S Strain:
* "S" stands for "smooth." This strain of bacteria has a smooth outer capsule made of polysaccharides.
* Causes disease: The capsule acts as a protective barrier against the host's immune system, allowing the bacteria to multiply and cause pneumonia.
R Strain:
* "R" stands for "rough." This strain lacks the smooth capsule.
* Does not cause disease: Without the capsule, the bacteria are easily recognized and destroyed by the host's immune system.
The Experiment:
* Frederick Griffith performed a series of experiments in the 1920s using these two strains.
* He injected mice with different combinations of S and R strains:
* S strain alone: Mice died.
* R strain alone: Mice lived.
* Heat-killed S strain: Mice lived.
* Heat-killed S strain + live R strain: Mice died.
The Key Finding:
* The dead S strain somehow transformed the harmless R strain into a disease-causing strain, even though the S strain was dead. This was the first evidence that genetic material could be transferred between organisms.
* Later, Avery, MacLeod, and McCarty identified the transforming principle as DNA.
In summary: The difference in disease-causing ability between the S and R strains of *Streptococcus pneumoniae* lies in the presence or absence of a protective capsule. The capsule allows the S strain to evade the host's immune system, leading to infection and disease.
