(i) Transforming Principle 

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

▪ 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. 

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

▪ When live R-type cells were injected into mice, they survived. 

▪ When heat-killed S-type cells were injected into mice, they survived and there were no symptoms of pnuemonia. 

▪ 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 and live S-type cells were obtained from mice. 

▪ 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.

(ii) Biochemical Characterisation of Transforming Principle 

▪ Oswald Avery, Colin MacLeod and Maclyn McCarty repeated Griffith’s experiment in an in vitro system in order to determine biochemical nature of transforming principle. 

▪ They reported that DNA from the heat-killed S-type bacteria caused the transformation of nonvirulent R-type bacteria into virulent S-type bacteria. 

▪ They also discovered that proteases and RNases did not affect transformation while DNases inhibited the process. 

▪ They concluded that DNA is the hereditary material.