In 1928, British bacteriologist Frederick Griffith had carried out a series of experiments using bacteria Streptococcus pneumoniae which causes pneumonia in human. In his experiments, Griffith used two related strains of bacteria, known as R (Rough, non-virulent) and S (Smooth, virulent).
1.When Griffith infected mice with the non-virulent bacteria (R strain), the mice did not develop pneumonia and survived.
2.When Griffith infected mice with the virulent bacteria (S strain), the mice developed and died.
3.When Griffith infected mice with heat-killed virulent bacteria (R strain), the mice did not develop pneumonia and survived as the bacteria had been died due to heat.
4.When Griffith infected mice with a mix of heat-killed virulent bacteria (S strain) and living non-virulent bacteria (R strain), the mice were found to developed pneumonia and died.
Griffith concluded that the R strain had been “transformed” into the lethal S strain by a “transforming principle” which allowed R strain to “transform” into S strain (smooth, polysaccharide capsule coated) bacteria and become virulent.