How do dormant spores know when to wake up?

Many of us have felt the urge to take a nap when times are stressful, and bacteria are no different. When faced with a long period of no food or water, some bacteria will enter dormancy, and reduce themselves down to a spore that may even be capable of remaining inactive for thousands of years. But how do these cells know when to wake up? The answer is that they can “count” and keep track of the number of times when nutrients become plentiful again. If a threshold is reached, indicating that conditions are persistently good, the cells reawaken.  Gurol Suel, of the University of California San Diego, explained to Will Tingle how the cells he’s been studying can count the number of times he “pulses” them with nutrients…

Gurol - We were very surprised to find out that when we give them these very tiny pulses, even though the spore does not wake up, it does not return to life, it counts the number of pulses that they observe in their environment. And they can not only count them, but they either sum them, They sum them, and then if these signals reach a threshold, then the spore initiates the program to return back to life. And so that means that even though they're completely dormant and remain dormant, they're capable of integrating information from the environment and processing it in a way where they're actually performing a calculation. They're summing signals over time that are spaced apart. And this is very, very fascinating because they're doing this without any type of energy that we would classically associate with the living cell. This concept is usually associated with complex systems that use a lot of energy. And we saw that this dormant cell can do the same type of calculation without any of those sort of energetic processes.

Will - And so what do you think is behind this ability to count how many pulses have already occurred?

Gurol - Yeah, so we were inspired by our previous work that we published several years ago where we discovered that bacteria can generate action potentials. Electrical signaling, just like neurons in the brain. And that was a very, very sort of eye-opening discovery for us. And that made us think that maybe since the spores, in their dormant state, do not have access to energy production or protein synthesis or gene expression, maybe they were using some kind of electrochemical potential that would be stored in the spore to do work in the dormant state without requiring new synthesis of energy. So when the spores are being made, the spores store electrochemical potential, and that means that they're separating charges across a capacitor, which in this case would be the membrane. And you can then have that potential energy available to you later to do some kind of work. But all you have to do is just release it. You don't have to do anything active, so to speak.

Will - So you're saying that these tiny bacteria have evolved a biological capacitor, like the one found in your car, as a way of storing the pulses and then counting them and then deciding when conditions are good enough to wake up. Is the number of pulses that reanimate these bacteria the same across the board?

Gurol - Yeah, that's a great question. So the short answer is no, No two cells are gonna be completely identical in every aspect that you can measure, so can never be completely the same. And because we're looking at a capacitive electrochemical potential, as you're saying, because this capacitor defines the ability of the spore to count and how many counts it needs to do before it reaches a threshold. If there are any two spores that have, starting off, slightly different electrochemical potential levels, then they will differ in the number of counts that they do before they jump back to life.

Will - And to open up the scope a bit, when we look for extraterrestrial life, it'd be nice to think that there'd be faster than light spaceships or advanced civilizations or something like that. But realistically, it's far more likely that if we do find something, it could well be similar to a dormant spore. So how does this research change any potential plans we have to deal with something other worldly if we found it in a dormant state.

Gurol - If we find any type of life form on another planet or one of the moons of Jupiter, I think it's very likely that it's gonna be something very primitive. And if you think about Mars, Mars had habitable conditions millions of years ago, but not anymore. So anything that would have been able to survive would have to be in some kind of an extremely dormant state. And bacterial spores may represent the closest thing that we may find on another planet, and I think it's very intriguing to see that even if we find something that looks like a spore, and if you would just to look at a spore, it looks like an inert object. It doesn't look like it's alive because it's exhibiting none of the typical sort of behaviors associated with a life form. If we look at spores and we can see that even when they're dormant, even when they appear inert, they're still able to ponder and calculate whether or not to come out of their shelter or their bunker state. I think it's very intriguing in terms of how we would approach and how we would deal with any potential objects that we may find on other planets and any of these missions that may suggest to be some sort of living life form.