In 1928, Frederick Griffith conducted a series of experiments with Streptococcus pneumoniae [a bacterium causing pneumonia].
He observed miraculous transformation in the bacteria during the course of his experiment, living organism (bacteria) had changed in physical form.
When bacterium grown on a culture plate, some produce smooth shiny colonies (S-strain) with mucous (polysaccharide) coat, while other produce rough colonies (R-strain) without coat or capsule.
When live S-strain bacteria were injected into mice, they died due to pneumonia.
When live R-strain bacteria injected into mice, they survived.
When heat killed S-strain bacteria were injected into mice, they survived and there were no symptoms of Pneumonia.
When he injected a mixture of heat killed S and live R-strain bacteria into mice, they died due to unexpected symptoms of Pneumonia and he recovered living as bacteria from the dead mice.
He concluded that R-strain bacteria had somehow been transformed by the heat killed Sstrain bacteria. Some transforming principle, transferred from the heat killed S-strain, had enabled the R-strain to synthesise a smooth polysaccharide coat and become virulent. But he was not able to understand the cause of this bacterial transformation.