Question

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

Answer 1

Frederick Griffith performed experiments on Streptococcus pneumoniae which is responsible for causing pneumonia. He observed that the bacteria produce two colonies as:

• R strain: rough colonies which are non-virulent.
  
• S strain: smooth colonies which are virulent and caused the death of mice.
  

He observed that S cells are virulent due to the production of a smooth polysaccharide coat which is absent in R cells. He performed the following experiment:

Mice + R- cell bacteria ----> Mice lived

Mice + S- cell bacteria ------> Mice died

Then he killed S- celled bacteria by heating which cause denaturation of its genetic material and then injected in mice.

Mice + Heat killed S- cells ------> Mice lived

Now he injected mice with heat killed S- cells and non-virulent R- cells.

Mice + Heat killed S- cells + R-cells ----->Mice died.

Then he recovered living S- cells from the dead mice.

Then he concluded that some transforming principle had been transferred from heat killed S- cell to living R- cells which enables R- cells to secrete polysaccharide coat and become virulent.