Griffith's experiment: transformation of pneumococci.

Pneumococci are bacteria that cause pneumonia. They occur in two strains: a disease-causing smooth strain (strain S), and a harmless rough strain (strain R). Strain S has a capsule on its cell surface; this capsule is absent from the harmless strain R (see Fact of life).

Griffith found that mice injected with live strain S soon died, but those injected with live strain R survived. Mice injected with dead strain S bacteria (killed by heat) аll survived. The results of this series of experiments were as expected. However, the results of Griffith's next series of experiments were thoroughly baffling: mice injected with a mixture of heat-killed strain S and live strain R died. Moreover, Griffith recovered live strain-S bacteria from the dead mice.

After many careful experiments, Griffith concluded that hereditary material had passed from the dead bacteria to the live bacteria. This changed harmless strain R bacteria into virulent strain S pathogens. This process is called transformation.