Streptococcus pneumonia

S - Strain 'inject into mice' mice die. 

R - strain 'inject into mice' mice alive. 

S - strain (heat killed) 'inject into mice' mice alive. 

R - strain (alive) + S (heat killed) strain'inject into mice' mice die. 

R strain transformed into virulent.

Detailed answer:

Transforming Principle:

Frederick Griffith (1928) conducted experiments with Streptococcus pneumoniae ( bacterium causing pneumonia).

(i) He observed two strains of this bacterium-one forming smooth shiny colonies (S-type) with capsule, while other forming rough colonies (R-type) without capsule. 

(ii) Capsulated S-strain was virulent and causes pneumonia whereas non capsulated R-strain was not virulent and does not cause pneumonia. 

(iii) When live S-type cells were injected into mice, they died due to pneumonia. 

(iv) When live R-type cells were injected into mice, they survived 

(v) When heat-killed S-type cells were injected into mice, they survived and there were no symptoms of pneumonia.

(vi) When heat-killed S-type cells were mixed with live R-type cells and injected into mice, they died due to unexpected symptoms of pneumonia. 

(vii) He concluded that heat-killed S-type bacteria caused a transformation of the R-type bacteria into S-type bacteria but he was not able to understand the cause of this bacterial transformation.