Griffith’s Experiment
Griffith experimented with Streptococcus pneumoniae bacteria. This bacteria causes Pneumonia.Two strains of this bacteria were used R-strain & S-strain.
- S strain
- Smooth mucous polysaccharide coat
- Resistant to immune system
- Virulent
- R strain
- Lacks the coat
- Destroyed by immune system of the host
- Non-virulent
The experiment was performed in multiple steps.
- S-strain (virulent) was injected into mouse. It was found that the mouse died of pneumonia
- R-strain (non-virulent) was injected into mouse. It was found that the mouse remained alive
- Heat killed S-strain (S-strain bacteria were killed by heating) was injected into mouse. It was found that the mouse remained alive
- Heat killed S-strain & live R-strain were injected into mouse. It was found that the mouse died of pneumonia
Griffith found that Live S-strain bacteria could be recovered from the dead mouse (Step 4).
Griffith thus arrived at the following conclusion:
- Something caused bacteria to change from one type (R) to another type (S)
- Some ‘Transforming principle’, transferred from heat-killed S strain to R strain and transformed it virulent.
However, the biochemical nature of the ‘Transforming principle’ was still unknown.
