Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [38]
After the E. coli populations had grown for a while, Luria took some bacteria from each one and spread them on petri dishes laced with viruses. He waited for epidemics to strike, and then for resistant E. coli colonies to emerge.
According to Lamarck, living things acquire new traits as they face new challenges, then pass these traits down to their offspring. If Luria’s E. coli obeyed Lamarck, the bacteria would acquire resistance after Luria exposed them to viruses. That would mean that once Luria had inoculated his virus-laden dishes, every microbe had the same small chance of evolving resistance. Luria ought then to have discovered a few resistant colonies in every dish. The experiment would have resembled a slot machine that pays out a lot of small wins.
If E. coli obeyed Darwin, on the other hand, the experiment would play out like a slot machine with a few big wins. According to Darwin’s followers, E. coli has a rare random chance of mutating every time it divides regardless of what it is experiencing. In other words, the bacteria in Luria’s experiment might have acquired resistance to viruses while they were growing in the flasks, long before Luria exposed them to the viruses. That head start would have produced a very different result for the experiment. If a mutation had emerged early on in one of the colonies, the mutant would have had a lot of time to produce offspring. When Luria took some of the bacteria from such a colony and placed them in a petri dish with viruses, a fair number of them would already be resistant. They would grow into many new colonies in the dish.
In some of the other flasks, resistant mutants would arise much later. They would have had less time to produce offspring by the time Luria exposed them to viruses. As a result, they’d produce fewer colonies in the petri dishes. And in still other flasks, no mutants would arise at all. Their bacteria would all die, leaving their dishes empty. So instead of a few colonies growing in most dishes—the Lamarckian prediction—Darwinian mutations would produce a few dishes loaded with colonies and the rest with few or none.
Luria let his slot machines spin, and then he began to count spots. When he was done, the verdict was clear: a few dishes were packed with colonies while many were empty. Life’s slot machine had paid out a handful of big jackpots. Darwin had won.
In 1943, Luria and Delbrück published these results, which would earn them shares in a Nobel Prize in 1969. Later generations of biologists would look back at Luria’s experiment as one of the greatest of the twentieth century. It provided compelling evidence that bacteria, like animals and plants, pass down their traits to their offspring through genes. It showed that those genes change spontaneously, and they can become more common in a population through natural selection. And the experiment became a powerful scientific tool: simply by counting colonies of bacteria, scientists can calculate how often mutations arise.
But when Luria and Delbrück first published the experiment, they did not bowl over the skeptics. Neo-Lamarckians remained unconvinced, pointing out that the researchers had had to rely on a lot of indirect clues to draw their conclusions. It was possible that the test tubes had not all been alike. Some might have had traces of soap or some other contamination that might have altered the bacteria. For another decade, microbiologists went on debating how bacteria adapted.
The controversy did not die until Joshua Lederberg, the scientist who discovered E. coli sex, tested the jackpot hypothesis with a new experiment. Lederberg and his wife, Esther, wrapped sheets of velvet around the ends of wooden cylinders that were as wide as a petri dish. The Lederbergs then