Technique Identifies Electricity-Producing Bacteria

Estimated reading time: 4 minutes

Living in extreme conditions requires creative adaptations. For certain species of bacteria that exist in oxygen-deprived environments, this means finding a way to breathe that doesn’t involve oxygen. 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.

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: The cells are much smaller than mammalian cells and extremely difficult to grow in laboratory conditions.

Now 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. They say that this property, known as polarizability, can be used to assess a bacteria’s electrochemical activity in a safer, more efficient manner compared to current techniques.

“The vision is to pick out those strongest candidates to do the desirable tasks that humans want the cells to do,” says Qianru Wang, a postdoc in MIT’s Department of Mechanical Engineering.

“There is recent work suggesting there might be a much broader range of bacteria that have [electricity-producing] properties,” adds Cullen Buie, associate professor of mechanical engineering at MIT. “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.” 

Just Between Frogs

Bacteria that produce electricity do so by generating electrons within their cells, then transferring those electrons across their cell membranes via tiny channels formed by surface proteins, in a process known as extracellular electron transfer, or EET.

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. Other techniques require rupturing a cell in order to purify and probe the proteins. Buie looked for a faster, less destructive method to assess bacteria’s electrical function.

For the past 10 years, his group has been building microfluidic chips etched with small channels, through which they flow microliter-samples of bacteria. Each channel is pinched in the middle to form an hourglass configuration. When a voltage is applied across a channel, the pinched section — about 100 times smaller than the rest of the channel — puts a squeeze on the electric field, making it 100 times stronger than the surrounding field. The gradient of the electric field creates a phenomenon known as dielectrophoresis, or a force that pushes the cell against its motion induced by the electric field. As a result, dielectrophoresis can repel a particle or stop it in its tracks at different applied voltages, depending on that particle’s surface properties.

Researchers including Buie have used dielectrophoresis to quickly sort bacteria according to general properties, such as size and species. This time around, Buie wondered whether the technique could suss out bacteria’s electrochemical activity — a far more subtle property.

“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.

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