Frederick Griffith’s 1928 pneumococcus experiment revealed the process later called bacterial transformation.
Griffith used two strains of Streptococcus pneumoniae. The smooth, capsulated strain caused disease in mice, while the rough, non-capsulated strain did not. Heat-killed smooth bacteria were harmless, but a mixture of heat-killed smooth bacteria and living rough bacteria killed the mice.
Live smooth bacteria were recovered from the dead animals. Griffith concluded that some heritable “transforming principle” had passed from the dead virulent cells to the living harmless cells.
He did not identify the substance as DNA. Avery and his colleagues later showed that DNA was responsible. Griffith’s experiment therefore demonstrated transformation as a biological phenomenon, not the molecular identity of the inherited material.