In eukaryotic organisms, genetic diversity is achieved by sexual reproduction, which allows the mixing of genomes through genetic exchange. Bacteria multiply by simple cell division in which two daughter cells result by division of one parent cell, and each daughter cell receives the full and identical genetic complement contained in the original parent cell. This process does not allow for the mixing of genes from other cells and leaves no means of achieving genetic diversity among bacterial progeny. Without genetic diversity and change, the essential ingredients for evolution are lost. However, because bacteria have been on earth for billions of years and microbiologists are witnesses to their ability to change on an all too frequent a basis, it is evident that these organisms are fully capable of evolving and changing.
Genetic change in bacteria is accomplished by three basic mechanisms: mutation, genetic recombination, and gene exchange between bacteria, with or without recombination. Throughout diagnostic microbiology and infectious diseases, there are numerous examples of the impact these genetic change and exchange mechanisms have on clinically relevant bacteria and the management of the infections they cause.
Mutation is defined as a change in the original nucleotide sequence of a gene or genes within an organism's genome, that is, a change in the organism's genotype. This change may involve a single DNA base within a gene, an entire gene, or several genes. Mutational changes in the sequence may arise spontaneously, perhaps by an error made during DNA replication. Alternatively, mutations may be induced by chemical or physical factors (i.e., mutagens) in the environment, or by biologic factors such as the introduction of foreign DNA into the cell. Changes in the DNA base sequence can result in changes in the base sequence of mRNA codons during transcription. This, in turn, can affect the types and sequences of amino acids that will be incorporated in proteins during translation.
Various outcomes may result from a mutation and are dependent on the site and extent of the mutation. For example, a mutation may be so devastating that it is lethal to the organism; the mutation therefore "dies" along with the organism. In other instances the mutation may be silent so that no changes in the organism's observable properties (i.e., the organism's phenotype) are detected. Alternatively, the mutation may result in a noticeable change in the organism's phenotype and the change may provide the organism with a survival advantage. This outcome, in Darwinian terms, is the basis for prolonged survival and evolution. Nonlethal mutations are considered stable if they are passed on from one generation to another as an integral part of the cell's genotype (i.e„ genetic makeup). Additionally, genes that have undergone stable mutations may also be transferred to other bacteria by one of the mechanisms of gene exchange. In other instances, the mutation may be lost through repair mechanisms in the cell that restore the original phenotype and genotype, or be lost spontaneously during subsequent cycles of DNA replication.
Besides mutations, bacterial genotypes can be changed through recombination. In this process, some segment of DNA that originated from one bacterial cell (Le., donor) enters a second bacterial cell (i.e., recipient) and is exchanged with a DNA segment of the recipient's genome. This is also referred to as homologous recombination because the pieces of DNA that are exchanged usually have extensive homology or similarities in their nucleotide sequences. Recombination involves a number of binding proteins, with the RecA protein playing a central role (Figure 2-8, ^4). After recombination, the recipient DNA consists of one original unchanged strand and the second from the donor DNA fragment that has been recombined.
Recombination is a molecular event that occurs frequently in many varieties of bacteria, including most of the clinically relevant species, and may involve any portion of the organism's genome. However, the recombinational event may go unnoticed unless the exchange of DNA results in a distinct alteration in the phenotype. Nonetheless, recombination is a major means by which bacteria may achieve genetic diversity and change.
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