms for human benefit.
Scientists have used the mechanisms of genetic transfer along with new technology from DNA research to perform genetic engineering on bacteria. They can use genes and specially engineered plasmids, called plasmid vectors, to make recombinant DNA in the laboratory.
Recombinant plasmids can then be used to transform bacteria such as Escherichia coli (E. coli). The bacteria will treat these recombinant plasmids just like ordinary plasmids, replicating them and, for expression vectors, expressing any genes included in them.
In this manner, bacteria can be used to produce a wide variety of products for medicine, agriculture, and industry. Genetic engineering and the products that result from it would not be possible without the knowledge of genetic transfer gained from studies of bacterial conjugation, transduction, and transformation.