Bacteria are simple, supposedly. Each one consists of a single microscopic cell. But together, these cells have a surprisingly rich social life, and are capable of unexpectedly complex behaviors. For example, a team of scientists led by Gürol Süel from the University of California, San Diego, now has shown that groups of bacteria can coordinate their actions and bolster their ranks by sending long-range electrical signals, not unlike those that course along our neurons and power our thoughts.
When bacteria find themselves on solid surfaces, they can secrete a large, slimy framework called a “biofilm,” which they then inhabit. These biofilms are everywhere. You find them on rocks and boat hulls, on shower heads and catheters, and on your teeth in the form of dental plaque. They are the equivalent of bacterial cities, where multicultural communities of microbes co-exist in a bustling three-dimensional world.
Two years ago, Süel’s colleague Jintao Liu noticed something odd about the way these cities emerge. By allowing a soil bacterium called Bacillus subtilus to create biofilms under controlled conditions, he realized that it did so in a jerky way. The biofilm would expand and stop, expand and stop, again and again, with each cycle taking two hours.
Without those pauses, the cells in the center of the biofilm would run out of food and starve to death. Those on the edges would then become more vulnerable. So by periodically stopping the outer individuals from multiplying, the bacteria allow time for nutrients to reach the central cells, which benefits the entire community.
For years, scientists have known that bacteria can coordinate their behavior by exchanging specific chemicals—a process known as quorum sensing. But Süel’s bacteria were doing something else. Arthur Pringle, another member of the team, realized that they were exchanging electrical messages rather than chemical ones.
On their surface, bacteria have small pores called ion channels, which allow electrically charged molecules to move in and out of the cells. When the bacteria in the center of the biofilms start to starve, they open some of these pores, allowing positively charged potassium ions to stream outwards. When neighboring cells detect these ions, they also open their pores and release their own potassium. The result is a wave of charged ions—an electrical pulse—that ripples through the biofilm, right to its edges.
This is very similar to what happens when neurons fire. They are lined with ion channels, too. When one opens, ions stream through and trigger nearby channels to open as well, creating a traveling electrical pulse. In neurons, this happens down the lengths of single cells. In the bacteria, it happens over large communities of cells. But otherwise, the principle is the same. Indeed, scientists have long studied bacterial ion channels to better understand their counterparts in neurons, without understanding how the microbes themselves use such channels.
Süel’s team provides one answer: The channels allow bacteria to talk to each other at a distance.
These messages can even extend beyond the boundaries of a biofilm. Jacqueline Humphries, one of the team’s members, has shown that once the wave of potassium ions hits the edge of a B. subtilis biofilm, it continues to diffuse outwards. When other B. subtilis cells in the surrounding hinterlands pick up these ions, they switch on a propeller-like motor and end up swimming towards the potassium source. Humphries could see this happening under a microscope, by tracking the bacteria with florescent dyes. As soon as the potassium wave reaches the boundaries of biofilm, the microbes outside would start swimming over.
“The amazing thing is that potassium ions are an essential currency for all cells,” says Humphries. So while she initially worked with B. subtilis, she realized that the electrical signals ought to be able to summon other bacteria of any species. To test that idea, she exposed the B. subtilis biofilms to Pseudomonas aeruginosa—another species from a very distant trunk of the bacterial family tree. These two species shouldn’t be able to communicate with each other through, say, quorum sensing. But electrical messages ought to work just fine.
“I remember sitting in the dark microscope room, waiting for the images to come in,” she says. When the P. aeruginosa cells started swimming towards the B. subtilis ones, she watched it happening. “It was a really satisfying moment.”
“This is amazing work that reshapes how we think about bacterial interactions and biofilm formation,” says Helen Blackwell, from the University of Wisconsin-Madison, who was not involved in the study. “It shows us a simple and generic way for many different bacteria to interact thorough electrical signals.”
Think about it this way. When bacteria communicate chemically, it’s like each family is speaking with its own language, relying on its own particular assortment of chemicals and receptors. By contrast, the electrical signals that Süel’s team discovered are more like mathematics—something universal. “It allows species to communicate across evolutionary divides and create mixed communities,” says Humphries. “It’s changed my perspective on biofilms.”
Many biofilms are incredibly ordered. Those in your mouth, for example, are structured like a rainforest, with some species living in the canopy, while others crowd in the floor. It’s natural to assume that they develop in this way. “But maybe that’s not the case,” says Humphries. “Maybe they just send out these ads, get whom they can, and then work out how to live together.”
“It’s a fascinating development,” says Julia van Kessel from Indiana University. She now wants to know how far the signals can travel, how they fare in different environments, and how they might activate the genes of the microbes they attract.
For Karine Gibbs from Harvard University, the critical question is: “What is the fate of cells pulled in by these signals?” she wonders. Some of them end up within the biofilm itself, but could they be eaten by the native cells? Are the biofilm bacteria luring in outsiders for food? Could they be “like the mythical Sirens calling ships to the shore?” Gibbs wonders.
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