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.
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