Griffith’s Experiment

Estimated time: 3 minutes

Maharashtra State Board: Class 12

Key Points: Griffith’s Experiment

Frederick Griffith (1928), a British medical officer, experimented on Streptococcus pneumoniae to find a cure for pneumonia.
Two strains were used - S-strain (virulent, smooth, pathogenic, encapsulated) and R-strain (non-virulent, rough, non-pathogenic, non-capsulated).
Four experiments - R-strain injected: the mouse survived. S-strain injected: mouse died. Heat-killed S-strain injected: the mouse survived. Live R-strain + Heat-killed S-strain injected: the mouse died.
In the 4th experiment, live S-strain bacteria were recovered from the dead mouse's blood, even though only heat-killed S-strain was used alongside the R-strain.
R-strain bacteria picked up something from the heat-killed S-strain, transformed into a virulent S-strain, and synthesised a smooth polysaccharide coat.
Griffith called the unknown substance responsible for this change the "transforming principle", believed to be some form of genetic material.
Griffith proved genetic material can transfer between bacteria, causing a permanent, heritable change, but did not identify what the transforming principle was (later proven to be DNA).