Griffith's Experiments

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In the late 1920s, Fred Griffith was attempting to develop a vaccine against a bacterium that causes pneumonia. To find out why two strains of the bacteria differed in their deadliness, he injected mice with four different mixtures.

Mice injected with R cells remained healthy. When Griffith examined blood samples from these mice, he found no live bacteria.

Mice injected with S cells developed pneumonia and died. When Griffith examined blood samples from these mice, he found live S cells.

Mice injected with S cells that were killed by exposure to high temperature remained healthy. When Griffith examined blood samples from these mice, he found no live bacteria.

Mice injected with a mixture of live R cells and heat-killed S cells developed pneumonia and died. When Griffith examined blood samples from these mice, he found both live R cells and live S cells.

Heat killed the S cells but did not destroy their hereditary material. Somehow, that material was transferred from the dead S cells to living R cells.