Frederick Griffith (1928) conducted experiments with Streptococcus pneumoniae (bacterium causing pneumonia). He observed two strains of this bacterium—one forming smooth shiny colonies ( -type) with capsule, while other forming rough colonies (R-type) without capsule. When live -type cells were injected into mice, they due to pneumonia. When live R-type cells were injected into mice, they survived. When heat-** -type cells were injected into mice, they survived and there were no symptoms of pneumonia. When heat- -type cells were mixed with live R-type cells and injected into mice, they due to unexpected symptoms of pneumonia. He concluded that heat-** -type bacteria caused a transformation of the R-type bacteria into -type bacteria but he was not able to understand the cause of this bacterial transformation.
