Harnessing electricity from bacteria

US engineers have reported progress in the quest to gather usable electricity from bacteria.

Scientists know that certain species of bacteria living in oxygen-deprived environments (including the human gut) have evolved a unique form of breathing that involves excreting and pumping out electrons. In other words, they actually produce electricity that could, in theory, be used to power equipment or purify water.

And with this knowledge, researchers are working, for example, to design effective microbial fuel cells or generate power from organic waste. NASA is even investigating whether bacteria could power future space missions.

What has hampered turning theory into such practical reality, however, is that it is hard to pin down the exact nature of a bacterium’s electrical properties. The cells are much smaller than mammalian ones, and extremely difficult to grow in laboratory conditions.

Now researchers from the Massachusetts Institute of Technology (MIT) in the US have developed a microfluidic technique they say can quickly process small samples of bacteria and gauge a specific property that’s highly correlated with microbes’ ability to produce electricity.

Writing in the journal Science Advances, they report that this property, known as polarisability, can be used to efficiently and safely assess bacteria’s electrochemical activity, allowing them to select the best candidate for a specific task.

“There is recent work suggesting there might be a much broader range of bacteria that have [electricity-producing] properties,” says mechanical engineer 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.”

Bacteria produce electricity by generating electrons in their cells and then transferring them 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 this electrochemical activity include growing large batches of cells and measuring the activity of EET proteins or rupturing a cell in order to purify and probe the proteins. Buie and MIT colleagues, including postdoc Qianru Wang, decided to try to find something faster and less destructive.

In their study, they used microfluidics to compare various strains of bacteria, each with a different, known electrochemical activity.

They flowed minute samples of each strain through an hourglass-shaped microfluidic channel and slowly amped up the voltage from zero to 80 volts. The resulting 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 via dielectrophoresis (the motion of a neutral particle caused by polarisation effects in a non-uniform electric field) and trap them in place.

Wang took note of the “trapping voltage” for each bacterial cell, measured the cell sizes, then used a computer simulation to calculate a cell’s polarisability – that is, how easy it is for a cell to form electric dipoles in response to an external electric field.

From her calculations, she discovered the more electrochemically active bacteria tended to have a higher polarisability, and this correlation occurred across all species tested.

“We have the necessary evidence to see that there’s a strong correlation between polarisability and electrochemical activity,” she says. “In fact, polarisability might be something we could use as a proxy to select microorganisms with high electrochemical activity.”