* S bacteria (smooth): These bacteria have a protective capsule, making them virulent (disease-causing).
* R bacteria (rough): These bacteria lack the protective capsule and are non-virulent.
The key point is that S bacteria were not always virulent. The R bacteria were not virulent because they lacked the capsule.
Here's why the S bacteria weren't always virulent:
* Strains: There are different strains of bacteria, some virulent and some non-virulent. The experiment likely used a specific strain of S bacteria that was virulent, while other strains might not be.
* Capsule Loss: Even within a virulent strain, bacteria can lose their capsule over time due to mutations or environmental factors. This would make them non-virulent.
Griffith's experiment:
Griffith injected mice with different combinations of these bacteria:
* Live S bacteria: Mice died
* Live R bacteria: Mice lived
* Heat-killed S bacteria: Mice lived
* Live R bacteria + Heat-killed S bacteria: Mice died
The results showed that the heat-killed S bacteria somehow transferred their virulence to the live R bacteria, allowing them to cause disease. This was later understood to be the process of bacterial transformation, where DNA from the dead S bacteria was taken up by the live R bacteria.
In summary:
The R bacteria were not virulent because they lacked the protective capsule. The S bacteria were not always virulent because different strains exist, and even within a strain, capsules can be lost. Griffith's experiment showed that dead, virulent S bacteria could transfer their virulence to non-virulent R bacteria through transformation.
