Final answer:
In Griffith's experiments, the S strain was virulent, and the R strain was non-virulent. Griffith's findings suggested that external DNA from the heat-killed S strain was taken up by the R strain, transforming it into a virulent strain.
Step-by-step explanation:
In Griffith's transformation experiments, the S and R strains of bacteria were virulent and non-virulent in nature, respectively.
The S strain, which is pathogenic, can cause death in mice due to its smooth appearance conferred by a polysaccharide capsule. In contrast, the R strain is non-pathogenic and is characterized by its rough appearance, which is due to a lack of such a capsule.
When Griffith injected a mouse with the heat-killed S strain and a live R strain, the mouse died, indicating that the R strain had been transformed into the virulent S strain.
Griffith concluded that external DNA from the heat-killed S strain was taken up by the R strain, changing its morphology and physiology.
Pneumonia was a serious cause of death in the wake of the post-WWI Spanish influenza pandemic, and Griffith was studying the possibility of creating a vaccine.
Griffith used two strains of pneumococcus (Diplococcus pneumoniae) bacteria which infect mice – a type III-S (smooth) which was virulent, and a type II-R (rough) strain which was nonvirulent.
The III-S strain synthesized a polysaccharide capsule that protected itself from the host's immune system, resulting in the death of the host, while the II-R strain did not have that protective capsule and was defeated by the host's immune system.