The first series of experiments were performed by a British physician F. Griffith in 1928, using the bacterium Diplococcus pneumoniae which causes pneumonia in mammals. There are two types of strains ; S-type, is capsulated and smooth and r-type is non-capsulated and rough.
When S-type of bacteria were injected into healthy mice, the
mice developed pneumonia and died. Thus S-type is virulent or pathogenic.
When R-type of bacteria were injected into healthy mice,
they did not cause pneumonia. Thus R-type is avirulent or non-pathogenic.
In the third step of experiments, heat killed S-type of
bacteria were injected into healthy mice, they did not cause disease and the
mice remained healthy.
When the mixture of heat killed S-type of bacteria and
living R-type bacteria was injected into healthy mice, the mice developed pneumonia
and died.
When blood sample from dead mice was cultured, colonies of
living S-type were obtained. Griffith concluded that living R-type of bacteria
must have picked up something from the surrounding medium that contains heat
killed S-type, and got changed to S-type. Therefore this change is due to the
phenomenon other than mutation, which he called transformation.
He named that something as transforming principle. It was
later proved that this transforming principle is DNA.