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.
Certain 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. Meaning these microbes can actually produce electricity. However, scientists didn’t have an efficient method for recognizing these specific microbes — until now.
“There is recent work suggesting there might be a much broader range of bacteria that have [electricity-producing] properties,” says 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.”
Existing techniques for probing bacteria’s electrochemical activity involve growing large batches of cells and measuring the activity of EET proteins. This is a highly meticulous and 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.
Buie’s research 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 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,” says Qianru Wang, a postdoc in MIT’s department of electrical engineering.
The MIT researchers used their microfluidic setup to compare various strains of bacteria, each with a different, known electrochemical activity. The strains included a “wild-type” or natural strain of bacteria that actively produces electricity in microbial fuel cells, and several strains that the researchers had genetically engineered. In general, the team aimed to see whether there was a correlation between a bacteria’s electrical ability and how it behaves in a microfluidic device under a dielectrophoretic force.
Through an imaging technique known as particle image velocimetry, 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 and trap them in place. Some bacteria were trapped at lower applied voltages, and others at higher voltages. 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. It was observed that bacteria that were more electrochemically active tended to have a higher polarizability. This remained true across all species of bacteria tested.
“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.
Collaborators on the team are currently using the method to test new strains of bacteria that have recently been identified as potential electricity producers.