The phage group (sometimes called the American Phage Group) was an informal network of biologists centered around Max Delbrück that contributed heavily to bacterial genetics and the origins of molecular biology in the mid-20th century. The phage group takes its name from bacteriophages, the bacteria-infecting viruses that group used as experimental model organisms. In addition to Delbrück, important scientists associated with the phage group include: Salvador Luria, Alfred Hershey, Seymour Benzer, Gunther Stent, James D. Watson, Frank Stahl, and Renato Dulbecco.
Origins of the phage group 
Bacteriophages had been a subject of experimental investigation since Felix d'Hérelle had isolated and developed methods for detecting and culturing them, beginning in 1917. Delbrück, a physicist-turned biologist seeking the simplest possible experimental system to probe the fundamental laws of life, first encountered phage during a 1937 visit to T. H. Morgan's fly lab at Caltech. Delbrück was unimpressed with Morgan's experimentally complex model organism Drosophila, but another researcher, Emory Ellis, was working with the more elementary phage. During the next few years, Ellis and Delbrück collaborated on methods of counting phage and tracking growth curves; they established the basic step-wise pattern of virus growth (the most obvious features of the lytic cycle).
The phage group started around 1940, after Delbrück and Luria had met at a physics conference. Delbrück and Luria began a series of collaborative experiments on the patterns of infection for different strains of bacteria and bacteriophage. They soon established the "mutual exclusion principle" that an individual bacterium can only be infected by one strain of phage. In 1943, their "fluctuation test", later dubbed the Luria-Delbrück experiment, showed that genetic mutations for antibiotic resistance arise in the absence of selection, rather than being a response to selection. That year, they also began working with Alfred Hershey, another phage experimenter. (The three would share the 1969 Nobel Prize in Physiology or Medicine, "for work on the replication mechanism and genetics of viruses".)
Delbrück, through his charm and enthusiasm, brought many biologists (and physicists) into phage research in the early 1940s. In 1944, Delbrück promoted the "Phage Treaty", a call for phage researchers to focus on a limited number of phage and bacterial strains, with standardized experimental conditions. This helped to make research from different laboratories more easily comparable and replicable, helping to unify the field of bacterial genetics.
Phage course at Cold Spring Harbor Laboratory 
Apart from direct collaborations, the main legacy of the phage group resulted from the yearly summer phage course at Cold Spring Harbor Laboratory. Beginning in 1945, Delbrück and others taught young biologists the fundamentals of phage biology and experimentation, instilling the phage group's distinctive math- and physics-oriented approach to biology. Many of the leaders of the emerging field of molecular biology were alumni of the phage course, which continued to be taught through the 1950s and 1960s.
- Morange, A History of Molecular Biology, pp 41-43
- Morange, A History of Molecular Biology, pp 43-44
- Morange, A History of Molecular Biology, pp 45-46
- History: The Phage Group, Cold Spring Harbor Laboratory, accessed May 4, 2007
- Morange, A History of Molecular Biology, pp 46-47
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