Free Microcosm by Carl Zimmer

proteins, which give rise to creatures uniquely able to experience the world, to shape their lives by words, rituals, images. And this pride colors our image of
E. coli.
    Surely
E. coli
must be all nature and no nurture. A colony descended from a single ancestor is just a billion genetically identical cousins, their behavior all run through the same genetic circuits.
E. coli
is just a single cell, after all, not a body made of a trillion cells that take years to develop.
E. coli
doesn’t grow up going to private school or searching for food on a garbage dump. It doesn’t wonder whether it might like snails for dinner. It’s just a bag of molecules. If it is genetically identical to another
E. coli,
then the two of them will live identical lives.
    This may all sound plausible, but it is far from the truth. A colony of genetically identical
E. coli
is, in fact, a mob of individuals. Under identical conditions, they will behave in different ways. They have fingerprints of their own.
    If you observe two genetically identical
E. coli
swimming side by side, for example, one may give up while the other keeps spinning its flagella. To gauge their stamina, Daniel Koshland, a scientist at the University of California, Berkeley, glued genetically identical
E. coli
to a glass cover slip. They floated in water, tethered by their flagella. Koshland offered them a taste of aspartate, an amino acid that attracts them and motivates them to swim. Stuck to the slide, the bacteria could only pirouette. Koshland found that some of the clones twirled twice as long as others.
    E. coli
expresses its individuality in other ways. In a colony of genetically identical clones, some will produce sticky hairs on their surface, and some will not. In a rapidly breeding colony, a few individual microbes will stop growing, entering a peculiar state of suspended animation. In a colony of
E. coli,
some clones like milk sugar, and others don’t.
    These differing tastes for lactose first came to light in 1957. Aaron Novick and Milton Weiner, two biologists at the University of Chicago, looked at how individual
E. coli,
respond to the presence of lactose. They fed
E. coli
a lactoselike molecule that could also trigger the bacteria to make beta-galactosidase. At low levels only a tiny fraction of the microbes responded by producing beta-galactosidase. Most did nothing.
    Novick and Weiner added more of the lactose mimic. The eager individuals remained eager. The reluctant ones remained reluctant. Only after the lactose mimic rose above a threshold did the reluctant microbes change. Suddenly they produced beta-galactosidase as quickly as the eager microbes.
    Somehow the bacteria were behaving in radically different ways even though they were all genetically identical. Novick and Weiner isolated eager and reluctant individuals and transferred them to fresh petri dishes, where they could breed new colonies of their own. Their descendants continued to behave in the same way. Eager begat eager; reluctant, reluctant. Novick and Weiner had found a legacy beyond heredity.
    There’s much to be learned about
E. coli
by thinking of it as a machine with circuitry that follows the fundamental rules of engineering. But only up to a point. Two Boeing 777s that are in equally good working order should behave in precisely the same way. Yet if they were like
E. coli,
one might turn south when the other turned north.
    The difference between
E. coli
and the planes lies in the stuff from which they are made. Unlike wires and transistors,
E. coli’
s molecules are floppy, twitchy, and unpredictable. They work in fits and starts. In a plane, electrons stream in a steady flow through its circuits, but the molecules in
E. coli
jostle and wander. When a gene switches on,
E. coli
does not produce a smoothly increasing supply of the corresponding protein. A single
E. coli
spurts out its proteins unpredictably. If its
lac
operon turns on, it may spit out six beta-galactosidase enzymes in