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.
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