'Death screams' of swarming bacteria help their comrades survive antibiotic attacks

AcrA proteins (red) bind to the outer membranes of E.coli (green).
AcrA proteins (red) bind to the outer membranes of E.coli (green). (Image credit: University of Texas at Austin)

Swarming bacteria "scream" when they die, warning neighboring bacteria of danger. 

These death shrieks aren't audible; rather, they are chemical alarms that the bacteria broadcast while on the verge of death, an action known as necrosignaling. 

Through necrosignaling, bacteria alert their swarming neighbors to the presence of a deadly threat, and thereby save the majority of the swarm (a bacterial colony that's on the move). When confronted by a threat such as antibiotics, the bacteria's chemical death cries can provide survivors enough time to acquire mutations that convey antibiotic resistance, scientists reported in a new study.

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Many species of bacteria swim about with the help of slender tail-like structures called flagella, which help them swiftly. And sometimes, bacteria such as Escherichia coli (E. coli) congregate in the billions and use their flagella to move together over solid surfaces, as a swarm. 

"Bacterial swarms are metabolically active and grow robustly," the researchers wrote. For that reason, the scientists suspected that swarms could also have their own mechanisms for evolving antibiotic resistance, which could differ from those of individual bacteria.

Researchers previously noted that when swarming bacteria encountered antibiotics, about 25% of the roaming colony died. The dead bacteria seemed to somehow protect the survivors — surviving cells appeared to actively move away from the antibiotics after a portion of the swarm died — but it was unclear what guided the bacteria's behavior. 

In the new study, scientists observed swarms of E. coli bacteria as they interacted with antibiotics, to unravel how dead cells might help save the rest of the swarm.

E. coli can gather in the billions as a swarm, moving together over solid surfaces. (Image credit: University of Texas at Austin)

Signals from the dead

The researchers found that dead E. coli in the swarm released a necrosignal: a protein that bound to the outer membranes of living cells in the swarm. 

With these dying chemical "screams," the dead bacteria activated mechanisms in the membranes of the live cells "to start pumping out the antibiotics," study co-author Rasika Harshey, a professor of molecular biosciences at the University of Texas at Austin, told Live Science in an email. This suggests that the compound communicated "a state of emergency," alerting the living bacteria to the presence of danger, according to the study.

The cascade of genes turned on by necrosignals not only protected the surviving swarm from antibiotics, but promoted future resistance to the compounds that killed their comrades. What's more, the scientists realized that subpopulations of swarm bacteria were genetically variable; some were more susceptible to the antibiotics than others. Swarms of bacteria may collectively cultivate different subpopulations as an evolutionary survival strategy — if new antibiotics kill the vulnerable members of the swarm, their deaths will help to protect the rest, the study authors wrote.

"Dead cells are helping the community survive," Harshey said. 

The findings imply that in dense bacteria swarms, exposure to low doses of antibiotics could actually promote the acquisition of antibiotic resistance — an important consideration for research into strategies for defeating bacterial infections, she added.

The findings were published online Aug. 19 in the journal Nature Communications.

EDITOR'S NOTE: This article was updated on Aug. 31 to correct the bacteria swarm's response to necrosignals as pump activation rather than flight. 

Originally published on Live Science.

Mindy Weisberger
Live Science Contributor

Mindy Weisberger is an editor at Scholastic and a former Live Science channel editor and senior writer. She has reported on general science, covering climate change, paleontology, biology, and space. Mindy studied film at Columbia University; prior to Live Science she produced, wrote and directed media for the American Museum of Natural History in New York City. Her videos about dinosaurs, astrophysics, biodiversity and evolution appear in museums and science centers worldwide, earning awards such as the CINE Golden Eagle and the Communicator Award of Excellence. Her writing has also appeared in Scientific American, The Washington Post and How It Works Magazine.