Question

Describe the series of experiments of F. Griffiths. Comment on the significance of the results obtained.

Answer 1

In 1928, British bacteriologist Frederick Griffith studied bacterial transformation. When one type of bacterial strain is converted into another type of bacterial strain, it is called bacterial transformation.

Complete answer:
Griffith conducted his experiment on Streptococcus Pneumoniae by injecting them in mice to develop a vaccine against pneumonia. For his experiment, he used two types of bacterial strains of Streptococcus Pneumoniae.

The R strain: the colonies of these bacteria have a rough appearance on the culture medium plate. Hence, it is called R = rough strain. it is a non-virulent strain, which does not cause disease.
The S strain: the colonies of these bacteria have a round and smooth appearance on the culture medium plate. Hence, this strain is called the S strain. It is a virulent strain, which causes pneumonia disease in mice.
The experiment was conducted in the following steps-
When R type strain was injected into mice, no disease appeared and the mice survived.
When S type strain was injected into mice, the disease appeared and the mice died due to disease.
When heat-killed S type strain was injected into mice, no disease appeared in mice and it survived.
When R type strain along with heat-killed S type strain was injected into mice, the disease appeared in mice and it died. it was very surprising because both the strains were not causing the disease.
Later on, it was concluded that some kind of genetic material (plasmid) was transferred from the heat-killed S type strain to living R type strain that converted the R type strain into virulent S type strain causing disease in mice leading to death. That is why it is called bacterial transformation.

R- Strain (Non-virulent)	Injected in Mice	Cause no disease	Mice survived
S- Strain (Virulent)	Injected in Mice	Cause disease	Mice died
Heat killed S-strain	Injected in Mice	Cause no disease	Mice survived
R- Strain and Heat killed S-strain	Injected in Mice	Cause disease	Mice died

Note:
This experiment was a milestone experiment because, for the first time, it was proved that the bacteria can be transformed from one type of strain into another type of a strain with the help of foreign genetic material or DNA.