Living in extreme conditions requires creative adjustments. For some species of bacteria found in oxygen deficiency, it means finding a way to breathe that does not involve oxygen. These hardy microbes that can be found deep within mines, at the bottom of lakes and even in the human intestine have developed a unique breathing that involves the excretion and pumping of electrons. In other words, these microbes can actually produce electricity.
Researchers and engineers are exploring ways to utilize these microbial power plants to run fuel cells and purify wastewater among other applications. However, attaching the electrical properties of a microbe 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 measure a specific property that is highly correlated with the ability of the bacteria to produce electricity. They say that this property, known as polarizability, can be used to assess the electrochemical activity of a bacterium in a safer and more effective way over current techniques.
"The vision is to select the strongest candidates to perform the desirable tasks that humans want the cells to do," said Qianru Wang, a postdoc in MIT's Department of Mechanical Engineering.
"There is new work that suggests that there may be a much wider range of bacteria that have [electricity-producing] properties," adds Cullen Buie, MIT Associate Professor. "A tool that allows you to probe these organisms can therefore be much more important than we thought. It's not just a small handful of microbes that can do this."
Buie and Wang have published their results today in Science Advances .
Right between frogs
Bacteria producing electricity do so by generating electrons in their cells and then transferring these electrons across their cell membranes via small channels formed by surface proteins in a process known as extracellular electron transfer or EET.
Existing techniques for probing the electrochemical activity of the bacteria involve growing large quantities of cells and measuring the activity of EET proteins ̵
For the past 10 years, his group has built microfluidic chips, etched with small channels, through which they flow microliter samples of bacteria. Each channel is adhered in the middle to form an hourglass configuration. When a voltage is applied across a channel, the cut section – approx. 100 times less than the rest of the canal – a clamp on the electric field, making it 100 times stronger than the surrounding field. The degree of the electric field creates a phenomenon known as dielectrophoresis, or a force that pushes the cell toward its movement induced by the electric field. As a result, dielectrophoresis can reject a particle or stop it in its traces at various applied voltages depending on the particle's surface properties.
Scientists, including Buie, have used dielectrophoresis to rapidly sort bacteria according to general characteristics such as size and species. This time, Buie wondered whether the technique could extinguish the bacterial electrochemical activity – a much more subtle trait.
"Basically, humans used dielectrophoresis to separate bacteria that were as diverse as, for example, a bird from a bird, while trying to distinguish between frog-siblings – tinier differences," Wang says.
In their new study, the researchers used their microfluidic set up to compare different bacterial strains, each with a different known electrochemical activity, the strains comprising a "wild-type" or natural strain of bacteria that actively produces electricity in microbial fuel cells. Generally, the team tried to see if there was a correlation between the electrical ability of the bacterium and how it behaves in a microfluidic unit under a dielectrophoretic force. The team flowed very small microlitre samples of each bacterial strain through hourglass-shaped microfluidic channel and slowly amplified the voltage across the can one, one volt per second, from 0 to 80 volts. Through an imaging technique, known as particle image velocimetry, they observed that the resulting electric field-driven bacterial cells pass through the channel until they approach the squeezed section, where the much stronger field acted to push back the bacteria via dielectrophoresis and trap them.
Some bacteria were trapped at lower applied voltages and others at higher voltages. Wang noted the "catch voltage" for each bacterial cell, measured their cell sizes, and then used a computer simulation to calculate a cell polarization – how easy it is for a cell to form electrical dipoles in response to an external electric field.
From his calculations, Wang discovered that bacteria that were more electrochemically active tended to have a higher polarization. She observed this correlation across all bacterial species tested by the group.
"We have the necessary evidence to see that there is a strong link between polarizability and electrochemical activity," Wang says. "In fact, polarization can be something we could use as a proxy to select microorganisms with high electrochemical activity."
Wang says that at least for the loads they have to measure, scientists can measure their electricity production by measuring their polarizability – something that the group can easily, effectively, and non-destructively track through their microfluidic technique.
Team employees are currently using the method to test new bacterial strains that have recently been identified as potential electricity generators.
"If the same tendency for correlation stands for the newer strains, then this technique may have wider use in clean energy generation, bioremediation and biofuel production," Wang says.
This research was partially supported by the National Science Foundation and the Institute for Collaborative Biotechnologies, through a US Army grant.