Training: Manipulation of Bacteria Without Genetic Engineering
A general procedure is to take a sample of bacteria from, at, or near, the site of contamination from which a pure culture is obtained in the laboratory and identified, using standard microbiology techniques. The ‘training’ may be required either to improve the bacterium’s tolerance to the pollutant or to increase the capabilities of pathways already existing in the bacterium to include the ability to degrade the pollutant, or a combination of both. Tolerance may be increased by culturing in growth medium containing increasing concentrations of the pol-lutant so that, over successive generations, the microbe becomes more able to withstand the toxic effects of the contaminant. Reintroduction of these bacteria to the polluted site should give them an advantage over the indigenous bacte-ria as they would be better suited to survive and remediate the contamination. Improving the microbe’s ability to degrade a contaminant, sometimes referred to as catabolic expansion, may be increased by culturing the bacteria in growth medium in which the contaminant supplies an essential part of the nutrition, such as being the only carbon source. Only bacteria which have undergone a mutation enabling them to utilise this food source will be able to survive and so the method effectively selects for the desired microbe; everything else having died.
It has been argued that under laboratory conditions where cultures of bacteria are isolated from each other to prevent cross-contamination, mutations are most likely to occur as a result of an error in DNA replication. This is far less likely to be the most prominent source of mutation in nature, as the microbes are constantly in close proximity with other organisms and, consequently, the opportunity for exchange of genetic material is enormous. In fact the process of DNA replication has a very high fidelity, the reasons for which are obvious. An increased rate of error may be forced upon the organism, speeding up the rate of mutation, by including a mutagen in the growth medium. A mutagen is a chemical which increases the rate of error in DNA replication, often by causing a very limited amount of damage to the DNA such that the DNA polymerase, the enzyme responsible for synthesising DNA, is unable to determine the correct base to add in to the growing nucleotide chain. If the error in the nascent strand cannot be recognised and corrected, the fault becomes permanent and is handed on through the generations.