Which bacterium did Frederick Griffith use in 1928 to demonstrate bacterial transformation?

Frederick Griffith used Streptococcus pneumoniae in 1928 to demonstrate bacterial transformation.

Griffith studied two pneumococcal forms in mice: a smooth, encapsulated strain that caused disease and a rough, non-encapsulated strain that did not. Heat-killed smooth bacteria were harmless, but mixing them with living rough bacteria killed the mice.

The surviving rough bacteria had acquired heritable instructions from the dead smooth cells and became encapsulated. Griffith called the unknown hereditary material a “transforming principle.” The experiment did not identify DNA directly, but it provided the first clear evidence that bacteria could acquire traits from other cells.

A 1944 study by Oswald Avery, Colin MacLeod, and Maclyn McCarty identified DNA as the transforming principle. Griffith’s experiment is therefore a foundation of molecular genetics, not an experiment demonstrating ordinary reproduction or bacterial mating.