Search for genetic material

In 1928, Frederick Griffith conducted a series of experiments with <i>Streptococcus pneumoniae</i>. When <i>Streptococcus pneumoniae</i> bacteria are grown on a culture plate, some produce smooth shiny colonies (S), while others produce rough colonies (R). Mice infected with S strain (virulent) die from pneumonia, but those infected with R strain do not develop pneumonia.

<i>S Strain → Inject into mice → Mice die</i>

<i>R strain → Inject into mice → Mice live</i>

Griffith killed the bacteria by heating them. When the heat-killed S strain bacteria were injected into mice, they did not kill the mice. When a mixture of heat-killed S strain and live R strain bacteria was injected into mice, the mice died. Griffith recovered living S bacteria from the dead mice.

<i>S Strain (heat killed) → Inject into mice → Mice live</i>

<i>S Strain (heat killed) + R strain (live) → Inject into mice → Mice die</i>

Griffith concluded that the R strain bacteria somehow were transformed by the heat-killed S strain bacteria. This must be due to the transfer of the genetic material. But he could not define the biochemical nature of genetic material.