Biomedical Engineering Reference
In-Depth Information
conjugation, but they are limited by plasmid compatibility and the survival of the plasmid in the
organism. MGEs that integrate into the chromosome, which also may include plasmids and
transposons carried on plasmids, have a better chance of staying in the organism and being
propagated. Finally, once the genes are in the host, there still remains the problem of gene
expression and successful protein folding. A plasmid may have a large host range, but still have
low expression of gene product (Kiesel et al.,
2007
).
The benefit of incorporating genes directly into indigenous microorganisms is that they are
already adapted for survival in that environment and there is no need for the inoculated host
bacteria to survive any longer than is necessary for gene transfer. There are a number of
examples of successful plasmid transfers for degradation of pollutants in the laboratory
(Top et al.,
1998
; Desaint et al.,
2003
; Bathe et al.,
2005
; Nancharaiah et al.,
2008
). However,
this procedure conceivably could lead to the unmitigated spread of the gene if no control is
engineered into the system. On the other hand, the genes might naturally be eliminated after the
pollutant is degraded and the selective pressure for the genes is removed.
Evidence of transposons and other MGEs abound in bacterial genomes (Springael and Top,
2004
;Shintanietal.,
2005
). The addition of specific MGEs simply accelerates the natural process of
evolution (directing the content of the MGE such that there is pollutant degradation). Still, under
current regulations and definitions,theuseofgenebioaugmentation comes under the same rulings
as GEMs. In the United States, under the USEPA's Toxic Substances Control Act (TSCA), the use of
“new” microorganisms must be reported to the USEPA (USEPA,
1997
).AccordingtotheMicrobial
Products of Biotechnology, Final Rule under TSCA Section 5 (USEPA,
1997
), new microorganisms
are those “created to contain genetic material from organisms in more than one taxonomic genera.”
Thus, different hosts of the same plasmid, even if the transfer occurred in the soil, are considered
new microorganisms and would have to be reported. The European Community has similar laws,
outlining the use of GMOs (EU,
2001
). The USEPA's concern is the risk involved with these
organisms due to “the significant likelihood of creating new combinations of traits, and the greater
uncertainty regarding the effects of such microorganisms on human health and the environment.”
These are concerns mirrored by the public and by researchers in the field (Kappeli and Auberson,
1997
; Urgun-Demirtas et al.,
2006
).
Clearly, the benefits of bioaugmentation can be increased by manipulating the degrading
microorganisms. The key is to increase their efficacy while making them environmentally safe
to use, whether by engineering programmed cell death or utilizing indigenous organisms.
For both current and future bioaugmentation methods, the site characteristics and economic
considerations play a major role in deciding what method will be appropriate. The following
section discusses the key steps involved in making such a decision.
1.4 MAKING THE DECISION TO BIOAUGMENT
When presented with a contaminated site, a series of decisions must be made as to whether
the site should be remediated and which remediation technique to use. If bioremediation is
selected, practitioners then must decide whether to bioaugment. This decision is discussed more
thoroughly in Chapter 4, but the general steps are summarized here. Bioremediation is one of
several proven remediation technologies that include physical, chemical and biological
approaches. It is important to understand that
in situ
bioremediation is not one technology,
but rather a suite of related techniques for exploiting or enhancing desired biological activities.
Therefore, even if bioremediation is selected, this does not imply bioaugmentation. An over-
view of the decision process taken before bioaugmentation is summarized in Figure
1.6
.
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