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Environmental Factor

October 2011

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Researchers discover how Geobacter remove uranium contamination

By Nancy Lamontagne
October 2011

left to right, Allison Speers, Ph.D., Sanela Lampa-Pastirk, Ph.D., Dena Cologgi, Ph.D., and Gemma Reguera, Ph.D.

Reguera and co-authors are pictured in their lab. Shown, left to right, are Ph.D. student Allison Speers, postdoctoral researcher Sanela Lampa-Pastirk, Ph.D., first author and Ph.D. student Dena Cologgi, and Reguera. Not shown, postdoctoral researcher Shelly Kelly, Ph.D. (Photo courtesy of Michael Steger, Michigan State University)

image of a Geobacter sulfurreducens cell

A Geobacter sulfurreducens cell uses its conductive pili to transfer electrons and reductively precipitate uranium (black precipitate). The figure has been manually colorized to highlight the cell (orange) and the pilus nanowires (yellow). (Graphic courtesy of Dena Cologgi and Gemma Reguera, Michigan State University)

A new study( Exit NIEHS funded in part by NIEHS has revealed that Geobacter bacteria use their conductive hair-like filaments or pili to clean up nuclear waste and other contamination. The study was led by Michigan State University researchers and appeared online Sept. 6 in the Proceedings of the National Academy of Sciences (PNAS).

Uranium contamination can occur at any step in the production of nuclear fuel and is also a problem in groundwater because of mining practices used during the Cold War era. Geobacter is known to immobilize uranium but, until now, scientists hadn't understood exactly how the bacteria performed this feat. Understanding the process could aid in the design of better uranium bioremediation strategies to limit human exposure to this dangerous element.

Conductive filiments are key to uranium removal

Microbiologist Gemma Reguera, Ph.D.( Exit NIEHS, and colleagues found that Geobacter's conductive pili, which function as microbial nanowires, are key for reducing the soluble form of uranium into a less-soluble form that may allow for easier removal. They made this discovery by growing the bacteria at temperatures that either induced or did not induce pili formation and comparing these wild-type bacteria with a mutant form lacking a pili-producing gene and an engineered strain into which the gene had been reintroduced.

The researchers found that Geobacter expressing pili removed more uranium from a solution than those without pili. The piliated bacteria deposited uranium outside their cells and kept the uranium from permeating their periplasm, the space just inside the external membrane. The piliated bacteria also had greater respiratory activity, were more viable, recovered from uranium exposure, and grew faster than pili-deficient strains. Non-piliated bacteria incorporated more uranium into their periplasm and cell membranes.

The pili also serve to shield Geobacter, allowing them to thrive in a harsh environment. “They are essentially performing nature's version of electroplating with uranium, effectively immobilizing the radioactive material and preventing it from leaching into groundwater,” Reguera said. The findings suggest that pili expression gives Geobacter an adaptive advantage when it is in a contaminated area.

Geobacter bacteria gain energy for growth by transferring electrons generated inside the cell to external electron acceptors, a process that could potentially be used for electricity generation. Reguera has filed patents to build on her research, which could lead to the development of microbial fuel cells that generate electricity while cleaning up contaminants.

In addition to funding( Exit NIEHS by the NIEHS Superfund Research Program, the team also received support from the U.S. Department of Energy.

Citation: Cologgi DL, Lampa-Pastirk S, Speers AM, Kelly SD, Reguera G( Exit NIEHS. 2011. Extracellular reduction of uranium via Geobacter conductive pili as a protective cellular mechanism. Proc Natl Acad Sci U S A. 108(37):15248-15252.

(Nancy Lamontagne is a staff writer with MDB, a contractor for the NIEHS Superfund Research Program and Worker Education and Training Program.)

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