The bacteria's cleaning power comes from their ability to "inhale" toxic metals and "exhale" them in a non-toxic form, explains team member Brian Lower, assistant professor in the School of Environment and Natural Resources at Ohio State University.
Using a unique combination of microscopes, researchers at Ohio State University and scientists from Austria, Sweden, Switzerland and the United States were able to see how the bacterium Shewanella oneidensis breaks down metal to chemically extract oxygen.
The study, published online this week in the journal "Applied and Environmental Microbiology," provides the first evidence that the Shewanella bacterium 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 use oxygen - to breathe.
"We use the oxygen we breathe to release energy from our food. But in nature, bacteria don't always have access to oxygen," said Lower. "Whether the bacteria are buried in the soil or underwater, they can rely on metals to get the energy they need. It's an ancient form of respiration."
Stained with dye, Shewanella oneidensis MR-1 bacteria are growing without oxygen on solid iron oxide. (Photo by J. Burns)
With better knowledge of the bacterium's abilities, scientists might one day engineer a Shewanella that would remediate nuclear waste. Since the bacteria can reduce chromium and uranium from the liquid phase to form insoluble compounds, they may be used to eliminate these two environmental pollutants from water.
"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," Lower said.
"This kind of respiration is fascinating from an evolutionary standpoint," he said, "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 U.S. Department of Energy is sponsoring the work in order to uncover new methods for treating waste from nuclear weapons production in the 1960s and 1970s. The agency estimates that more than 2,500 billion liters of U.S. groundwater are contaminated with uranium as a consequence of nuclear weapons production.
Shewanella is naturally present in the soil, and can be found at nuclear waste sites such as the Hanford Nuclear Reservation in central Washington state, said Lower.
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 to test how the bacterium responded to the metallic mineral hematite.
An atomic force microscope places a tiny tip above the surface being studied to measure how much the tip rises and falls as it passes over the surface. It can measure features smaller than a nanometer, a billionth of a meter, and detect atomic forces between the probe tip and the surface material.
The scientists combined the atomic force microscope with an optical microscope to get a precise map of the bacteria's location on the hematite.
For this study, 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 enable the bacteria to "breathe" hematite, Lower explained.
Lower's coauthors on the paper come from Corning, Inc.; Pacific Northwest National Laboratory; Johannes Kepler University of Linz, Austria; Ecole Polytechnique Federale de Lausanne, Switzerland; and Umea University, Sweden.
In the future, Lower and his partners want to test whether the Shewanella bacterium leaves OmcA on the cell surfaces when exposed to uranium and technetium, a radioactive metal produced in large quantities by nuclear fission. Technetium spreads more readily than many other radioactive substances, but despite of the importance of understanding its toxicity in animals and humans, experimental evidence is thin.
As a result of nuclear fuel reprocessing, technetium has been discharged into the sea in a number of locations along the British coast, and some seafood contains tiny but measurable quantities.
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