Griffith work on

Ans : Streptococcus pneumoniae bacteria.

Explanation:

Griffith experimented with Streptococcus pneumoniae bacteria. This bacteria causes Pneumonia.Two strains of this bacteria were used R-strain & S-strain.

• S strain
  • Smooth mucous polysaccharide coat
  • Resistant to immune system
  • Virulent
• R strain
  • Lacks the coat
  • Destroyed by immune system of the host
  • Non-virulent

i. Streptococcus pneumoniae (pneumococcus) bacteria were grown on a culture plate, some produces smooth shiny colonies (S) while others produces rough colonies (R). This was because the S strain bacteria has a mucous (polysaccharide) coat, while R strain does not.
Mice infected with the S strain (virulent) die from pneumonia infection but mice infected with the R strain do not develop pneumonia.
S strain ---> Inject into mice ----> Mice die
R strain ----> Inject into mice ----> Mice live

The virulet S strain bacteria were killed by heating them. It was observed that heat killed S strain bacteria injected into mice did not kill them.
S strain (heat-killed) ----> Inject into mice ----> Mice live

When a mixture of heat-killed S and live R bacteria was injected, the mice died.
S strain (heat-killed) + R strain (live) ----> Inject into mice ----> Mice died. Moreover, living S bacteria was recovered from the dead mice.

The experment concluded that the R strain bacteria had somehow been transformed by the heat-killed S strain bacteria and that some ‘transforming principle’, transferred from the heat-killed S strain, had enabled the R strain to become virulent. It was thought that this must be due to the transfer of the genetic material.