Transfer of newly synthesized plasmid DNA through intercellular bridge
Figure 2-8 Genetic recombination (A). The mechanisms of gene exchange between bacteria: transformation (B), transduction (C), and conjugational transfer of chromosomal (D) and plasmid (E) DNA.
As just mentioned, an organism's opportunity for undergoing recombination depends on the acquisition of "foreign" DNA from a donor cell. The three mechanisms by which bacteria physically exchange
DNA include transformation, transduction, and conjugation.
Transformation. Transformation involves recipient cell uptake of free DNA released into the environment
Figure 2-9 Photomicrograph of Escherichia coli sex pilus between donor and recipient ceil. (Courtesy C Brlnton, From Brock TD et al, editors: Biology of microorganisms, Englewood Cliffs, NJ, 1994, Prentice Hall.)
when another bacterial cell (i.e., donor) dies and undergoes lysis (see Figure 2-8, B), This DNA, which had constituted the dead cell's genome,- exists as fragments in the environment. Certain bacteria are able to take up this free DNA, that is, are able to undergo transformation. Such bacteria are said to be competent. Among the bacteria that cause human infections, competence is a characteristic commonly associated with members of the genera Haemophilus, Streptococcus, and Neisseria.
Once the donor DNA usually as a singular strand, gains access to the interior of the recipient cell, recombination with the recipient's homologous DNA can occur. The mixing of DNA between bacteria via transformation and recombination plays a major role in the development of antibiotic resistance and in the dissemination of genes that encode factors essential to an oiganism's ability to cause disease, Additionally, gene exchange by transformation is not limited to organisms of the same species, thus allowing important characteristics to be disseminated to a greater variety of medically important bacteria.
Transduction. Transduction is a second mechanism by which DNA from two bacteria may come together in one cell thus allowing for recombination (see Figure 2-8, C), This process is mediated by viruses that infect bacteria (l.e„ bacteriophages). In their "life cycle" these viruses integrate their DNA into the bacterial cell's chromosome, where viral DNA replication and expression is directed. When the production of viral products is completed, viral DNA is excised (cut) from the bacterial chromosome and packaged within protein coats. The viruses are then released when the infected bacterial cell lyses. In transduction, the virus not only packages its own DNA but may also package a portion of the donor bacterium's DNA.
The bacterial DNA may be randomly incorporated with viral DNA (generalized transduction), or it may only be incorporated along with adjacent viral DNA (specialized transduction). In either case, when the viruses infect another bacterial cell they release their DNA contents, which may include bacterial donor DNA. Therefore, the newly infected cell is the recipient of donor DNA introduced by the infecting bacteriophage and recombination between DNA from two different cells may occur.
Conjugation. The third mechanism of DNA exchange between bacteria is conjugation. This process occurs between two living cells, involves cell-to-cell contact, and requires mobilization of the donor bacterium's chromosome. The nature of intercellular contact is not well characterized in all bacterial species capable of conjugation. However, in E. coli contact is mediated by a sex pilus (Figure 2-9), The sex pilus originates from the donor and establishes a conjugative bridge that serves as the conduit for DNA transfer from donor to recipient cell. With intercellular contact established, chromosomal mobilization is undertaken and involves DNA synthesis. One new DNA strand is produced by the donor and is passed to the recipient, where a strand complementary to the donor strand is Synthesized (see Figure 2-8, D). The amount of DNA transferred depends on how long the cells are able to maintain contact, but usually only portions of the chromosome are transferred. In any case, the newly introduced DNA is then available to recombine with the recipient's chromosome.
In addition to chromosomal DNA, genes encoded in nonchromosomal genetic elements, such as plasmids and transposons, may be transferred by conjugation (see Figure 2-8, E). Not all plasmids are capable of conjugative transfer, but for those that are, the donor plasmid usually is replicated so that the donor retains
O O Plasmids • Transposon
O O Plasmids • Transposon
Potential for subsequent dissemination of plasmids and transposons to a variety of other recipients
Figure 2-10 Pathways for bacterial dissemination of plasmids and transposons, together and independently.
a copy of the plasmid that is being transferred to the recipient. Plasmid DNA may also become incorporated into the host cell's chromosome.
In contrast to plasmids, transposons do not exist independently within the cell. Except when they are moving from one location to another, transposons must be incorporated into the chromosome and/or into plasmids. these elements are often referred to as "jumping genes," because of their ability to change location within, and even between, the genomes of bacterial cells. Transposition is the process by which these genetic elements excise from one genomic location and insert into another. Transposons carry genes whose products help mediate the transpositional process as well as genes that encode for some other characteristic such as antimicrobial resistance. In the cases of both plasmids and transposons, homologous recombination between the genes of these elements and the host bacterium's chromosomal DNA can occur but is not necessary. Therefore, these elements provide an alternative means for those organisms that cannot accommodate recombination to maintain foreign DNA in their genome.
Plasmids and transposons play a key role in genetic diversity and dissemination of genetic information among bacteria. Many characteristics that significantly alter the activities of clinically relevant bacteria are encoded and disseminated on these elements. Furthermore, as shown in Figure 2-10, the variety of strategies that bacteria can use to mix and match genetic elements provides them with a tremendous capacity to genetically adapt to environmental changes, including those imposed by human medical practices. A good example of this is the emergence and widespread dissemination of resistance to antimicrobial agents among clinically important bacteria. Bacteria have used their capacity for disseminating genetic information to establish resistance to most of the commonly used antibiotics (see Chapter 11 for more information regarding antimicrobial resistance mechanisms).
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