Write a brief note on Griffith’s experiment on Streptococcus pneumoniae.

Explanation

Frederick Griffith (1928) a British doctor was studying the pathogenicity of different strains of Streptococcus pneumoniae. It has two strains-(i) Virulent cause pneumonia, has S-type (Smooth) of bacteria, covered by sheath of mucilage. (ii) Non-virulent do not produce the disease, has R-type (Rough) of bacteria, devoid of sheath of mucilage.
Griffith found that on injecting live R-type, bacteria did not produce the disease while live S type caused pneumonia and the death in mice. However, when heat-killed S-type injected, they did not produce the disease. Finally Griffith injected-a combination of live-R type and heat-killed S type bacteria in to mice. While some mice survived, others developed the disease of pneumonia and died. Autopsy of dead mice showed that they possessed both the type of bacteria (Virulent-S-type and non-virulent-R-type) in living form though the mice had been injected with dead virulent (S type) and living non-virulent (R-type) bacteria.

From the above experiment Frederick Griffith concluded that the occurrence of living S-type virulent bacteria is possible only by their formation from R-type non-virulent bacteria, which pick up the trait of virulence from dead-bacteria. This phenomenon is called Griffith Effect or transformation. Griffith proposed that the transforming principle is a chemical substance released by heat killed S-type, which changed the R-type into S-bacteria. It was a permanent change as the new S-type formed only S-type progeny.