Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [5]
Lederberg drew out samples of the bacteria and put them on fresh petri dishes. Now he withheld the four nutrients they could not make themselves: threonine, proline, methionine, and biotin. Neither of the original mutant strains could grow in the dishes. If their descendants were simply copies of their ancestors, Lederberg reasoned, they would stop growing as well.
But after weeks of frustration—of ruined plates, of dead colonies—Lederberg finally saw E. coli spreading across his dishes. A few microbes had acquired the ability to make all four amino acids. Lederberg concluded that their ancestors must have combined their genes in something akin to sex. And in their sex they proved that they carried genes.
Two E. coli having bacterial sex
In the years that followed, the discovery would allow scientists to breed E. coli like flies and to probe genes far more intimately than ever before. Twelve years later, at the ancient age of thirty-three, Lederberg would share the Nobel Prize in Medicine with Tatum and Beadle. But in 1946, when he picked up his petri dishes and noticed the spots that appeared to be the sexual colonies he had dreamed of, Lederberg allowed himself just a single word alongside the results in his notebook: “Hooray.”
HOST AND PARASITE
While Lederberg was observing E. coli having sex, other scientists were observing it getting sick. And they were learning things that were just as important about the nature of life.
The first scientist to appreciate just how revealing a sick E. coli could be was not a biologist but a physicist. Max Delbrück had originally studied under Niels Bohr and the other pioneers of quantum physics. In the 1930s it seemed as if a few graceful equations could melt away many of the great mysteries of the universe. But life would not submit. Physicists like Delbrück were baffled by life’s ability to store away all of the genes necessary to build a kangaroo or a liverwort in a single cell. Delbrück decided to make life—and in particular, life’s genes—his study.
“The gene,” Delbrück proposed, “is a polymer that arises by the repetition of identical atomic structures.” To discover the laws of that polymer, he came to the United States, joining Morgan’s laboratory to breed flies. But the physicist in Delbrück despised the messy quirks of Drosophila. He craved another system that could provide him with far more data and was far simpler. As luck would have it, another member of Morgan’s lab, Emory Ellis, was studying the perfect one: the viruses that infect E. coli.
The viruses that infect E. coli were too small for Delbrück and Ellis to see. As best anyone could tell, they infected their bacterial hosts and reproduced inside, killing the microbes and wandering off to find new victims. The new viruses seemed identical to the old, which suggested that they might carry genes. Delbrück and Ellis set out to chart the natural history of E. coli’s viruses.
To study the viruses—known as bacteriophages—Delbrück and Ellis could look only for indirect clues. If they added viruses to a dish of E. coli, the viruses invaded the bacteria and replicated inside them. The new viruses left behind the shattered remains of their hosts and infected new ones. Over a few hours spots formed on the dish where their victims formed transparent pools of carnage. “Bacterial viruses make themselves known by the bacteria they destroy,” Delbrück said, “as a small boy announces his presence when a piece of cake disappears.”
Although the signs of the viruses were indirect, there were a lot of them. Billions of new viruses could appear in a dish in a few hours. The power of Delbrück and Ellis’s system attracted a small flock of young scientists. They called themselves the Phage Church, and Delbrück was their pope. The Phage Church demonstrated that E. coli’s bacteriophages were not