Genetic Manipulation - Environmental Biotechnology: Theory and Application - page 214

their fitness for that changed environment, and so they may be 'trained' by the
artificially accelerated expansion of pre-existing pathways. The final option is
that they may be genetically engineered. Organisms which represent the 'norm',
frequently being the most abundant members occurring in nature, are described as
'wild type'. Those with DNA which differs from this, are described as mutant.
Alteration can be by the normal processes of evolution which constantly pro-
duces mutants, a process which may be accelerated artificially, or by deliberate
reconstruction of the genome, often by the introduction of a gene novel to that
organism. This latter route is the basis of genetic engineering (GE) which has
several advantages over traditional breeding or selection techniques. The process
is specific, in that one gene, or a selected group of genes, is transferred and so
the mutation is quite precise. There is flexibility in the system in that, depending
on the modifications made to the genome, a new product may be produced or the
level of expression of the existing product or products may be altered in quantity
or proportions to each other. Another advantage often quoted is that GE allows
genes to be transferred between totally unrelated organisms. The preceding dis-
cussion suggests that this is not a phenomenon unique to GE, but it is at least
defined and specific.
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 iden-
tified, 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