Avery–MacLeod–McCarty experiment

The Avery–MacLeod–McCarty experiment was an experimental demonstration, reported in 1944 by Oswald Avery, Colin MacLeod, and Maclyn McCarty, that DNA is the substance that causes bacterial transformation, in an era when it had been widely believed that it was proteins that served the function of carrying genetic information (with the very word protein itself coined to indicate a belief that its function was primary). It was the culmination of research in the 1930s and early 20th Century at the Rockefeller Institute for Medical Research to purify and characterize the "transforming principle" responsible for the transformation phenomenon first described in Griffith's experiment of 1928: killed of the virulent strain type III-S, when injected along with living but non-virulent type II-R pneumococci, resulted in a deadly infection of type III-S pneumococci. In their paper "Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types: Induction of Transformation by a Desoxyribonucleic Acid Fraction Isolated from Pneumococcus Type III", published in the February 1944 issue of the Journal of Experimental Medicine, Avery and his colleagues suggest that DNA, rather than protein as widely believed at the time, may be the hereditary material of bacteria, and could be analogous to genes and/or viruses in higher organisms.

With the development of serological typing, medical researchers were able to sort bacteria into different strains, or types. When a person or test animal (e.g., a mouse) is inoculated with a particular type, an immune response ensues, generating antibodies that react specifically with antigens on the bacteria. Blood serum containing the antibodies can then be extracted and applied to cultured bacteria. The antibodies will react with other bacteria of the same type as the original inoculation. Fred Neufeld, a German bacteriologist, had discovered the pneumococcal types and serological typing; until Frederick Griffith's studies bacteriologists believed that the types were fixed and unchangeable from one generation to the next.

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