Faced with starvation and stressful conditions, some bacteria enter a dormant state in which vital processes stop. Locking themselves into deep dormancy allows these cells, called spores, to withstand extremes of heat, pressure and even the harsh conditions of outer space.
Finally, when conditions become favorable, spores that may have been dormant for years can wake up in minutes and come back to life.
The spores wake up by rehydrating and restarting their metabolism and physiology. But until now scientists did not know whether spores can monitor their environment “during sleep” without waking up. In particular, it was not known how spores deal with vague environmental cues that do not clearly indicate favorable conditions. Would the spores simply ignore these mixed conditions or would they take note?
Biologists from the University of California, San Diego have solved this mystery in a new study published in the journal Science. Researchers in the School of Biological Sciences discovered that spores have an extraordinary ability to assess their environment while remaining in a physiologically dead state. They found that the spores use stored electrochemical energy, acting like a capacitor, to determine if conditions are right to return to normal life.
“This work changes the way we think about spores, which were considered inert objects,” said Gürol Süel, professor in the Department of Molecular Biology. “We show that cells in a deeply dormant state have the ability to process information. We discovered that spores can release their stored electrochemical potential energy to perform a calculation on their environment without the need for metabolic activity.”
Many bacterial species form spores (partially dehydrated cells surrounded by a tough protective layer) as a survival strategy that allows them to remain dormant for thousands of years. This remarkable ability makes them a threat in the form of bacterial anthrax, as well as a contamination hazard in medicine and the food industry.
Süel and his colleagues tested whether dormant spores of Bacillus subtilis could detect short-lived environmental signals that were not strong enough to trigger a return to life. They discovered that the spores were able to count such small inputs and if the sum reached a certain threshold, they would decide to come out of the dormant state and resume biological activity.
Developing a mathematical model to help explain the process, the researchers discovered that the spores use a mechanism known as integrate and fire, based on fluxes of potassium ions to assess the surrounding environment. They found that the spores responded even to short-lived favorable signals that were not sufficient to trigger an exit from dormancy. Instead of waking up, the spores released some of their stored potassium in response to each small input and then added up consecutive favorable signals to determine if conditions were right for exit. This cumulative signal processing strategy can reveal whether external conditions are truly favorable and prevents spores from “jumping the gun” into a world of unfavorable conditions.
“The way spores process information is similar to how neurons in our brain work,” said Süel. “In both bacteria and neurons, small, short inputs add up over time to determine whether a threshold is reached. Upon reaching the threshold, spores initiate their return to life, while neurons trigger a action potential to communicate with other neurons.” Interestingly, spores can perform this signal integration without requiring any metabolic energy, while neurons are among the most energy-dependent cells in our body.
The researchers believe the new information about the spores challenges popular ideas about cells in extremely dormant states that appear dead. These findings have implications for evaluating life in objects such as meteors, as well as space missions searching for evidence of life.
“This work suggests alternative ways to deal with the potential threat posed by pathogenic spores and has implications for what to expect from extraterrestrial life,” said Süel, who has affiliations with the San Diego Center for Systems Biology, the BioCircuits Institute and the Microbiomes Innovation Center. . “If scientists find life on Mars or Venus, it’s likely to be in a dormant state, and now we know that a life form that seems completely inert may still be able to think about its next steps.”
Authors of the paper include Kaito Kikuchi, Leticia Galera-Laporta, Colleen Weatherwax, Jamie Lam, Eun Chae Moon, Emmanuel Theodorakis, Jordi Garcia-Ojalvo, and Gürol M Süel.
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