Bacterial spores cost nearly 10 billion ATP to make and revive, energy budget finds

A bacterial spore is a survival capsule, dried almost solid and wrapped in protein armor with its chemistry switched off ^[1]. It survives boiling, radiation and the vacuum of space; NASA once left Bacillus spores bolted to a satellite for nearly six years, and the ones shielded from sunlight revived when they came back to Earth ^[2][3]. The puzzle the study takes on is why evolution drops a trait it has kept for roughly 3 billion years ^[4].

The answer turns on a cost that is easy to overlook. Even where food is abundant and a cell's sporulation genes sit idle, those genes still get copied every time it divides, and copying DNA takes energy ^[17]. A lineage that deletes genes it is not using spends a little less at each division, and across thousands of generations that saving compounds into more descendants than the lineage that keeps them ^[8]. To check whether evolution would notice, the team gathered published data on Bacillus subtilis, a harmless soil bacterium, and fed it into a model of a starving population ^[9][10].

Most of a spore's price is opportunity cost, not fuel it burns outright: raw material that could have become a new cell goes into armor instead, and a bacterium making a spore stands still while its neighbors divide ^[13]. The full round trip, forming a spore and later reviving it, comes to nearly 10 billion ATP molecules, among the costliest things a bacterium does ^[11]. ATP is the cell's energy currency, spent on every job, and treating it as money let the authors weigh a spore against a swimming tail ^[19]. Growing that flagellum and swimming off to find food costs a fraction as much ^[12].

The bill is also lopsided. The mother cell pays about 87% of the construction, feeds material to the spore through a narrow channel, then dissolves to release it ^[16]. That leaves the spore carrying roughly 13% of its own build ^[1], and it wakes up nearly broke. A dormant spore holds almost none of the fuel it needs to germinate, short by close to five orders of magnitude ^[14]. What it packs instead is raw material: small proteins around its DNA that it can break down for amino acids, and carbon compounds it can start burning within minutes of getting wet ^[15].

To finish waking, a spore has to find food outside. Building the first complete energy budget of that process shows where the spore is weakest, and the authors say it could help fight the bacteria behind some of the hardest hospital infections to treat ^[18].

The study is a cost accounting: it prices sporulation from published measurements on a benign soil species and runs them through a population model ^[10]. The authors say the same accounting applies to any costly trait in any organism ^[20].