Frederick Griffith: studied two strains of Streptococcus pneumonia:
S (smooth) strain and R (rough) strain.
S strain can cause pneumonia in mice, it’s pathogenic, or disease-causing, due to the cells having an outer capsule that protects them from the animals immune system.
R strain doesn’t have that capsule, therefore it’s nonpathogenic.
To test for the pathogenicity trait, Griffitth injected mice with the two strains
Living S cells were injected (pathogenic control) -> Mouse died
Living R cells were injected (nonpathogenic control) -> Mouse healthy
Heat-killed S cells were injected (nonpathogenic control) -> Mouse healthy
Mixture of heat-killed S cells and living R cells injected -> Mouse died
In blood of moise, living S cells were found. They were able to reproduce and make more S cells.
Conclusion: Living R bacteria had transformed into the pathogenic S bacteris by an unknown substance from the dead S cells the enabled R cells to make capsules
The Science Notebook 