Microcosm by Carl Zimmer Page B

clusters that coat the intestines. The clusters not only block invaders but also prevent individual microbes from penetrating the lining of the gut. All this biochemical goodwill makes sense—after all, we and
E. coli
are members of the same collective.
    TOGETHERNESS
    In 2003, Jeffry Stock and his colleagues at Princeton University put
E. coli
in a maze. The maze, which measured less than a hundredth of an inch on each side, had walls of plastic and a roof of glass. The scientists submerged it in water and then injected
E. coli
into the entrance. The bacteria began to spin their flagella and swim. Stock’s team had added a gene for a glowing protein to each
E. coli
so they could follow their trail as the microbes wandered through the labyrinth.
    At first the bacteria seemed to move randomly. But they gradually gathered together and began to swim in schools. Some of the schools got trapped in a dead end, where the bacteria were content to stay with one another. The other bacteria swam after them, and after two hours the dead end was filled with a huddled mass of glowing microbes.
    To figure out how the bacteria were finding one another, the Princeton scientists set mutants loose in the maze. They found that
E. coli
can congregate as long as their microbial tongues taste the amino acid serine. It just so happens that in the normal course of its metabolism,
E. coli
casts off serine in its waste. Scientists had known of the microbe’s attraction to serine since the 1960s, but they had generally assumed that it had something to do with the microbe’s search for food.
E. coli’
s sociable flocking in the maze raised another possibility: its tongue may be tuned to find other
E. coli.
    Not long ago
E. coli
and most other bacteria were considered loners. After all, they seemed to lack the sort of glue that holds societies together: a way to communicate. They cannot write e-mail; they cannot shake their tail feathers; they cannot sing across a desert at dawn. But
E. coli
does have a kind of language of its own and its own kind of society.
    E. coli’
s social life has been overlooked for decades because most biologists have been more interested in the bare basics of its existence: how it feeds, grows, and reproduces. They’ve perfected the recipe for getting
E. coli
to do all three things as fast as possible. The warm, oxygen-rich, overfed life
E. coli
enjoys in the lab favors individual microbes that can breed quickly. But it bears little resemblance to
E. coli’
s normal existence. Although each person eats about sixty tons of food in a lifetime,
E. coli
may starve for hours or days. When it does get the chance to eat, it may be presented with a low-energy sugar barely worth the effort it takes to break down.
E. coli
may have to compete with other microbes for every molecule. At the same time, it must withstand assaults from viruses, predators, and man-made dangers such as antibiotics. Its host may become ill, devastating its entire habitat. One of the best ways to withstand all these catastrophes is to join forces with other
E. coli.
    Once they gather, the bacteria may do a number of things. Under some conditions a group of
E. coli
will sprout a new kind of flagellum, one that’s far longer than its ordinary tail. The new flagella join together, tethering millions of bacteria into a single seething mass. Instead of swimming, they swarm across a surface, squirting out molecules that soak up water and create a carpet of slime. Swarming allows
E. coli
to glide across a petri dish or, scientists suspect, across an intestinal wall.
    E. coli
can also settle down and build a microbial city. Scientists have long been aware that bacteria can form a cloudy layer of scum on their flasks, known as a biofilm. Biofilms simply annoyed biologists at first. But a closer look revealed biofilms to be marvelously intricate structures. All microbes can make biofilms, and scientists suspect that the vast majority of microbes spend most of their