When biologists describe a brain cell firing, they often invoke a sizzle of electricity passing from cell to cell. This is because brain cells, or neurons, work by sending electric pulses. In fact, all of our nerve cells work by using electricity. For example, when we poke someone the nerves in our finger, arm, and spinal cord relay electricity from fingertip to brain. Those impulses tell us what the objects we’ve poked feel like.
What if bacteria are using electricity in the same way?
No, you aren’t forgetting a crucial fact about bacteria—they don’t have neurons. Bacteria are single-celled organisms, so they can’t even have specialized cells, let alone cells complex enough to relay information between organs. However, a paper out in PNAS this week shows that bacteria may also use electricity to respond to mechanical force.
Surprisingly, bacteria seem to have electrical fluctuations much like our neurons. You can watch bacteria “blink” with these voltage changes thanks to the work of CU Boulder professor Dr. Joel Kralj. Kralj designed a fluorescent sensor that lights up in response to changes in voltage across bacterial cell membranes. This sensor led to a surprising discovery: bacteria blinking cheerfully away under a microscope.
Giancarlo Bruni, a third year PhD student in MCDB and Dr. Kralj’s first graduate student, was studying these voltage changes—which the Kralj lab calls “voltage transients”—in E. coli. In his first few experiments, Bruni watched bacteria in real time using the voltage sensor described above as well as a calcium sensor. He found that voltage transients were correlated with calcium rushing into the bacterial cells. A change in voltage preceded a change in calcium, suggesting the voltage change was causing the ion change.
Calcium is an incredibly important ion in eukaryotes, because it is one of the ways that cells control the electricity that allows neurons and muscle cells to function. Cells can create electrical pulses because movement of molecules across cell membranes is regulated extremely carefully—the most stringent TSA checkpoint pales in comparison. Charged molecules, or ions, can only move across membranes under certain conditions. To maintain a negative membrane potential, which is typical for neurons and bacteria alike, cells use this membrane regulation to keep more negative ions (or fewer positive ions) inside of them compared to the surrounding liquid.
In response to some stimuli (say, you get poked and the nerve cells under your skin respond), cells depolarize, switching their negative membrane potential to positive. Ions such as calcium can then rush into the cell. That rush of calcium is a crucial aspect of how some animal cells work.
Bruni wondered if the change in voltage and calcium in bacteria was a response to something about the environment, in the same way that our mammalian neurons respond to the environment by changing their membrane potential. He decided he could test this idea by changing the mechanical force being placed on them. So, he poked them.
Yes, literally. He poked the bacteria with a small pad of agar—a gel like substance—and watched what happened to the calcium concentration. Sure enough, the calcium concentration inside cells went up. This suggests that the calcium is acting as a “mechanosensor”. It may be telling the bacteria something about their physical surroundings.
Bruni thinks that poking the bacteria with agar is a good way to test how bacteria respond to their surroundings in natural environments.
“You have this little bacteria swimming around, and all of a sudden it runs into a soft, juicy cell,” said Bruni. “When it hits this cell, it needs a reason to stop and realize that it’s on something that might be a food source. One thing we could be simulating is landing on a potential food source.”
Bruni wasn’t able to find a protein that was, by itself, responsible for the voltage transients. Instead, he thinks that several proteins might be involved.
“We think maybe it’s a pleiotropic thing where there are lots of channels involved in pushing calcium and other ions around,” said Bruni.
Next, Bruni investigated how the protein content of cells changes after being poked. By tagging proteins with fluorescent molecules he could track how much of those proteins were present in each condition—pre-poke and post-poke. The concentration of ten proteins (of 16 tested) changed significantly post-poke, providing further proof that the cells respond physiologically to mechanical force.
It’s possible that responding to stimuli by altering membrane potential and ion concentrations is an ancient evolutionary trait, existing across all three domains of life. Brains and nerve cells are certainly eukaryotic inventions, but what if using voltage as a response mechanism evolved a lot earlier than any of us suspected?
When asked what the most exciting part of this project had been, Bruni answered with a straight face, “the poke experiment.”
“I really just poked the cells with an agarose pad to see what would happen,” he said. Such is science—breakthroughs come from strange places.
By Alison Gilchrist