Frederick Griffith (England) in 1928 took an experiment to vaccinate mice against pneumonia by injecting the mice with heat-killed bacteria of encapsulated strain.
For this purpose, he worked with two strains of Streptococcus pneumonia. One is virulent (pathogenic) and other is avirulent. The virulent strain has a polysaccharide capsule in its cell wall.
The capsule can cause disease (pneumonia) by preventing phagocytosis. And the avirulent strain doesn’t cause disease as it has no capsule.
He performed four different experiments in four different way as following:
First, he injected the mice with living encapsulated or smooth (S) bacteria. As a result, the mice died and he found colonies of encapsulated bacteria in the dead mice and isolated them from the mice.
Second, he injected the mice with dead encapsulated or smooth (S) bacteria. The mice were alive and he found no colonies from the mice.
Third, he injected the mice with living non-capsulated or rough (R) bacteria. The mice were alive. And he isolated a few colonies from the mice.
Fourth, he mixed living non-capsulated rough (R) bacteria with dead encapsulated smooth (S) bacteria (second+third) and injected the mice with this mixture. The mice died and he found colonies of encapsulated bacteria in the dead mice and isolated them from the mice.
What has happened in the fourth experiment? He expected the mice to alive, but it died. Why? He showed the reason that the genetic material (genes) from dead encapsulated bacteria was transferred to living non-capsulated bacteria. Thus the genetic material altered the living non-capsulated bacteria genetically and the progeny of living non-capsulated bacteria became encapsulated.
Later, the scientists had tried to determine which component causes this transformation. In the United States three scientists Oswald T. Avery, Colin M. MacLeod and Maclyn McCarty in 1944 performed an experiment for the molecular explanation of transformation. After a series of experiment, they declared that the genetic material was DNA.