1928
Frederick Grifith demonstrated the genetic transformation principle.Griffith worked with two strains of the bacterium pneumococcus. The virulent strain of the bacteria possessed a smooth polysaccharide capsule that was necessary for infection. The colonies of the virulent strain appeared smooth. A nonvirulent strain contained a rough capsule and was incapable of causing infection. Colonies of the nonvirulent strain appeared rough. If mice were injected with the S (smooth) strain of pneumococcus they died after several days; however, when injected with the R strain of bacteria they lived. Griffith also injected mice with heat killed S strain and found that the mice would survive. When he mixed the heat killed S strain and the R strain and then injected the combination into mice, Griffith found that the mice died. Griffith thought that some "transforming principle" had been transferred from the S strain of bacteria to the R strain causing the R strain to acquire virulence by synthesis of a new polysaccharide coat.
