Biomedical Engineering Reference
In-Depth Information
strengths of contributory organisms to optimise their functional isolated potential.
Inserting the bacterial genes responsible for plastic production into Arabidopsis
and Brassica plants, avoids the expense of feed, since photosynthesis naturally
provides the necessary carbon and the metabolic flow of intermediates from fatty
acid and amino acid synthesis is redirected for plastic production. The PHBV
biosynthesised in plant plastids appears to be of a marketable type and quality,
though the yield was relatively low. The process continues to be the subject of
ongoing research with the archaeon, Haloferax mediterranei , becoming one of
the most promising candidates for commercial production of these plastics (Liu
et al ., 2009).
Clearly, when it becomes a realistic and widespread industrial proposition,
returning to the intervention triangle model to consider the agricultural benefits
of specifically environmental biotechnology, it will represent a major advance
in both 'clean' production and pollution control. While work continues on the
former, in many ways the latter is already achievable.
Microbial pesticides
Chemical pesticides are problematic for many reasons. Firstly, although some
degradation occurs as described in Chapter 4, pesticides are notoriously recalci-
trant and consequently their use may lead to a build up of chemicals damaging to
the environment. This is an increasing problem with the intensive drive towards
more and more cost effective crop production. Secondly, insects are known
to develop resistance to pesticides and so new and in some ways, more poi-
sonous chemicals might be introduced to maintain the same level of effectiveness.
Thirdly, chemical pesticides are rarely targeted to specific problematic species
and may kill other organisms of no harm or even of some benefit to the crop
plants. Balanced natural environments have an equilibrium between assailant and
victim, however it may take a commercially unacceptable time for this balance
to establish, sometimes incurring quite extreme swings in either direction. For
example, one season may see a flourishing of citrus trees due to an outbreak of
disease leading to a dearth of swallowtail butterflies, the caterpillars of which
feed voraciously on citrus. The lack of insect host reduces the level of infection,
leading to a recovery of the butterfly population the following year and, conse-
quently, seriously damaged citrus trees. One means by which insect numbers are
controlled in nature is by bacteria which produce toxins killing the insect which
consumes it. Although this may serve to create the balance described above, it
may not be sufficient to satisfy commercial crop production. Sufficient time may
elapse between ingestion of the toxin by the larva and its ensuing death, for it to
have caused considerable damage by feeding on the crop. Perhaps the best stud-
ied pesticidal bacterial toxin is the δ -endotoxin from Bacillus thuringiensis .This
protein, frequently abbreviated to 'Bt toxin', is active against some members of
the Lepidoptera (butterflies and moths), Diptera (flies, midges and mosquitoes)
and Coleoptera (beetles) families and has been used in its native, unmodified
form as a pesticide for many years. There are several strains of the bacterium
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