Delbrück , Max

(1906–1981) German-born American physicist and molecular biologist

The son of a history professor, Delbrück trained as a physicist first in his native city of Berlin, and then at Tübingen, Bonn, and Göttingen, where he completed his doctorate in 1930. After spending the period from 1931 to 1933 in Copenhagen, Delbrück was appointed to the Kaiser Wilhelm Institute for Chemistry, Berlin. He left Germany for America in 1937, working first at the California Institute of Technology and from 1940 until 1947 at Vanderbilt University, Nashville. Delbrück returned to Cal Tech in 1947 and remained there as professor of biology until his retirement in 1976. He became a naturalized American citizen in 1945.

While at Copenhagen, under the influence of Niels Bohr, Delbrück's interest was diverted from atomic physics to questions about the nature of life. In the late 1930s he began to work with bacteriophages, the viruses discovered by D'Herelle that infect and destroy bacteria. They were relatively simple, reproduced quickly, and were easy to handle; an ideal organism, Delbrück argued, in which to study the mechanisms of replication and development.

In 1939, with E. Ellis, he first demonstrated the phenomenon of ‘one-step growth’. Working with the phage T4 he found that “a virus particle enters a bacterial cell and after a certain period (between 13 and 40 minutes, depending on the virus, on the dot for any particular type), the bacterial cell is lysed and 100 particles are liberated.” How can one particle, Delbrück asked, become 100 in a mere 20 minutes?

Soon after he began to collaborate with Salvador Luria. In 1943 they published a paper, Mutations of Bacteria from Virus Sensitivity to Virus Resistance. How, they asked, do bacteria acquire resistance to lethal phage? Is it induced by contact, or does it arise from a fortunate mutation? Luria and Delbrück realized that the dynamics of bacterial growth would differ in each case. The number of resistant strains found in bacterial colonies exposed to phage should fluctuate more than if the resistance was induced. Delbrück worked out the statistics and Luria performed the experiment; the results clearly revealed that bacteria underwent mutations.

Delbrück went on to show in 1945, in collaboration with W. Bailey, that phage can reproduce sexually. They were working with the two viruses T2 and T4r, both of which could be bred in bacterium B. They found that:

T2 formed small colonies and attacked bacterium A.

T4r formed large colonies and attacked bacterium C.

When both T2 and T4r were bred together in B, the parent types produced two new strains:

Strain 1: formed small colonies and attacked bacterium C.

Strain 2: formed large colonies and attacked bacterium A.

Obviously, Delbrück concluded, “the parents had got together and exchanged something.”

By this time Delbrück had begun to be recognized as the leader of what became known as the phage group. From 1945 onwards he ran an annual summer phage course at Cold Spring Harbour Laboratory, New York, which was attended over the years by most of the leading molecular biologists of the following decade. For Delbrück himself, however, the mid-1950s seemed to be a good time to move on. He turned to the study of sensory mechanisms in the fungus Phycomyces. It grew towards the light, against gravity, and into the wind. How did it sense these stimuli? What range of light did it respond to? These and other questions were tackled by Delbrück and his coworkers in what was soon called the Phycomyces group. His last published paper in 1981 was in this field and proposed that the chemical photoreceptor ofPhycomyces was a flavin and not, as had been supposed, a carotene.

For his earlier work with the phage group Delbrück shared the 1969 Nobel Prize for physiology or medicine with Salvador Luria and Alfred Hershey.