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Bacteria seen swimming the electron shuffle

Researchers have captured the bacterium Shewanella's behavior on film

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3:56pm, December 7, 2009
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New videos have caught bacteria in the act of a completely new behavior. A study appearing in the Dec. 14 Proceedings of the National Academy of Sciences finds that Shewanella cells briefly touch an electron-accepting surface, lift off and swim furiously, and then return to the metal surface.

The researchers call this flighty new behavior electrokinesis, and think it may be a way for bacteria to dump built-up electrons before taking off in search of food, much like a whale surfacing for a breath before diving. Understanding this frenetic movement may help scientists design better microbial fuel cells, which harness these electron-shuttling bacteria to produce energy.

“As far as we know, it is a new behavior,” says study coauthor and microbiologist Ken Nealson of the University of Southern California in Los Angeles. “It’s a new way of thinking about what bacteria do. The really great thing is that it’s probably opened up 10 times more questions and created 10 times more hypotheses than we had when we started.”

Until now, Shewanella bacteria were thought to lead relatively simple lives, sometimes moving around and at other times sticking to a surface and depositing electrons on it. But the new study finds that the lifestyle of Shewanella is complicated.

In the new study, researchers led by Howard Harris also of the University of Southern California videotaped Shewanella cells as they came into contact with materials that could store electrons, such as manganese oxide and electrodes. From the movies, it became apparent that Shewanella cells were having what study coauthor Orianna Bretschger of the J. Craig Venter Institute in San Diego calls “touch-and-go” interactions with the surface. The cells “touch the surface, then quickly swim away, only to return again later for another fleeting interaction,” she says. What’s more, when researchers varied the electrode’s potential to accept more electrons from the bugs, the cells swam away even faster, the researchers found.

To show that the behavior was new, and not a form of chemotaxis, in which bacteria sense different amounts of chemicals in their surroundings, the researchers tested Shewanella mutants that lacked the genes necessary for chemotaxis. These mutants performed electrokinesis just as well as the normal cells, indicating that the new behavior is not like chemical sensing, Nealson says. The bacteria don’t need to sense and approach the electron-accepting surface before interacting with it.

Although the researchers don’t yet know why the bacteria engage in this behavior, one idea is that it helps the microbes balance the needs to breathe and to eat. Shewanella cells get energy by shuffling electrons to a surface that can accept them. In this case, the place where the bugs respire, or exchange electrons, may be different from where they find food. Bretschger likens the situation to an abalone diver who takes a breath of air and then dives deep down for the delicacy.

Since Shewanella cells respire on the surface of an electron acceptor such as metal, depositing all their electrons onto a metal is like taking a deep breath. With their electron storage tank empty, the cells are ready to swim away looking for food. As the cells’ electron sinks become full again, the cells return to the metal and dump their load.

The team “captured a very interesting behavior of Shewanella on film,” comments Jeffrey Gralnick of the University of Minnesota in St. Paul. Because researchers knew that Shewanella needed to offload electrons in order to get the energy to swim, the increased swimming after they bump into an electron acceptor “makes perfect sense,” he says.

Understanding this behavior may help scientists develop new microbial fuel cells, Nealson says. Such devices could generate small amounts of electricity while purifying water at a treatment plant. Swimming behavior is not ideal for such a system, so figuring out how to change the bugs’ behavior would be helpful. “This is exactly what you would like the bugs in the fuel cell not to do.”

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