Bacteria may be ancient organisms, but don't call them primitive. Despite being unicellular, they can behave collectively—sharing nutrients with neighbors, moving in concert with others and even committing suicide for the greater good of their colony. Molecules that travel from cell to cell enable such group behavior in a signaling process called quorum sensing. Now new evidence reveals that bacteria may have another way to “talk” to one another: communication via electrical signaling—a mechanism previously thought to occur only in multicellular organisms.
In 2010 molecular biologist Gürol Süel, now at the University of California, San Diego, set out to understand how a soil bacterium called Bacillus subtilis could grow into massive communities of more than a million cells and still thrive. He and his colleagues found that once the colony reaches a critical size, bacteria on the periphery stop reproducing to leave core cells with a sufficient nutrient supply.
But that observation led to the question of how the edge cells receive word to cease dividing. In a recent follow-up study, Süel discovered that the intercellular signals in this case were in fact electrical. The messages travel via ion channels, proteins on a cell's surface that control the flow of charged particles—in this case, potassium ions—into and out of a cell. The opening and closing of these channels can change the charges of neighboring cells, inducing them to release such particles and thereby relaying electrical signals from one cell to the next. “We've known that bacteria had ion channels and people have assigned them different functions, but only in the context of the single cell,” Süel says. “Now we're seeing that they're also being used to coordinate behavior over millions of cells.” The study appears in the journal Nature.
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Electrical signaling of this type is also how neurons in our brain pass along information. This and other findings are therefore prompting scientists to reevaluate their assumptions about single-celled life. “Bacteria have been thought of as limited because they are not multicellular,” says Steve Lockless, a biologist at Texas A&M University who was not involved in the study. But as unicellular organisms increasingly offer evidence of multifaceted behaviors, that may not be the case for much longer.