Off The Cliff

E. coli caught Theodor Escherich's eye thanks to its gift for multiplication—the way a single microbe can give rise to a massive, luxurious growth in a matter of hours. If the bacteria Escherich discovered had continued to reproduce at that rapid rate, they would have soon filled his flasks with a solid microbial mass. In a few days they could have taken over the Earth. But E. coli did something else. It began to grow more slowly, and then, within a day, it stopped.

All living things could, in theory, take over the planet. But we do not wade through forests of puffballs or oceans of fleas. A species' exponential growth quickly slams into the harsh reality of this finite world. As E. coli's population grows denser, the bacteria use up oxygen faster than fresh supplies can arrive. Their waste builds up around them, turning toxic. This collision with reality can be fatal. As E. coli runs out of its essential nutrients, its ribosomes get sloppy, producing misshapen protein that attacks other molecules. The catastrophe can ripple out across the entire microbe. To continue to grow under such stress would be suicidal, like driving a car over a cliff.

Instead, E. coli slams on the brakes. In a matter of seconds it stops reading its genes and destroys all the proteins it's in the midst of building. It enters a zombielike state called the stationary phase. The microbe begins to make proteins to defend against heat, acid, and other insults even as it stops making the enzymes necessary for feeding. To keep dangerous molecules from slipping through its membranes, E. coli closes off many of its pores. To protect its DNA, E. coli folds it into a kind of crystalline sandwich. All of these preparations demand a lot of energy, which the microbe can no longer get from food. So E. coli eats itself, dismantling some of its own energy-rich molecules. It even cannibalizes many of its ribosomes, so it can no longer make new proteins.

The threats faced by a starving E. coli are much like the ones our own cells face as we get old. Aging human cells suffer the same sorts of damage to their genes and ribosomes. People who suffer Alzheimer's disease develop tangles of misshapen proteins in their brains—proteins that are deformed in much the same way some proteins in starving E. coli are deformed. Life not only grows and reproduces. It also decays.

Although humans and microbes face the same ravages of time, it's the microbe that comes out the winner. If scientists pluck out a single E. coli in a stationary phase and put it in a flask of fresh broth, it will unpack its DNA, build new proteins, and resume its life with stately grace. Scientists can leave a colony of E. coli in a stationary phase for five years and still resurrect some viable microbes. We humans never get such a second chance.

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