Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [24]
My distaste for snails is a minor example of a major fact: life is full of differences. We humans differ from one another in ways too many to count. We are shy and bold, freckled and pale, truckers and hairdressers, Buddhists and Presbyterians. We get cancers in third grade and live for a century. We have fingerprints.
Scientists have only a rough understanding of how this diversity arises. We are not merely the output of software written in a programming code of DNA. As we develop in the womb, our genes interact with signals from our mothers. The environment continues to influence those genes in unpredictable ways after birth. The food we eat, the air we breathe, the traumas and joys and boredom of childhood, and all the rest have an influence on which genes become active. Our differences are not just hard to trace but a source of pride. We can produce greatness of all kinds: Babe Ruths and Frédéric Chopins, Mae Wests and Marie Curies. They are products of our complexity, of a species in which each individual carries 18,000 genes that can become 100,000 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