Some bacteria species in oxygen-deprived environments—such as at the bottom of a lake or deep within a cave—are able to survive without oxygen, by generating electrons within their cells and then transferring the electrons across cell membranes through tiny channels formed by surface proteins, a process known as extracellular electron transfer (EET).
New research suggests that it could one day be possible to harness that electricity.
A team of engineers from the Massachusetts Institute of Technology (MIT) has developed a new method to process extremely small samples of bacteria and decipher specific properties that are highly correlated with the 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,” Qianru Wang, a postdoc in MIT’s Department of Mechanical Engineering, said in a statement.
“There is recent work suggesting there might be a much broader range of bacteria that have [electricity-producing] properties,” Cullen Buie, an associate professor of mechanical engineering at MIT, said in a statement. “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.”
In the past, researchers have sought ways to use these microbes for a variety of applications, including running fuel cells and purifying sewage water. However, because the microbes’ cells are so small and difficult to grow in the lab, scientists have struggled to find a way to harness this power.
Some of the flawed techniques used in the past including growing large batches of cells and measuring the activity of EET proteins in a very time-consuming and detail oriented process, as well as rupturing a cell in order to purify it and probe the proteins.
However, the research team created microfluidic chips etched with small channels that are pinched in the middle to form an hourglass configuration, so when a voltage is applied across one of the channels, the pinched section puts a squeeze on the electric field to make it about 100 times stronger than the surrounding field.
This creates a phenomenon called dielectrophoresis where a force that pushes the cell against its motion is induced by the electric field.
When dielectrophoresis is occurring, the particle is repelled at different applied voltages depending on the particle’s surface properties. While the researchers have used dielectrophoresis to sort bacteria based on size and species in the past, they hoped they could also use the phenomenon to examine bacteria’s electrochemical activity, which can be subtle to observe.
“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 said.
Now, the team used the microfluidic setup to compare different strains of bacteria that each contained a different, known electrochemical activity, including a natural strain of bacteria that actively produces electricity in microbial fuel cells and several genetically engineered strains.
The researchers flowed very small microliter samples in each strain through the channel and slowly amped up the voltage across the channel one volt per second from zero to 80 volts. The researchers then used particle image velocimetry to observe that the electric field propelled bacterial cells through the channel until they approached the pinched section, where the stronger field acted to push back on the bacteria through dielectrophoresis and trap them in place.
They found that some bacteria was trapped lower applied voltages, while others were trapped at higher voltages and the bacteria that were more electrochemically active had a higher polarizability.
“We have the necessary evidence to see that there’s a strong correlation between polarizability and electrochemical activity,” Wang said. “In fact, polarizability might be something we could use as a proxy to select microorganisms with high electrochemical activity.”
The researchers are now trying to use the technique to test new strains of bacteria that have been recently identified as potential electricity producers.
“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 said.
The study was published in Science Advances.