91Ó°ÊÓ

Whena bacteria cell takes up a naked DNA (Deoxyribonucleic acid) molecule or fragment from the medium, it gets incorporated into the recipient chromosome. It becomes a part of the host chromosome. This process is called bacterial transformation.

Under natural conditions, when bacteria cell lyses, it releases its genetic material in the surrounding. When these DNA fragments come in contact with another bacterial cell, it results in bacteria transformation.

Griffith selected two strains ofS. pneumonia: the R type and the S type.The R strain was non-pathogenic, while the S strain was pathogenic.The presence of the outer capsule on the S type S. pneumoniaaccounts for the pathogenicity of this strain,which was absent in the R strain.

He injected the R and S strain into the mouse and studied the effect of these strains on the mouse.When he injected the mouse with living S cells in the first panel, he observed that the mouse died.

In the second panel, he injected the mouse with living R cells, and he observed that the mouse was alive. In the third panel, he injected the mouse with heat-killed S cells, and he observed that the mouse was alive.

In the fourth panel, he injected the mouse with a mixture of heat-killed S cell and living R cells; he observed that the mouse was dead.

When Griffith injected the mouse with the mixture of living R cells and heat-killed S cells, he was expecting that the mouse would be alive because when he injected heat-killed S cells alone in the mouse, the mouse was alive. When he injected with the living R cell alone, then also the mouse survived.

However, he was surprised to see the result because the mouse died in the fourth panel. To know the cause of the death, Griffith obtained the blood sample from the dead mouse. He found living R cells were present in the blood of the mouse.

He concluded that some heritable factors transformed from the dead S cells into the living R cells, making them pathogenic as they developed outer capsule-like S cells. Thus, the R cells became pathogenic due to the uptake and incorporation of the S cell DNA fragment into the R cell genome.