Describe the various steps of Griffith’s experiment that led to the conclusion of the ‘transforming principle’.

Griffith’s experiment –

• He grew Streptococcus pneumonia bacteria on a culture plate.

• Some produced soft colonies (S) i.e. S-strain bacteria have a mucous coat.

• Some produced rough colonies (R) i.e. R-strain bacteria don’t have any mucous coat.

• Mice infected with S-strain (virulent) died from pneumonia while mice infected with R-strain didn’t.

• He killed the bacteria by heating them. He observed that heat-killed S-strain bacteria didn’t kill the mice.

• Then he injected the mice with a mixture of heat-killed S and live R bacteria, the mice died.

• Moreover, he recovered living S bacteria from the dead mice.

• He concluded that R strain bacteria had somehow been transformed by heat-killed S strain bacteria.

• Some ‘transforming principle’ transferred from the heat-killed S strain bacteria, had enabled the R strain to synthesise a smooth mucous coat and become virulent.