Question

Explain Griffith's experiment on bacteria transformation.

Answer 1

Frederick Griffith’s 1928 experiment used two strains of the pneumonia-causing bacterium Diplococcus pneumoniae:

• S strain: smooth, capsulated and virulent; it caused disease in mice.
• R strain: rough, non-capsulated and non-virulent; it did not cause disease.

Experiments and observations

1. Live S bacteria injected into mice caused pneumonia and the mice died.
2. Live R bacteria did not cause disease, and the mice survived.
3. Heat-killed S bacteria did not cause disease, and the mice survived.
4. A mixture of heat-killed S bacteria and live R bacteria caused disease; the mice died. Live S bacteria were recovered from them.

Griffith proposed that a substance from the dead S cells had entered the live R cells and transformed them into virulent, capsulated S cells. He called this substance the transforming principle. The experiment demonstrated bacterial transformation, although it did not identify the substance; later work by Avery, MacLeod and McCarty showed that the transforming principle was DNA.