Griffith's Transformation Experiment

The first experimental clue to the identity of the genetic material came not from any deliberate search for it, but from a study of pneumonia in mice carried out by the British medical officer Frederick Griffith in 1928. Griffith worked with two different strains of the bacterium Streptococcus pneumoniae. The first, called the S (smooth) strain, is covered by a polysaccharide (mucous/capsule) coat, which gives its colonies a smooth, glistening appearance on a culture plate and — critically — protects the bacterium from a host's immune defences, making the S strain virulent (disease-causing). The second, called the R (rough) strain, lacks this protective polysaccharide coat, so its colonies look rough and irregular, and because it has no protection from the host's immune system, the R strain is non-virulent (harmless).

Griffith injected mice with four different preparations and recorded whether each mouse lived or died. Mice injected with live S-strain bacteria died, as expected, because the virulent, capsule-protected S bacteria multiplied unchecked in the host. Mice injected with live R-strain bacteria survived, since the non-virulent R strain could not overcome the host's immune defences. Mice injected with heat-killed S-strain bacteria (S bacteria first killed by heating, so the capsule protein and polysaccharide were destroyed/denatured before injection) also survived — heat-killing had rendered the S bacteria harmless, exactly as expected. The critical, unexpected result came from the fourth preparation: when Griffith injected a mixture of heat-killed S bacteria together with live, harmless R bacteria, the mice died — and, on autopsy, Griffith recovered LIVE, virulent S-strain bacteria from the dead mice's blood. …