of the Problem
Chlorinated solvents such as trichloroethylene (TCE)
are used in a wide range of industrial processes
and in the creation of consumer goods such as furniture
coverings, plastic food wrap, and even decaffeinated
coffee. Prior to their regulation,
industrial wastes containing high levels of chloroethenes
were disposed of haphazardly. These disposal practices
have made chloroethenes common ground water contaminants.
Since the 1980’s researchers have identified
a number of microorganisms that degrade groundwater
contaminants through a process called cometabolism.
The microbes occur naturally in soil and groundwater
situ bioremediation. Oregon State University
Arp and Peter
Bottomley are leading WRHSRC
studies on cometabolism of chlorinated solvents
such as trichloroethylene (TCE). Their goal is
to maximize the contaminant degrading potential
strains of bacteria.
Aerobic Cometabolism with Butane-Grown Microorganisms
Trichloroethylene (TCE) is a common
and persistent contaminant in soil and groundwater. Human exposure
to TCE has been linked to kidney and liver damage and TCE is
a possible carcinogen. Because of the health risks associated
with TCE exposure, considerable effort has been made to remove
TCE contamination from soil and ground water.
on highlighted words and illustrations
or links will pop up.
Aerobic cometabolism is an emerging cleanup technology that
utilizes microorganisms to degrade TCE and other chloroethenes.
The term cometabolism indicates that transformation of the contaminants
is a secondary reaction (Figure
The microbes consume a hydrocarbon, such as butane for their
needs. In the process, they produce enzymes that fortuitously
degrade other compounds such as chloroethenes. One cleanup
approach is to add the microbes and/or their food source
to an aquifer and exploit their ability to transform contaminants.
State University professors Daniel
Arp and Peter
Bottomley (Figure 2) are
leading studies of three types of bacteria that
TCE and other chloroethenes. They are developing a better
understanding of the biology of the bacteria and the chemistry
of the cometabolic reactions – essential information for
utilizing the bacteria in bioremediation.
The team’s focus is on bacteria that use butane as an
energy source and produce butane monoxygenase (BMO), an enzyme
that degrades a wide range of substances including chloroethenes.
In the case of TCE, the BMO enzyme oxidizes TCE to TCE epoxide
1). The oxidation of TCE is a reductant consuming process,
it "uses up" the substance that provides electrons for
the reaction. As a result, the sustained cometabolism of chloroethenes
requires the presence of an appropriate substrate to donate reductant
and support BMO enzyme activity.
One WRHSRC project compared the ability of different
sources of reductant to drive the oxidation of dichloroethenes
by the bacterium
P. butanovora. Butane, the energy source for the bacteria,
can serve as the source of reductant. However, this creates
that derives energy and the
secondary oxidation reaction that degrades the chloroethenes.
Adding an alternate substrate
eliminates this competition and can increase the efficiency of
the dechlorination reaction. In comparisons of several organic
acids, graduate student Dave Doughty found that lactate supported
the highest initial transformation rates and sustained transformation
for the longest period of time (Doughty et al., in press).
Cometabolism of chloroethenes can stop or slow because the compounds
and their breakdown products (such as TCE epoxide) inactivate
the BMO enzyme or damage or kill the bacteria itself. In a second
WRHSRC project, the team studied the TCE transformation ability
of three strains of bacteria that each produces a different type
of BMO enzyme. In a recent paper in Applied Microbial and
Cell Physiology, the team compares the TCE transformation
ability of each bacterium and evaluates its effect on BMO enzyme
and cell physiology (Halsey et al., 2005). “One interesting
Graduate student Kim Halsey “is that P. butanovora,
the bacterium that was initially most efficient at degrading
was also the bacterium most negatively affected by TCE epoxide.” In
comparison, the bacterium that was the slowest TCE transformer,
M. vaccae, was able to sustain TCE degradation
the longest. This result suggests that strains that are slower
co-metabolizers may be more effective for sustaining bioremediation.
The team is continuing to investigate the biochemistry of the
BMO enzymes. For example, one current project focuses on making
specific mutations within the BMO enzyme of P. butanovora and
the mutation’s effects on the bacteria’s ability
to transform TCE.
For More Information
Peter Bottomley or Dr.
Daniel Arp or review
the following references:
Arp DJ (1999) Butane metabolism by butane-grown Pseudomonas
butanovora. Microbiology 145:1173–1180.
Arp DJ, Yeager CM, Hyman MR (2001) Molecular and cellular
fundamentals of aerobic cometabolism of trichloroethylene.
Doughty DM, Sayavedra-Soto LA, Arp DJ, and
Bottomley PJ (in press) Effects of dichloroethene
isomers on the induction and activity of butane monooxygenase
bacterium, ‘Pseudomonas butanovora’,
Applied and Environmental Microbiology.
Halsey KH, Sayavedra-Soto LA, Bottomley PJ, Arp DJ
(2005) Trichloroethylene degradation by butane-oxidizing bacteria causes a spectrum
of toxic effects. Applied Microbiology and Biotechnology [Epub
ahead of print 2005 Mar 8].