Image: Bacterium
Pacific Northwest National Laboratory
This image shows the bacterium Shewanella oneidensis growing on the surface of the iron oxide mineral hematite. Scientists found that the proteins bacteria like this one use to digest toxic metals are on the outside of the bacteria's cell as well as in an ooze-like material that comes out of the cell.
updated 3/27/2009 12:43:21 PM ET 2009-03-27T16:43:21

Just like humans effortlessly suck up oxygen and breathe out carbon dioxide, some bacteria take in toxic metals and release non-toxic versions.

With that natural ability, these microbes have caught the eye of scientists who'd like to put them to work cleaning up nuclear waste sites.

A new study brings that goal one step closer to reality. Researchers have identified and located two proteins that give certain bacteria the power to detoxify dangerous metals, including uranium, chromium and technetium.

"This was a big step," said Brian Lower, a microbiologist at The Ohio State University in Columbus. "These microorganisms are commonly occurring. If we can harness their abilities to clean up these sites, that would be one way to help prevent harm to the ecosystem, the water supply, and ourselves."

Lower and colleagues studied a bacterium called Shewanella oneidensis. About 20 years ago, Shewanella was one of the first groups of microorganisms found to metabolize toxic metals. Ever since, scientists have been trying to figure out how they do it.

To investigate, Lower's team used a new technique called antibody-recognition force microscopy. Based on the proteins they suspected they'd find, the researchers put protein-matching antibodies at the tip of a probe. Then, they used the probe to scan the surfaces of individual Shewanella. When the antibodies found a match, a microscope zeroed in and took high-resolution images.

The images confirmed what the scientists had expected to find: Both of the proteins they looked for were there, and both proteins — both types of enzymes — were sitting on the outside of the bacteria's cell as well as in an ooze-like material that comes out of the cell.

In the human body, enzymes stay inside cells, because that's where biological processes happen. But the minerals in metals are insoluble, which means they can't easily penetrate a cell membrane. Shewanella gets around this problem by putting their metal-digesting enzymes outward.

The new findings, reported in the journal Applied and Environmental Microbiology, give some insight into the history of life on Earth, Lower said. Shewanella probably represent one of the earliest forms of respiration.

"These organisms may give us a glimpse of what life was like before oxygen existed," he told Discovery News.

The work also opens up the tantalizing possibility that some day, scientists will be able to use Shewanella for remediation by, for example, engineering bacteria that produce more of their proteins and then dumping the microbes on contaminated sites.

Just as intriguing, Lower added, is evidence that Shewanella generate a small amount of electricity as they eat waste, giving them potential as biofuel cells.

Plenty of challenges remain. For example, once Shewanella has had its way with a toxic metal, the metal goes from soluble in water to insoluble. That means the solid will stay on site and out of the water supply. But scientists don't yet know if the material will remain insoluble and stay put indefinitely. Efficiency is another important goal.

"It's a matter of outsmarting the microbes so they do what you want them to do rather than what they want to do," said Kenneth Nealson, professor of Earth and Biological Sciences at the University of Southern California in L.A. His lab is working to do just that. Progress, he said, is encouraging.

"I think in 10 or 15 years," he said, "Some of these systems will be at a level where we may very well be able to clean up major water bodies and things like that."

© 2012 Discovery Channel


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