(1) In 1928, Frederick Griffith, carried out experiments with bacterium Streptococcus pneumoniae (which causes pneumonia in humans and other mammals).
(2) Griffith used two strains of Streptococcus pneumonia:
- S-type (Virulent, smooth, pathogenic and encapsulated).
- R-type (Non-virulent, rough, nonpathogenic and non-capsulated).
(3) Experiments carried out by E Griffith:
- Mice were injected with R-strain bacteria and they survived (no pneumonia).
- Mice injected with S-strain bacteria developed pneumonia and died.
- When heat-killed S-strain bacteria were injected in mice, the mice survived.
- On injecting a mixture of heat-killed Sbacteria and live R bacteria, the mice died.
(4) Griffith obtained live S-strain bacteria from the blood of the dead mice.
(5) He concluded that the live R-strain bacteria must have picked up something (which he called transforming principle) from the heat killed S bacterium, and got changed into Stype. Transforming principle allowed R-type to synthesize capsule and it became virulent.
(6) Thus, F. Griffith first demonstrated genetic transformations.
