Biomedical Engineering Reference
In-Depth Information
illustrating the USEPA's use of regulation to arrive at a better understanding of the impacts of
GMOs (Sayre and Seidler,
2005
). Ideally, regulations would allow research to proceed under
realistic field conditions and facilitate the use of “safe” technologies while still protecting the
environment and the public. One way to sidestep this issue was suggested in a study that used
killed genetically-modified
Escherichia coli
that had over-expressed atrazine chlorohydrolase
to remediate a site contaminated with atrazine (Strong et al.,
2000
).
The success of GEMs in the field remains uncertain. Since their creation and optimization
would have occurred in the laboratory under favorable and perhaps unrealistic conditions, there
is always some doubt whether inoculated GEMs will be able to survive in natural environments.
However, it appears that some GEMs may have specific advantages over indigenous organ-
isms, such as tolerance for high levels of a pollutant, or simply not affected by the other
microflora (Lenski,
1993
; Ripp et al.,
2000
; Bott and Kaplan,
2002
). In one field study, the
bacterium
Pseudomonas fluorescens
HK44, containing a bioluminescent gene (lux) within the
promoter for naphthalene catabolic genes, was used to both degrade and monitor the presence
and degradation of naphthalene (Ripp et al.,
2000
). The hurdles encountered during this
endeavor have been reviewed, and the use of GEMs in general has been discussed in recent
reviews (Sayler and Ripp,
2000
; Cases and de Lorenzo,
2005
). In another field release,
Pseudomonas putida
W619-TCE, known to degrade TCE, was inoculated in the roots of poplar
trees to reduce TCE transpiration during phytoremediation (Weyens et al.,
2009
). These
technologies are still new and uncertain, and the regulations controlling them are expected to
be revised periodically.
1.3.2.2 Gene Bioaugmentation
Bioremediation, in its most simplistic form, relies on enzymes that catalyze biodegradation.
These enzymes are proteins that are coded by genes carried in the bioremediating organism.
In gene bioaugmentation, the goal is to circumvent the problems inherent in sustaining
inoculated organisms in the contaminated system and instead encourage the uptake of the
genes themselves into the indigenous microbes.
Catabolic mobile genetic elements (MGEs) are ideal for gene bioaugmentation. MGEs are
pieces of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) that can be easily transmitted
between organisms and include plasmids, transposons, bacteriophage-related elements and
genomic islands. Degradation genes often are found on MGEs. For example,
Dehalococcoides
strains involved in chlorinated ethene degradation can transfer reductive dehalogenation genes
on MGEs, possibly phages (West et al.,
2008
). Two recent reviews on the topic have compiled
lists of existing MGEs (Top et al.,
2002
; Nojiri et al.,
2004
).
The most likely method to accomplish gene bioaugmentation potentially is to inoculate the
contaminated media with organisms carrying MGEs. These organisms could then transfer the
MGE to the indigenous microbes, and the fate of the added organisms would be unrelated to
the degradation of the pollutant. There are three general methods by which the inoculated
strains could transfer DNA - transformation, conjugation and transduction. Bacteria in the
contaminated medium that are naturally competent could incorporate extracellular DNA
directly through natural transformation. Conjugation involves the direct transfer of genetic
material from one cell to another, but conjugation is limited by the compatibility of the donor
and receiving bacteria. Finally, transduction uses a bacteriophage (bacterial virus) to transfer
genetic material between organisms.
There are several hurdles for successful gene bioaugmentation. First, the donating organ-
isms must survive long enough to transfer the genetic material. Second, the DNA must be
compatible with the accepting strains. Plasmids are one type of MGE that can be transferred by
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