Delbrück, Max (1906-1981) was a German-born American biologist who shared the 1969 Nobel Prize in physiology or medicine with American biologists Alfred Day Hershey and Salvador Edward Luria. Delbrück received the prize for his pioneering work on bacteriophages (viruses that attack bacteria). This work partly focused on the genetic function of bacteriophages. Delbrück's studies led other scientists to discover the structure and importance of DNA (deoxyribonucleic acid), the substance that makes up genes in the cells of living organisms.

Max Ludwig Henning Delbrück was born in Grunewald, Germany, now a suburb of Berlin. He was the youngest of seven children of Hans Delbrück, a history professor at the University of Berlin and for many years an editor and political columnist for the journal Prussian Yearbooks. Delbrück's mother, Liana Tiersch Delbrück, was the daughter of a surgery professor and the granddaughter of chemist Baron von Liebig, who helped establish the field of organic chemistry, the study of compounds that contain carbon. Grunewald, where Delbrück grew up, was a comfortable suburb of Berlin, but its families were not spared the troubles of World War I (1914-1918) or the political disorder, inflation, and impoverishment in the years that followed.

As a boy, Delbrück became interested in mathematics and astronomy. After graduating from the Grunewald high school, he went to the University of Tübingen in 1924 to study astronomy. He left there to take advantage of free tuition at his father's school. Not content, he moved on to the University of Bonn, returned to the University of Berlin, and eventually entered the University of Göttingen, where he remained.

Göttingen had become a center for the new field of quantum mechanics, the field of physics that describes the structure of atoms and the motion of atomic particles. This stimulating atmosphere would greatly influence young Delbrück. Faculty members he met included Hungarian-born theoretical physicist Eugene Paul Wigner, who later became a United States citizen, and German physicist Max Born, who played a major role in developing quantum mechanics. Both would later earn Nobel Prizes in physics. Delbrück worked out mathematical proofs for the chemical bonding of lithium. In 1930, he received his doctorate in physics.

Delbrück went on to spend three years in England, Switzerland, and Denmark. He did postgraduate studies at the University of Bristol in England for a year and a half. A Rockefeller Foundation Fellowship afforded Delbrück six months working under Danish physicist Niels Bohr at the University of Copenhagen, followed by another six months with Austrian theoretical physicist Wolfgang Pauli at the University of Zurich. Bohr had developed a theory about the structure of the atom. Pauli would later win the 1945 Nobel Prize in physics for his proposal in 1925 of a rule explaining the behavior of electrons in atoms.

Bohr aroused Delbrück's interest in the relation between physics and biology by suggesting that some of the attributes of physics could be found to be the same in biology. This greatly influenced Delbrück's outlook.

Delbrück returned to Berlin in 1932 and became an assistant to Lise Meitner. an Austrian-born physicist who played an essential role in the discovery of nuclear fission (the splitting of the nucleus of an atom). Meitner was studying neutron irradiation of uranium with Otto Hahn, a German chemist who discovered the process of nuclear fission in 1938. Scientists in the United States soon used that discovery in developing the first atomic bomb.

Delbrück met regularly in Berlin with a group of physicists and biologists that shared an interest in genetics. In an influential 1935 paper, Delbrück and his research partners proposed that genes be viewed as molecules.

Delbrück immigrated to the United States in 1937, when he received a second Rockefeller Fellowship, which he used to study biology and genetics at the California Institute of Technology (Caltech) in Pasadena. There, working with geneticist Thomas Hunt Morgan, he investigated the genetics of the fruit fly (Drosophila melanogaster), an organism commonly used in research because it has a short life span, and many generations can be studied without waiting very long.

At Caltech, Delbrück began assisting on phage research. He became interested in the genetics of the bacteriophage, a type of virus that infects bacterial cells. Its name means “bacteria eater.” In a pioneering paper in 1939, he helped explain how phages multiply in individual cells. This work launched a new era of virus research. After World War II (1939-1945) started, he decided to remain in the United States. Back in Germany, several members of Delbrück's family were persecuted for their resistance to the Nazi regime.

In 1940, Delbrück accepted a position as instructor of physics at Vanderbilt University in Nashville. He continued his bacteriophage research at Vanderbilt for the next seven years. Delbrück and Mary Adeline Bruce, whom he had met at Caltech, married in 1941. They had four children. Delbrück became a United States citizen in 1945.

Around 1940, Delbrück met Salvador Luria at a meeting of the American Association for the Advancement of Science in Philadelphia. Luria was conducting bacteriophage research at the College of Physicians and Surgeons of Columbia University in New York City. Delbrück visited Luria's laboratory, and the two men planned a series of experiments. They shared their work through correspondence and occasional meetings.

In the early 1940's, scientists knew little about the nature of viruses, the microscopic organisms that are a major cause of disease. Delbrück, Luria, and Hershey, who was associated with Washington University School of Medicine in St. Louis, Missouri, had each performed research on bacteriophages.

Most bacteriophages have an elaborate structure—a spherical head containing a nucleic acid and a hollow tail made of protein—that enables them to penetrate the tough cell walls of bacteria. When a bacteriophage enters a bacterium, the tail penetrates the cell wall first. Then the nucleic acid in the head moves through the tail and into the cell.

Since viruses cannot be seen with an ordinary microscope, Luria, Delbrück, and Hershey obtained images of bacteriophages with the Newly invented electron microscope. In 1943, using a technique perfected by Delbrück, Luria discovered that viruses mutate, or change their characteristics from one generation to another. Working independently, Delbrück and Hershey discovered in 1946 that different strains of bacteriophage may exchange genetic material (genes), if more than one strain infects the same bacterial cell. The phenomenon, which they called genetic recombination, was the first experimental evidence of recombination of DNA in viruses.

During 10 years of collaboration and independent study, the trio showed that the phage particle, which has a shell of protein, is essentially DNA. Hershey in 1952 proved that when the phage invades a cell, the DNA sheds its protein coating and takes over the genetic mechanism of the host, forcing it to reproduce new viruses instead of new cells. By this discovery of viral replication, the three biologists uncovered a process basic to all life. This opened the door to numerous discoveries of how more complex organisms reproduce and pass on hereditary characteristics.

Luria, Delbrück, Hershey, and other scientists wrote up guidelines for bacteriophage research. They encouraged other investigators in the field to concentrate on seven bacteriophages that infect a certain colon bacterium. That way, experimental results from different laboratories could be compared.

Caltech appointed Delbrück professor of biology in 1947, and the National Academy of Sciences elected him a member in 1949. Delbrück, Hershey, and Luria shared the 1969 Nobel Prize in physiology or medicine “for their discoveries concerning the replication mechanism and the genetic structure of viruses.”

Delbrück remained at Caltech until his retirement in 1977, when he was appointed to the institute's board of trustees. From 1961 to 1963, he served as a guest professor at the University of Cologne in West Germany, where he helped to establish the university's Institute of Genetics.

Several top level and smaller universities awarded Delbrück honorary degrees. He also won a number of prestigious science prizes, including the Kimber Genetics Award of the National Academy of Sciences in 1964. He was a member of the National Academy of Sciences, the American Academy of Arts and Sciences, the Royal Danish Academy, the Royal Society of London, and the French Academy of Sciences.