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Streptococcus pneumoniae, often known as pneumococcus, is a bacterium that can cause serious infections like pneumonia, meningitis, and septicemia. It was central to Griffith's transformation experiments because it naturally occurs in multiple strains with distinct characteristics. These characteristics allowed for clear differentiation and analysis of their effects when injected into mice. Streptococcus pneumoniae can be found in two forms: virulent (S strain) and non-virulent (R strain), each playing a crucial role in demonstrating genetic transformation.

The virulent strain of Streptococcus pneumoniae, also known as the S strain, has a smooth, polysaccharide capsule that makes it particularly deadly. This capsule protects the bacteria from the host's immune system, allowing it to cause severe infections. In Griffith's experiments, when he injected the live S strain into mice, the mice developed pneumonia and died. The smooth capsule was thus a key factor in the bacteria's virulence or ability to cause disease.

The non-virulent strain of Streptococcus pneumoniae, known as the R strain, has a rough appearance under a microscope because it lacks the protective polysaccharide capsule. Without this capsule, the bacteria are unable to evade the host's immune system effectively. When Griffith injected this non-virulent R strain into mice, the mice survived. This non-lethal characteristic demonstrated that the R strain alone could not cause disease, highlighting the importance of the capsule in bacterial virulence.

Genetic transformation refers to the process by which a bacterium takes up genetic material from its environment and incorporates it into its own genome. In Griffith's experiments, he observed that mixing heat-killed virulent S strain bacteria with live non-virulent R strain bacteria resulted in the death of mice and the presence of live S strain bacteria in their bodies. This indicated that some 'transforming principle' from the heat-killed S strain was taken up by the R strain, transforming it into a virulent form. This groundbreaking discovery showed that genetic information could be transferred between different bacterial strains, paving the way for further research in molecular genetics.