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Griffith s experiment

Griffith's experiment was conducted in 1928 by Frederick Griffith in which he used two varieties of pneumococcus; one with a smooth polysaccharide coat (infectious S strain), and the other without (harmless mutant R strain). Griffith injected mice with two different strains of each bacteria (IIS, IIIS, IIR and IIIR) and observed that the mice injected with the strains with out the polysaccharide coat (mutant R strain) lived, while the mice injected with the polysaccharide coat (S strain) died.

IIR strain can mutate into IIS, and IIIR into IIIS and vice versa, but not IIR into IIIS and vice versa.

However, if the mice were injected with heat-killed IIIS strain pneumococcus, they survived. But if they were injected with both heat killed IIS strain and living IIIR strain pneumococcus, they died. Both IIS and IIS strains pneumococcus were isolated from the mice’s blood, suggesting that they could not have resulted from the mutation of IIIR strain pneumococcus.

This was one of the first experiments suggesting that bacteria are capable of transferring genetic information, otherwise known as the “transforming principle”.

See also: Genetics, Hershey-Chase experiment, Oswald Theodore Avery

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