Jan 14 2019
Creative adaptations are needed to live in adverse conditions. In this context, certain bacterial species existing in oxygen-deprived environments have to find a way to breathe that does not involve oxygen.
A microfluidic technique quickly sorts bacteria based on their capability to generate electricity. (Image credit: Qianru Wang)
These robust microorganisms, which exist at the bottom of lakes, deep within mines, and also in the gut of humans, have evolved a special form of breathing in which electrons are excreted and pumped out. To put this in simpler terms, these microorganisms can, in fact, generate electricity.
Engineers and scientists are investigating ways to leverage these microbial power plants to purify sewage water and run fuel cells, among other applications. However, it has been a challenge to pin down the electrical properties of a microbe. Compared to mammalian cells, the cells of a microbe are relatively smaller and very hard to grow in laboratory settings.
MIT engineers have now created a microfluidic method that can rapidly process tiny bacterial samples and measure a particular property that is highly associated with bacteria’s ability to generate electricity. According to them, this property, called polarizability, can be used for assessing the electrochemical activity of bacteria in a safer and more efficient way when compared to present methods.
The vision is to pick out those strongest candidates to do the desirable tasks that humans want the cells to do.
Qianru Wang, Postdoc, Department of Mechanical Engineering, MIT
There is recent work suggesting there might be a much broader range of bacteria that have [electricity-producing] properties. 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.
Cullen Buie, Associate Professor, Department of Mechanical Engineering, MIT
Wang and Buie have published their study results in Science Advances.
Just between frogs
Electricity-producing bacteria do so by creating electrons inside their cells and subsequently transferring those electrons across their cell membranes through small channels created by surface proteins, in a process called extracellular electron transfer, or EET.
In current techniques used for studying the electrochemical activity of bacteria, large batches of cells have to be grown and the activity of EET proteins have to be measured—a meticulous and time-intensive process. Similarly, in other methods, a cell has to be ruptured to purify and analyze the proteins. In order to assess the electrical function of bacteria, Buie searched for a faster and less destructive method.
For the past decade, his team has been constructing microfluidic chips etched with tiny channels, via which they flow microliter-samples of bacteria. To form an hourglass configuration, each channel is pinched in the center. Upon applying a voltage across a channel, the pinched section, which is approximately 100 times tinier than the rest of the channel, squeezes the electric field, rendering it 100 times sturdier than the surrounding field. The electric field gradient produces a phenomenon called dielectrophoresis, or a force that thrusts the cell against its motion caused by the electric field. Consequently, dielectrophoresis can either repel a particle or halt it in its tracks at varied applied voltages, based on the surface properties of that particle.
Dielectrophoresis was used by scientists including Buie to rapidly sort bacteria in accordance with general properties, like species and size. Around this time, Buie speculated whether the method would be able to suss out bacteria’s electrochemical activity, which is known to be a far more elusive 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.
Qianru Wang, Postdoc, Department of Mechanical Engineering, MIT
An electric correlation
In their latest work, the investigators applied their microfluidic setup to compare different strains of bacteria, each with a known but different electrochemical activity. The strains contained a natural or “wild-type” bacterial strain that actively creates electricity in microbial fuel cells, and a number of strains that have been genetically engineered by the researchers. On the whole, the researchers wanted to check whether a correlation existed between a bacteria’s electrical ability and the way it acts in a microfluidic device under a dielectrophoretic force.
First, microliter samples of each bacterial strain was allowed to flow via the hourglass-shaped microfluidic channel and the voltage was gradually increased across the channel, 1 V for each second, from 0 to 80 V. Then using an imaging technique called particle image velocimetry, the researchers noted that the ensuing electric field pushed the bacterial cells via the channel until they reached the pinched section, where the relatively stronger field acted to thrust back on the bacteria through dielectrophoresis and hold them in place.
While some bacteria were trapped at higher voltages, others were trapped at lower applied voltages. Wang noted the “trapping voltage” for each bacterial cell, determined their cell sizes, and subsequently utilized a computer simulation to measure a cell’s polarizability—that is, how easily a cell creates electric dipoles in response to an external electric field.
Based on her calculations, Wang found that more electrochemically active bacteria are inclined to have higher polarizability. She noted this correlation across all bacterial species tested by the group.
“We have the necessary evidence to see that there’s a strong correlation between polarizability and electrochemical activity,” stated Wang. “In fact, polarizability might be something we could use as a proxy to select microorganisms with high electrochemical activity.”
According to Wang, at least for the strains measured by them, scientists can measure their electricity generation by determining their polarizability—something that the team can efficiently, easily, and nondestructively track through their microfluidic method.
The team’s collaborators are presently using the technique to test innovative bacterial strains that have newly been identified as promising 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.
Qianru Wang, Postdoc, Department of Mechanical Engineering, MIT
The study was partly supported by the National Science Foundation and the Institute for Collaborative Biotechnologies, through the U.S. Army grant.