The Unity Of Life

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Escherich originally dubbed his bacteria Bacterium coli communis: a common bacterium of the colon. In 1918, seven years after Escherich's death, scientists renamed it in his honor. By the time it got a new name, it had taken on a new life. Microbiologists were beginning to rear it by the billions in their laboratories.

In the early 1900s, many scientists were pulling cells apart to see what they were made of, to figure out how they turned raw material into living matter. Some scientists studied cells from cow muscles, others sperm from salmon. Many studied bacteria, including E. coli. In all of the living things they dissected, scientists discovered the same basic collection of molecules. They focused much of their attention on proteins. Some proteins give life its structure—the collagen in skin, the keratin in a horse's hoof. Other proteins, known as enzymes, usher other molecules into chemical reactions. Some enzymes split atoms off molecules, and others weld molecules together.

Proteins come in a maddening diversity of complicated shapes, but scientists discovered that they also share an underlying unity. Whether from humans or bacteria, proteins are all made from the same building blocks: twenty small molecules known as amino acids. And these proteins work in bacteria much as they do in humans. Scientists were surprised to find that the same series of enzymes often carry out the same chemical reactions in every species.

"From the elephant to butyric acid bacterium—it is all the same!" the Dutch biochemist Albert Jan Kluyver declared in 1926.

The biochemistry of life might be the same, but for scientists in the early 1900s, huge differences seemed to remain. The biggest of all was heredity. In the early 1900s, geneticists began to uncover the laws by which animals, plants, and fungi pass down their genes to their offspring. But bacteria such as E. coli didn't seem to play by the same rules. They did not even seem to have genes at all.

Much of what geneticists knew about heredity came from a laboratory filled with flies and rotten bananas. Thomas Hunt Morgan, a biologist at Columbia University, bred the fly Drosophila melanogaster to see how the traits of parents are passed on to their offspring. Morgan called the factors that control the traits genes, although he had no idea what genes actually were. He did know that mothers and fathers both contributed copies of genes to their offspring and that sometimes a gene could fail to produce a trait in one generation only to make it in the next. He could breed a red-eyed fly with a white-eyed one and get a new generation of flies with only red eyes. But if he bred those hybrid flies with each other, the eyes of some of the grandchildren were white.

Morgan and his students searched for molecules in the cells of Drosophila that might have something to do with genes. They settled on the fly's chromosomes, those strange structures inside the nucleus. When chromosomes are given a special stain, they look like crumpled striped socks. The stripes on Drosophila chromosomes, Morgan and his students discovered, are as distinctive as bar codes. Chromosomes mostly come in pairs, one inherited from each parent. And by comparing their stripes, Morgan and his students demonstrated that chromosomes can change from one generation to the next. As a fly's sex cells develop, each pair of chromosomes embrace and swap segments. The segments a fly inherited determined which genes it carried.

There was something almost mathematically abstract about these findings. George Beadle, one of Morgan's graduate students, decided to bring genes down to earth by figuring out exactly how they controlled a single trait, such as eye color. Working with the biochemist Edward Tatum, Beadle tried to trace cause and effect from a fly's genes to the molecules that make up the pigment in its eyes. But that experiment soon proved miserably complex. Beadle and Tatum abandoned flies for a simpler species: the bread mold Neurospora crassa.

Bread mold may not have obvious traits such as eyes and wings, but it does produce many enzymes, some of which build amino acids. To see how the mold's genes control those enzymes, Beadle and Tatum bombarded it with X-rays. They knew that when fly larvae are exposed to X-rays, the radiation mutates some of their genes. The mutations produce new traits—extra leg bristles or a different eye color—which mutant flies can pass down to their offspring.

Beadle and Tatum now created bread mold mutants. Some were unable to produce certain types of amino acids because they now lacked a key enzyme. But if Beadle and Tatum mated the mutant bread mold with a normal one, some of their offspring could make the amino acid once more. Beadle and Tatum concluded in 1941 that behind each enzyme in bread mold there is one gene.

