Researchers are studying some common soil bacteria that “inhale” toxic
metals and “exhale” them in a non-toxic form.
The bacteria might one day be used to clean up toxic chemicals left over
from nuclear weapons production decades ago.
Using a unique combination of microscopes, researchers at Ohio State Univ.
and their colleagues were able to glimpse how the Shewanella oneidensis
bacterium breaks down metal to chemically extract oxygen.
The study, published in Applied and
Environmental Microbiology, provides the first evidence that Shewanella
maneuvers proteins within the bacterial cell into its outer membrane to contact
metal directly. The proteins then bond with metal oxides, which the bacteria
utilize the same way we do oxygen.
The process is called respiration, and it’s how living organisms make
energy, explained Brian Lower, assistant professor in the School
of Environment and Natural Resources
at Ohio State. We use the oxygen we breathe to
release energy from our food. But in nature, bacteria don’t always have access
to oxygen.
“Whether the bacteria are buried in the soil or underwater, they can rely on
metals to get the energy they need,” Lower said. “It’s an ancient form of
respiration.”
“This kind of respiration is fascinating from an evolutionary standpoint,
but we’re also interested in how we can use the bacteria to remediate nasty
compounds such as uranium, technetium, and chromium.”
The last two are byproducts of plutonium. The United States Department of
Energy is sponsoring the work in order to uncover new methods for treating
waste from nuclear weapons production in the 1960s and ’70s.
Shewanella is naturally present in the soil, and can in fact be
found at nuclear waste sites such as the Hanford
site in the state of Washington, Lower explained.
With better knowledge of the bacterium’s abilities, scientists might one day
engineer a Shewanella that would remediate such waste more
efficiently.
“For instance, if you could enhance this bacterium’s ability to reduce
uranium by having it make more of these key proteins, that could perhaps be one
way to clean up these sites that are contaminated,” he said.
The danger at such waste sites is that the toxic metals are soluble, and so
can leak into the local water supply. But these bacteria naturally convert the
metals into an insoluble form. Though the metals would remain in place, they
would be stable solids instead of unstable liquids.
For this study, Lower and his colleagues used an atomic force microscope
(AFM) to test how the bacterium responded to the metallic mineral hematite.
They combined the AFM with an optical microscope to get a precise map of the
bacteria’s location on the hematite.
Though the bacteria are very small, they are still thousands of times bigger
than the tip of an AFM probe. So the microscope was able to slide over the
surface of individual bacteria to detect protein molecules on the cell surface
and in contact with the metal.
The researchers coated their probe tip with antibodies for the protein OmcA,
which they suspected Shewanella would
use to “breathe” the metal.
Whenever the probe slid over an OmcA protein, the antibody coating would
stick to the protein. By measuring the tiny increase in force needed to pull
the two apart, the researchers could tell where on the bacteria surface the
proteins were located.
The microscope detected OmcA all around the edges of the bacteria, wherever
the cell membrane contacted the hematite—which suggests that the protein does
indeed enable the bacteria to “breathe” hematite. The protein was even present
in a gelatinous ooze that was seeping from the bacteria. This suggests that Shewanella
might create the ooze in order to obtain energy from a wider portion of the
metal than it can directly touch, Lower said.
In the future, he and his partners want to test their new microscope
technique on other types of cells. They also want to test whether Shewanella
produces OmcA on the cell surface when exposed to uranium and technetium.
