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

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.