If you're just tuning in, on Tuesday I offered five free signed copies of my new book Microcosm: E. coli and the New Science of Life to readers if they sent in a question. I was quite stoked to see the huge reaction. I can tell from the quality of the questions that the sheer volume was not just the result of the lust for a free book. While I can only answer five questions today, I think most people who asked one will find that parts of the book touch on it. So--without further ado, let's dive in. (This is the first of five posts I'll deliver today.) 1. Frank asks:
Why E. coli? From a historical perspective, why do we study E. coli? There are countless easily culturable microbes out there, so how did the scientific community select this particular species as "the model" for microbiology?
This is one of the strangest parts of the story of E. coli. This microbe isn't just the model for microbiology. It's a model for a lot of the biology common to all living things, from the genetic code to the creation of new copies of DNA to the process by which food is turned into living matter. Scientists have identified the basic functions of most of E. coli's genes, which is a lot more than we can say even for human genes. If you type in "Escherichia coli" into PubMed, the search engine for the National Library of Medicine, you get 253,128 papers. Another favorite species, Drosophila melanogaster, sometimes (wrongly) called the fruit fly, brings up only 29,918. So you might think there must have been some eminently rational plan to select E. coli to become the creature science knows best. But there wasn't. It was discovered by Theodor Escherich, a pediatrician. In 1885 he delivered a lecture announcing the discovery of a rod-shaped microbe in the diapers of healthy babies. He was struck by how fast it grew on all sorts of food--milk, potatoes, blood. Scientists in the early 1900s used it to study metabolism, but they also used a lot of other bacteria. It was one among many. A few scientists in the late 1930s and early 1940s changed that. These were scientists who had especially deep questions about how life works. Max Delbruck wanted to know what genes are. George Beadle and Edward Tatum wanted to know how genes produced traits. They wound up with E. coli almost by accident. Tatum wanted to safe, fast-growing microbe that could build a lot of amino acids by itself. He and Beadle planned to blast such a microbe with X-rays to create mutations, and see whether the microbe lost the ability to make one of those amino acids. He chose a strain of E. coli called K-12 that had been isolated from a diptheria patient and had been used in microbiology classes at Stanford ever since. Max Delbruck, down at Caltech, wanted to find something simpler than flies in which he could study genes. He discovered that another Caltech scientist, Emory Ellis, was infecting E. coli with viruses from sewer water. Ellis was really interested in viruses that might cause cancer in people, but figuring out how viruses infected E. coli seemed like a good place to start. So Delbruck and Ellis began to investigate how viruses could use E. coli to make new copies of themselves. It certainly didn't hurt that E. coli was safe, grew fast, thrived in oxygen, and otherwise made life easy on scientists who studied it. But its success also came through a peculiar snowball effect. A young graduate student named Joshua Lederberg came to Tatum's lab to study his E. coli mutants, in the hopes of discovering that bacteria have sex. Tatum's bacteria just so happened to swap genes. Now scientists began to use their sex life to study genes, by pulling the microbes apart in the act and seeing which genes had made the jump. Scientists began to map E. coli's genes. They discovered in E. coli the switches that turn genes on and off. In other words, a new science called molecular biology was born. Soon scientists were choosing E. coli to study so they didn't have to reinvent the wheel. It helped that so much of biology is the same from species to species. As the French E. coli biologist Jacques Monod declared, what is true for E. coli is true for the elephant. But in an important sense, E. coli was the accidental victor.