This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A spore is a bacterium's survival capsule: dried almost solid and wrapped in protein armor, with its chemical processes shut down. Bacteria enter this state of hibernation when environmental conditions are unfavorable.
What's left is close to indestructible: It can survive boiling, desiccation, radiation and the vacuum of space. NASA once bolted Bacillus spores to the outside of a satellite and left them in orbit for nearly six years; the ones shielded from sunlight were able to come back to life upon returning to Earth. Out in the dirt, bacteria have handed down this spore-forming ability for roughly 3 billion years.
But if you put one of these bacteria in a comfortable laboratory flask and feed it well, within a few thousand generations it will lose the ability to make spores altogether. Why would a trait this useful, conserved for 3 billion years, vanish in a few thousand generations? This paradox has bothered microbiologists like me for decades.
In our research published in the journal Proceedings of the National Academy of Sciences, my colleagues Jay Lennon and William Shoemaker and I think the answer comes down to energy: We added up what it costs a bacterium to go dormant and wake up again, and the energy bill is large enough for evolution to push it to lose its spore-making genes if they aren't being used. This same arithmetic runs on any costly trait in any organism. Building the first complete energy budget of forming and sending out a bacterial spore also shows where a spore is most vulnerable—information that could help fight the bacteria behind some of the hardest hospital infections to treat.
Cells run on a molecule called adenosine triphosphate, or ATP, which acts like a rechargeable battery. Because every job in a cell is paid for with ATP, we can treat this molecule like a real currency: It lets you directly compare the cost of producing a spore with the cost of producing a swimming tail. Starting with Bacillus subtilis, a harmless soil bacterium, we gathered information from the scientific literature on which genes switch on as it builds spores, hour by hour, as well as how many copies of each type of protein one bacterium carries.
Proteins are what a cell actually builds—the armor, the machinery, the enzymes that do every job—and they are also where most of a cell's energy goes, which makes them the main thing to follow in energy accounting. These data let us price every step of making a spore—copying genetic material, reading genes, assembling proteins, building membranes—into one bill, and a second bill for bringing a spore back to life as a new bacterium. We then fed those numbers into a mathematical model of a starving population of Bacillus to ask what the cost of a spore does to a bacterial colony and whether evolution would notice—that is, whether a bacterium that quietly deleted its spore-making genes would end up with more descendants than one that kept them.
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