Living in extreme conditions requires creative adaptations. These hardy microbes, which can be found deep within mines, at the bottom of lakes, and even in the human gut, have evolved a unique form of breathing that involves excreting and pumping out electrons. In other words, these microbes can actually produce electricity. The study has been published in Science Advances.

Scientists and engineers are exploring ways to harness these microbial power plants to run fuel cells and purify sewage water, among other uses. But pinning down a microbe's electrical properties has been a challenge. MIT engineers have developed a microfluidic technique that can quickly process small samples of bacteria and gauge a specific property that's highly correlated with bacteria's ability to produce electricity.

"The vision is to pick out those strongest candidates to do the desirable tasks that humans want the cells to do," says Qianru Wang.  "There is recent work suggesting there might be a much broader range of bacteria that have [electricity-producing] properties," adds Cullen Buie. "Thus, a tool that allows you to probe those organisms could be much more important than we thought. It's not just a small handful of microbes that can do this."

Researchers use dielectrophoresis to quickly sort bacteria

Existing techniques for probing bacteria's electrochemical activity involve growing large batches of cells and measuring the activity of EET proteins — a meticulous, time-consuming process.  Researchers including Buie have used dielectrophoresis to quickly sort bacteria according to general properties, such as size and species. "Basically, people were using dielectrophoresis to separate bacteria that were as different as, say, a frog from a bird, whereas we're trying to distinguish between frog siblings — tinier differences," Wang says.

An electric correlation

The researchers used their microfluidic setup to compare various strains of bacteria, each with a different, known electrochemical activity. They observed that the resulting electric field propelled bacterial cells through the channel until they approached the pinched section, where the much stronger field acted to push back on the bacteria via dielectrophoresis. Wang took note of the "trapping voltage" for each bacterial cell, measured their cell sizes, and then used a computer simulation to calculate a cell's polarizability — how easy it is for a cell to form electric dipoles in response to an external electric field

"We have the necessary evidence to see that there's a strong correlation between polarizability and electrochemical activity," Wang says. "In fact, polarizability might be something we could use as a proxy to select microorganisms with high electrochemical activity."

Wang says that, at least for the strains they measured, researchers can gauge their electricity production by measuring their polarizability — something that the group can easily, efficiently, and nondestructively track using their microfluidic technique. "If the same trend of correlation stands for those newer strains, then this technique can have a broader application, in clean energy generation, bioremediation, and biofuels production," Wang says.