University of Minnesota
May 6, 2013
An apparatus to measure bacteria using light to make electricity.
Electricity from bacteria opens a world of possibilities
Alchemists never managed to change base metals into gold, but all around us microbes achieve the next best thing.
Using primitive electricity, some bacteria can change the electrical state of metals—notably iron, but also uranium, manganese, and arsenic.
And that makes them gold to University of Minnesota researchers Daniel Bond and Jeffrey Gralnick, both associate professors in the Department of Microbiology and members of the U's BioTechnology Institute.
"Our labs focus on the fundamental question of what kinds of bacteria are capable of changing metals from one state to another, and how they do it," says Bond. "Only bacteria can do this—they control the chemistry of soils and water by altering the [electrical] states of most elements."
Electron thieves and donors
How they do it is the big question, but in short (not the electrical kind), they either steal electrons from, or donate them to, atoms of metal. These movements of electrons, which carry a negative charge, constitute an electric current.
These currents, typically generated when bacteria break down organic compounds, can be used to build biosensors that signal the presence of specific pollutants or chemicals in water or soil. But bacteria in Bond's and Gralnick's labs also hold potential for detoxifying metals.
"Some bacteria from the Soudan mine [in northern Minnesota] can take electrons from [soluble] metals to make rusts, which precipitate out of the water," says Gralnick. "They use the electrons to make energy … much like the way humans do. But they don't grow easily in the lab."
These "iron-oxidizing" bacteria make their living stealing electrons from iron, and combining it with oxygen to form rust. Gralnick and Bond have found a way to grow them by substituting an electrode for iron. Using a battery, they charge the electrode with electrons and put it in a chamber with the bacteria. The bacteria flock to the electrode and start slurping up the electrons; this immediately registers as a current whose strength reflects the activity of the bacteria.
But the bacteria the researchers study the most are the "iron-reducing" bacteria, which behave in the opposite way by donating electrons (acquired from organic compounds) to iron in rust. This makes the iron water-soluble again, and can also lead to the formation of interesting new minerals, such as magnetic particles.
The iron-reducers are good model organisms for bacteria that use the same biochemical process to donate electrons to potentially very toxic metals—uranium, for example.
"With those extra electrons, the uranium precipitates out of solution," Gralnick notes. "We have colleagues in the UK who test bacteria for the ability to [donate electrons] and change the solubility of radioactive compounds."
The researchers are also asking fundamental questions about how microbial communities, which are integral to ecosystems everywhere, evolved to share electrons. For example, they use a gold electrode and an engineered strain to force two bacterial species to grow only in the presence of each other. The engineered species needs to donate electrons, but can't do so to gold. So it donates them to the other species, which passes the electrons to the electrode; that way, both can live. The researchers are able to study how microbial communities work together when electrons are the primary currency, and say this research could lead to new biotechnology applications.
The field of bacteria interfaced with electrodes got off the ground—or into it—with the Office of Naval Research "because these bacteria can be used to generate electrical power in sediments deep in the ocean," says Bond. "Thus, the Navy, and other branches of the Defense Department, strongly support this work for energy and sensing applications."
For example, what if the microbes activated their electricity-generating biochemical pathways in response to chemicals in the environment?
"If we can harness the precision of bacterial sensing, I'd love to use bacteria as an early warning system for pollutants," says Bond. "But you can only build these organisms if you understand how they work."
And that's what he and Gralnick are all about.
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