Life's list grows longer. It stores information in genes. It needs barriers to stay alive. It captures energy and food to build new living matter. But if life cannot find that food, it will not survive for long. Living things need to move—to fly, squirm, drift, send tendrils up gutter spouts. And to make sure they're going in the right direction, most living things have to decide where to go.
We humans use 100 billion neurons bundled in our heads to make that decision. Our senses funnel rivers of information to the brain, and it responds with signals that control the movements of our bodies. E. coli, on the other hand, has no brain. It has no nervous system. It is, in fact, thousands of times smaller than a single human nerve cell. And yet it is not oblivious to its world. It can harvest information and manufacture decisions, such as where it should go next.
E. coli swims like a spastic submarine. Along the sides of its cigar-shaped body it sprouts about half a dozen propellers. They're shaped like whips, trailing far behind the microbe. Each tail (or, as microbiologists call it, flagellum) has a flexible hook at its base, which is anchored to a motor. The motor, a wheel-shaped cluster of proteins, can spin 250 times a second, powered by protons that flow through its pores into the microbe's interior.
Most of the time, E. coli's motors turn counterclockwise, and when they do their flagella all bundle together into a cable. They behave so neatly because each flagellum is slightly twisted in the same direction, like the ribbons on a barber's pole. The cable of flagella spin together, pushing against the surrounding fluid in the process, driving the microbe forward.
E. coli can swim ten times its body length in a second. The fastest human swimmers can move only two body lengths in that time. And E. coli wins this race with a handicap, because the physics of water is different for microbes than for large animals like us. For E. coli, water is as viscous as mineral oil. When it stops swimming, it comes to a halt in a millionth of a second. E. coli does not stop on a dime. It stops on an atom.
About every second or so, E. coli throws its motors in reverse and hurls itself into a tumble. When its motors spin clockwise, the flagella can no longer slide comfortably over one another. Now their twists cause them to push apart; their neat braid flies out in all directions. It now looks more like a fright wig than a barber's pole. The tumble lasts only a tenth of a second as E. coli turns its motors counterclockwise once more. The flagella fold together again, and the microbe swims off.
The first scientist to get a good look at how E. coli swims was Howard Berg, a Harvard biophysicist. In the early 1970s, Berg built a microscope that could follow a single E. coli as it traveled around a drop of water. Each tumble left E. coli pointing in a new random direction. Berg drew a single microbe's path over the course of a few minutes and ended up with a tangle, like a ball of yarn in zero gravity. For all its busy swimming, Berg found, E. coli manages to wander only within a tiny space, getting nowhere fast.
E. coli's flagellum is driven by motorlike proteins that spin in its membrane.
Offer E. coli a taste of something interesting, however, and it will give chase. E. coli's ability to navigate is remarkable when you consider how little it has to work with. It cannot wheel and bank a pair of wings. All it can do is swim in a straight line or tumble. And it can get very little information about its surroundings. It cannot consult an atlas. It can only sense the molecules it happens to bump into in its wanderings. But E. coli makes good use of what little it has. With a few elegant rules, it gets where it needs to go.
E. coli builds sensors and inserts them in its membranes so that their outer ends reach up like periscopes. Several thousand sensors cluster together at the microbe's front tip, where they act like a microbial tongue. They come in five types, each able to grab certain kinds of molecules. Some types attract E. coli, and some repel it. An attractive molecule, such as the amino acid serine, sets in motion a series of chemical reactions inside the microbe with a simple result: E. coli swims longer between its tumbles. It will keep swimming in longer runs as long as it senses that the concentration of serine is rising. If its tumbles send it away from the source of serine, its swims become shorter. This bias is enough to direct E. coli slowly but reliably toward the serine. Once it gets to the source, it stays there by switching back to its aimless wandering.
Scientists began piecing together E. coli's system of sensing and swimming in the 1960s. They chose E. coli's system because they thought it would be easy. They could take advantage of the long tradition of using mutant E. coli to study how proteins work. And once they had solved E. coli's information processors, they would be able to take what they had learned and apply it to more complex processors, including our own brains. Forty years later they understand E. coli's signaling system more thoroughly than that of any other species. Some parts of E. coli's system turned out to be simple after all. E. coli does not have to compute barrel rolls or spiral dives. Its swim-and-tumble strategy works very well. Every E. coli may not get exactly where it needs to go, but many of them will. They will be able to survive and reproduce and pass the run-and-tumble strategy on to their offspring. That is all the success a microbe needs.
Yet in some important ways, E. coli's navigation defies understanding. Its microbial tongue can detect astonishingly tiny changes in the concentration of molecules it cares about, down to one part in a thousand. The microbe is able to amplify these faint signals in a way that scientists have not yet discovered. It's possible that E. coli's receptors are working together. As one receptor twists, it causes neighboring receptors to twist as well.
E. coli may even be able to integrate different kinds of information at the same time—oxygen climbing, nickel falling, glucose wafting by. Its array of receptors may turn out to be far more than just a microbial tongue. It may be more like a brain.
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