Question

Describe Frederick Griffith's experiment on Streptococcus pneumoniae. Discuss the conclusion he arrived at.

Answer 1

Hint: Transformation is one of the processes of transferring genetic material through the cell membrane in the form of naked deoxyribonucleic acid (DNA) between microbial cells resulting in genetic alteration.

Complete Answer:
- Transformation was first performed by British bacteriologist Frederick Griffith in 1928. He discovered transformation of the pneumococcal cells (Streptococcus pneumoniae) cells. In the experiment, Griffith cultured Streptococcus pneumoniae bacteria and bacteria showed two patterns of growth after the growth. One culture plate, smooth shiny colonies (S) were observed while in the other rough colonies (R) were seen. This difference was due to the presence of mucous coat in S strain bacteria.
- Further, he injected both S and R strains to mice. The one which was infected with the S strain developed pneumonia and died while those infected with the R strain stayed alive.
- In the next stage, the heat-killed the S strain was injected into mice, but the mice stayed alive. Then, he mixed the heat-killed S and live R strains. The mixture was injected into mice and he found mice died. He also found living S strain bacteria in the dead mice.
- After the observation, Griffith concluded that R strain bacteria had been transformed by S strain bacteria and the R strain became virulent. He also concludes that the transformation was inheritable. And he assumed this transforming principle as genetic material i.e., DNA.

Note: Transformation occurs naturally in the environment. The 67 prokaryotic species are able to perform the transformation naturally. Like in Micrococcus luteus (Actinobacteria phylum), competence develops during the mid-late exponential growth phase and in addition triggered by amino acids starvation. A type of transformation known as natural transformation has helped bacterial populations to overcome the challenge of maintaining population numbers. During harsh and extreme environmental changes, some bacterial genera release DNA from cells into the environment that is taken up by competent cells.