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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