In Frederick Griffith’s 1928 experiment, transformation made harmless pneumococcal bacteria become disease-causing.
Griffith studied two forms of Streptococcus pneumoniae: a smooth, capsule-forming strain that caused disease and a rough strain that normally did not. Heat-killed smooth bacteria could not cause disease by themselves, and live rough bacteria were also harmless.
When the two were injected together into mice, the mice died, and live smooth bacteria were recovered. Griffith concluded that a heritable “transforming principle” had passed from the dead cells to the living rough cells.
He did not identify the principle as DNA. Avery, MacLeod, and McCarty later showed that DNA was responsible. Griffith’s result nevertheless provided an early experimental demonstration that bacteria could acquire inherited biological properties from their surroundings.