When a mutation occurs in a population of microbes, the total number of cells containing the change may be relatively small (for example, on the order of 1 x 10-10). However, natural and unnatural (artificial) selection may quickly increase the percentage of the population that contains the new gene. Natural selection occurs as a result of differential reproduction.
If a microbe containing the mutation is better able to survive and reproduce than those lacking this change, the mutant will increase naturally in frequency through time. Selection might also affect the frequency of mutants in a mixed culture of penicillin-resistant and susceptible microbes.
By adding penicillin to the culture, the drug will act as a selective agent and rapidly destroy the susceptible cells. In only a few hours, the entire population might be composed of penicillin-resistant microbes.
The combined action of natural and artificial selection on resistant cells has resulted in an increase in drug-resistant pathogens throughout the world. The selection of resistant mutants, however, does not totally explain the emergence of resistant organisms.
Other genetic mechanisms involving the acquisition of new genes play an important role in this phenomenon. Selective agents do not act directly on the genotype of microbes. Selection is based on how the agent interacts with the phenotype of the cell.
If the genes are inactive for any reason, the microbe will be adversely affected by the selective agent. Thus, a mutation that is beneficial to the microbe, but not phenotypically expressed, can be lost from the population.
Since the phenotype of a cell is the result of the expressions of its functioning gene combinations of genes that are generated in a cell population. Any process that results in the integration of new combinations of genes together in a single cell is called genetic recombination.
When fertilization (conjugation) occurs in eukaryotic organisms, genes donated by each of the parents are recombined into a single cell—the zygote. Since unique genes (due to mutations) and gene package (due to crossing-over and independent assortment) are brought together in a single cell (genetic recombination), this new individual will have gene combinations (a genotype) not found in either parent.
In prokaryotes there is no true fusion of cells. However, a portion of a DNA molecule from a donor may be transferred to a recipient cell to form a partial zygote called a merozygote.
The incoming segment of DNA, the exogenote or exttachromosomal, DNA, may join into the recipient DNA, the endogenote, by breakage and reunion. This process results in a recombinant strand, of DNA since the original genes of the recipient have been replaced by the new exogenote genes.
As a result, the genetic makeup of the cell has new genes and new gene combinations that may enable it to survive better in a changed environment. We now know that the exogenote DNA does not always join with the endogenote by genetic recombination, but may remain free in the cytoplasm of the recipient.
This separate loop of DNA may function in the host without replicating. As a result, only one cell in the population will contain the exogenote at any particular time. As the population increases, the genetic uniqueness of this single microbe may be overshadowed by the other cells. However, self- replicating exogenotes may remain in the population indefinitely.
These separate loops of genetic material replicate on their own and are distributed to the newly forming cells during binary fission in which the same manner as the host chromosomal genes. Cells which contain functional self-replicating exogenotes demonstrate a great amount of genetic variation not seen in cells that lack these extra genes.
The possession of these genes may be a selective advantage because of increased genetic variety. It is also an advantage to be able to transfer exogenote and endogenote genes to other cells to maintain this variety.
The three methods of transferring exogenote and endogenote DNA that have been identified in bacteria are transformation, conjugation and transduction.