Q.In the light of Griffith's experiment, explain the action of two strains of Diplococcus pneumoniae and give his conclusion.

Live S-strain killed mice; live R-strain did not; heat-killed S-strain did not; but heat-killed S-strain mixed with live R-strain killed the mice, and live S-type bacteria were recovered.

Griffith's mixed-strain experiment revealed bacterial transformation by a heritable 'transforming principle' (later shown to be DNA).

Frederick Griffith worked with two strains of Diplococcus (Streptococcus) pneumoniae: the S-strain (Smooth), which has a polysaccharide capsule, is virulent, and causes fatal pneumonia when injected into mice; and the R-strain (Rough), which lacks a capsule, is non-virulent, and is harmless when injected. He carried out four experiments: (1) mice injected with live S-strain bacteria died, showing S is pathogenic; (2) mice injected with live R-strain bacteria survived, showing R is non-pathogenic; (3) mice injected with heat-killed S-strain bacteria alone survived, showing that killing the S-strain destroys its ability to cause disease; (4) mice injected with a mixture of heat-killed S-strain and live R-strain bacteria unexpectedly died — and, crucially, live, virulent S-strain bacteria (with a capsule) were recovered from their blood, even though only dead S-strain and live (non-capsulated) R-strain bacteria had been injected. …