In 1915 English bacteriologist Frederick William Twort of the University of London discovered discovered bacteriophages, a type of virus that attacks bacteria (the term bacteriophage was coined by Félix d’Herelle, who in 1917 independently confirmed Twort’s discovery).
The discovery of bacteriophage began an immensely fruitful line of research that produced, among other things, Avery’s demonstration that DNA is the basic material responsible for genetic transformation (1944) and Alfred Hershey and Martha Chase’s “Waring Blender” experiment showing that DNA is the carrier of genetic information in virus reproduction (1952). Much of this work was done by members of the “phage group,” founded in 1940 by Max Delbrück, Salvador Luria and Hershey. The establishment of the group’s annual summer “phage course” at Cold Spring Harbor in 1945 attracted a great number of researchers to the field, one of whom was the young James Watson, who studied with Delbrück at Cal Tech and obtained his Ph.D. in 1950 at Indiana University under Luria. Watson began his scientific career by investigating bacterial viruses, attempting to study the fate of DNA of infecting virus particles.
“The greater number of workers assimilated into the Phage Group through the Cold Spring Harbor course, as well as the easier access to new tools such as radioactive tracers and ultracentrifuges, engendered more rapid progress during the next seven years. In 1952 the fifty or so stalwarts, gathered at the Abbaye de Royaumont near Paris for the first International Phage Symposium, knew by then that the phage DNA is the sole carrier of the hereditary continuity of the virus and that the details uncovered hitherto concerning the physiology and genetics of phage reproduction were to be understood in terms of the structure and function of DNA. In the very next year, the discovery of the Watson-Crick structure of DNA and the proposed mechanism of its replication provided the fundament for that understanding” (Stent, “Introduction: Waiting for the paradox,” in Phage and the Origins of Molecular Biology, ed. J. Cairns, G. Stent and J. Watson , 3-8, quoting from p. 6).
Twort, "An investigation on the nature of ultra-microscopic viruses," The Lancet 2 (1915) 1241-43.
Brock, The Emergence of Bacterial Genetics, 113-14. Judson, The Eighth Day of Creation, 45-46.