Frederick Griffith was using streptococcus pneumoniae which is known as pneumococcus.Pneumococcus is a bacterium that can cause pneumonia . Griffith used two strains of the bacterium which is the S strain.
S strain produces smooth, shiny colonies and is virulent.Virulent means highly infectious.On the other hand the R strain which produces rough colonies and is harmless. Strain S has a polysaccharide coat which is a capsule that surrounding each cell. The R strain is genetically identical except that it carries a mutation that prevents it from making the polysaccharide coat .So the mutation in a gene affects the ability of the bacterium to make the coat.
This causes the change in the virulence state of the bacterium. There are several types of S strains.Each of it comes with a significant chemical mixture of the polysaccharide coat.
Griffith worked with IIS and IIIS strains, which have type II and type III coats .The S – type cells mutate into R- type cells , and the R- type cells mutate into S- type cells. The mutations are highly specific. This means that if a IIS cell mutates into an R cell , then that R cell can mutate back only into a IIS cell , not a IIIS cell.This shows that this transformation are very distinct. Griffith did experiments with mice where they are injected with different strains of the bacterium .
He carefully observed the effects different strains of the bacterium on the mice. When mice were injected with IIR bacteria.??R bacteria is the bacteria derived by mutation from IIS bacteria.This is when the mice lived. When mice were injected with living IIIS bacteria, the mice died, and living IIIS bacteria could be isolated from their blood. However , if the IIIS bacteria were killed by heat before injection, the mice lived.
These experiments showed that the bacteria had both to be alive and to have the polysaccharide coat to be virulent and kill the mice. In his experiment , Griffith injected mice with a mixture of living IIR bacteria and heat killed the IIIS bacteria. The mice died, and living IIIS bacteria were present in the blood .These bacteria could not have function by mutation of the R bacteria, because mutation would have produced IIS bacteria. Griffith concluded that some IIR bacteria had somehow been transformed into smooth, virulent IIIS bacteria by interaction with the dead IIIS bacteria.
Genetic material from the dead IIIS bacteria had been added to the genetic material in the living IIR bacteria . Griffith believed that the unknown agent responsible for the change in the genetic material was a protein ;but this was false, and he turned out to be wrong . He had no experimental evidence one way or the others as to the material acting as the agent bringing about the genetic change . So Griffith called this agent the transforming principle.