Question

Explain the experiment performed by Griffith on Streptococcus pneumoniae. What did he conclude from this experiment?

Answer 1

In 1928, the British bacteriologist Frederick Griffith conducted a series of experiments using the bacterium Streptococcus pneumoniae to understand the nature of heredity and search for a vaccine for pneumonia.

During the experiment, Griffith cultured Streptococcus pneumonia bacteria, which showed two patterns of growth. One culture plate was made up of smooth, shiny colonies (S) while the other consisted of rough colonies (R). The difference was due to the appearance of the mucous coat in S strain bacteria, whereas the R strain bacteria lacked them.

• Experiment: In Griffith’s experiment, he injected both S and R strains into mice. The one which was infected with the S strain developed pneumonia and died, while those infected with the R strain remain alive.
  In the second stage, Griffith heat-killed the S strain bacteria and injected them into mice, but the mice remained alive. Then, he mixed the heat-killed S with live R strains. This mixture was injected into mice, and they died. Additionally, he found living S strain bacteria in dead mice.
• Conclusion: supported by the observation, Griffith concluded that the R-strain bacteria had been transformed by the S-strain bacteria. The R strain inherited a particular ‘transforming principle’ from the heat-killed S strain bacteria that built them virulently. And he concluded this transforming principle as genetic material.