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ITHACA, N.Y. -- An antibiotic-resistant bacterium, isolated from sewer
sludge by Cornell University scientists, is pointing the way to better
water-pollution cleanup strategies.
The bacterium, coccoid Strain 195, perfectly reduces the toxic pollutants
tetrachloroethene and trichloroethene (also known as PCE, or perc, and TCE,
respectively) to nontoxic ethene gas, the Cornell researchers report in the
June 6 issue of Science. But the talented bug alone may not be magic
bullet against the second-most-common ground water pollutant in North
America.
Still, knowing how Strain 195 works its magic in the laboratory will lead
to better pollution cleanup strategies in the field, predicted James M.
Gossett, the Cornell professor of civil and environmental engineering who
started Cornell microbiologist Stephen H. Zinder on the search for the
perfect dechlorinator, a quest that began in a sewage treatment plant.
Zinder, a professor of microbiology, as well as graduate student Xavier
Maymo-Gatell and other colleagues, used funding from the Cornell Center for
Advanced Technology in Biotechnology and the U.S. Air Force to isolate
Strain 195 from an enriched, mixed culture of bacteria that Gossett and his
co-workers had developed from a sludge sample. The sludge came from
Ithaca's old sewage-treatment plant, a microbe-rich relic from the days
when dry-cleaning solvents and industrial degreasing agents were flushed
down the drain.
An anaerobic bacterium, Strain 195, "uses tetrachloroethene much like we
use oxygen," Zinder said, describing the reduction process that leaves
little more than ethene, also known as ethylene, the natural gas that
causes fruit to ripen.
Until the Cornell discovery, reported in the Science article "Isolation of
a Bacterium that Reductively Dechlorinates Tetrachloroethene to Ethene,"
the best available bioremediation organisms could only reduce
tetrachloroethene to vinyl chloride. "They were turning a suspected
carcinogen into a known carcinogen," Gossett said. In carefully controlled
laboratory conditions, however, Strain 195 goes all the way.
That doesn't mean the Cornell bioremediation researchers will start
injecting truck loads of Strain 195 into pollution sites. For one thing,
195 is difficult to grow by itself; it seems to need chemical collaboration
with other bacteria, and the pollution cleanup process may benefit from a
little enhancement with extra hydrogen, nitrogen, phosphorous or even
vitamins.
"Strain 195 needs a lot of vitamin B-12. It doesn't have the capability to
make B-12," Gossett said, "and we wouldn't have known about those needs
without isolating the organism."
Armed with new knowledge about 195 and other anaerobic pollution-eaters, a
Cornell team led by Gossett and Zinder is better prepared to recommend
bioremediation protocols. Their mission, funded again by the U.S. Air
Force, will take them to military airbases with subterranean pollution
problems.
Among the bioremediation test sites are a former B-52 airbase in
Plattsburgh, N.Y., where planners hope to replace Air Force operations with
an eco-industrial park, and a still-functioning base in Fallon, Nev., where
the U.S. Navy's "Top Gun" fighter pilots train. Like many other airbases,
the New York and Nevada facilities harbor concentrations of toxins,
especially around pits where crash-and-rescue personnel dumped jet fuel for
firefighting practice -- and threw in leftover chlorinated solvents "to get
rid of them."
If chlorinated solvents were that easy to destroy, Gossett said, then
tetrachloroethene, trichlorothene and other related chemicals would not be
the number-two ground water pollutant (after petroleum hydrocarbons).
However, Strain 195 and other microorganisms may already be hard at work at
some pollution sites, a possibility that the Cornell team can investigate,
now that they know what to look for.
"As we piece together the family tree for these organisms, we can develop
gene probes and ask: 'Who's here? Which dechlorinators are working here?'
And that will tell us whether some enhancement would be helpful or whether
the problem will take care of itself," Gossett said.
The Cornell researchers have learned enough about Strain 195 to show that
the tiny pollution-eater has a limb of its own on the bacterial family
tree. Equipped with an unusual cell wall structure that makes it
completely resistant to most common antibiotics, Strain 195 may be a unique
match for some of the most irreducible compounds that modern chemistry has
devised.
"It's as if this bug were born to dechlorinate," Zinder said.
"Then what was it doing before we invented chlorinated ethylenes?"
Gossett asked. "It's true, there are some natural levels (of chlorinated
ethylenes) in nature, but not in the high concentrations normally needed to
support growth."
The evolutionary origin of Strain 195 is just one of the questions
on the research agenda for the Cornell microbiologists.
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EDITORS: An electron micrograph of the bacterium is available by special
arrangement. Please contact Cornell News Service for further information.
--------------------------------
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