Overview
This lecture discusses Frederick Griffith’s 1928 experiment with Streptococcus pneumoniae, highlighting its impact on genetics and discovery of bacterial transformation.
Background and Importance
- Frederick Griffith studied Streptococcus pneumoniae to understand bacterial pneumonia, especially after the deadly 1918 flu pandemic.
- Many flu deaths were due to secondary bacterial infections like Streptococcus pneumoniae.
- Griffith’s work provided the first evidence of horizontal gene transfer—specifically, bacterial transformation.
- The experiment paved the way for discovering DNA as hereditary material.
Streptococcus pneumoniae Strains
- The "rough" strain lacks a capsule, is non-pathogenic (non-virulent), and does not cause disease.
- The "smooth" strain has a capsule, is virulent (deadly), and can evade phagocytosis, causing severe disease.
Griffith’s Experiment Steps
- Control: Injecting mice with live rough (non-pathogenic) strain did not cause illness.
- Injecting mice with heat-killed smooth (virulent) strain did not cause illness; this functions as a vaccine.
- Injecting mice with live rough strain mixed with heat-killed smooth strain killed the mice.
- Living, encapsulated, pathogenic bacteria were recovered from dead mice in mixed-strain group.
- Transfer of the recovered bacteria to new mice also caused disease and death.
Mechanism Discovered: Transformation
- Griffith concluded something (later known as DNA) was transferred from dead smooth cells to live rough cells.
- This process, called transformation, allows bacteria to take up naked DNA from their environment and acquire new traits, such as capsule formation and pathogenicity.
Key Terms & Definitions
- Streptococcus pneumoniae — Bacterial species causing pneumonia.
- Capsule — Protective outer layer enabling bacteria to evade immune system.
- Virulent — Capable of causing disease.
- Non-pathogenic — Not causing disease.
- Bacterial transformation — Uptake of naked DNA by bacteria, resulting in genetic change.
- Horizontal gene transfer — Movement of genetic material between organisms other than by descent.
Action Items / Next Steps
- Review bacterial transformation in upcoming content.
- Remember key experiment steps and their implications for DNA as genetic material.