A hazy but consistent picture of genes was emerging—at least a picture of the genes of animals, plants, and fungi. But there didn't seem to be a place for bacteria in the picture. The best evidence for genes came from chromosomes, and bacteria seemed to have no chromosomes at all. Even if bacteria did have genes, scientists had little hope of finding them. Scientists could study a fly's genes thanks to the fact that flies reproduce sexually. A fly's chromosomes get cut up and shuffled in different combinations in its offspring. Scientists could not run this sort of experiment on bacteria, because bacteria did not have sex. They seemed to just grow and then split in two. Many researchers looked at bacteria as simply loose bags of enzymes—a fundamentally different kind of life.

It would turn out, however, that all life, bacteria included, shares the same foundation. E. coli would reveal much of that unity, and in the process it would become one of the most powerful tools biologists could use to understand life.

The transformation started with a simple question. Edward Tatum wondered if the one-gene, one-enzyme rule he discovered in mold applied to bacteria. He decided to run the mold experiment again, this time directing his X-rays at bacteria. For his experiment, Tatum chose a strain of E. coli called K-12. It had been isolated in 1922 from a California man who suffered from diphtheria, and it had been kept alive ever since at Stanford University, where it was used for microbiology classes.

Tatum's choice was practical. Like most strains of E. coli, K-12 is harmless. E. coli is also versatile enough to build all of its own amino acids and many other molecules. For food, it needs little more than sugar, ammonia, and some trace minerals. If E. coli used a lot of enzymes to turn this food into living matter, Tatum would have plenty of targets for his X-rays. He might succeed in creating only a few mutants of the sort he was looking for, but thanks to E. coli's luxurious growth he'd be able to see them. A single mutant could give rise to a visible colony in a day.

Tatum pelted colonies of E. coli with enough X-rays to kill 9,999 of every 10,000 bacteria. Among the few survivors he discovered mutants that could grow only if he supplied them with a particular amino acid. Helped along, the mutants could even reproduce, and their offspring were just as crippled. Tatum had gotten the same results as he had with bread mold. It looked as if behind every enzyme in E. coli lurked a gene.

It was a profound discovery, but Tatum remained cautious about its significance. It now seemed that bacteria had genes, but he could not say for sure. The best way to prove that a species had genes was to breed males and females and study their offspring. But E. coli seemed sadly celibate. "The term 'gene' can therefore be used in connection with bacteria only in a general sense," Tatum wrote.

The connection became far stronger when a somber young student arrived at Tatum's lab at Yale. Joshua Lederberg was only twenty-one years old when he began to work with Tatum, but he had a grand ambition: to find out whether bacteria had sex. As part of his military service during World War II, Lederberg had spent time in a naval hospital on Long Island, where he examined malaria parasites from marines fighting in the Pacific. He had gazed down at the single-celled protozoans, which sometimes reproduced by dividing and sometimes by taking male and female forms and mating. Perhaps bacteria had this sort of occasional sex, and no one had noticed. Others might mock the idea as a fantasy, but Lederberg decided to take what he later called "the long-shot gamble in looking for bacterial sex."

When Lederberg heard about Tatum's work, he realized he could look for bacterial sex with a variation on Tatum's experiments. Tatum was amassing a collection of mutant E. coli K-12, including double mutants—bacteria that had to be fed two compounds to survive. Lederberg reasoned that if he mixed two different double mutants together, they might be able to pick up working versions of their genes through sex.

Lederberg started work at Yale in 1946. He selected a mutant strain that could make neither the amino acid methionine nor biotin, a B vitamin. The other strain he picked couldn't make the amino acids threonine and proline. Lederberg put the bacteria in a broth he stocked with all four compounds so that the mutant microbes could grow and multiply. They mingled in the broth for a few weeks, with plenty of opportunity for hypothetical sex.

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."

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