Question

Explain the Griffith’s experiment in detail with diagram.

Answer 1

• In 1928, Frederick Griffith performed an experiment with Streptococcus pneumoniae (bacterium responsible for causing pneumonia).
• There are two strains of the bacteria:

1. S-strain whose cells produce a capsule of polysaccharides (capsulated). This strain is virulent (pathogenic) and causes pneumonia.
2. R-strain whose cells lack a capsule (non-capsulated) and are avirulent or non-pathogenic.

• Griffith performed his experiment by injecting the above strains of bacteria into mice and found the following results:

1. R-strain (non-virulent) bacteria were injected into mice, the mice suffered no illness because R-strain was non-pathogenic.
2. S-strain (virulent) bacteria were injected into mice, the mice developed pneumonia and finally died.
3. Then, Griffith injected heat-killed S-strain bacteria into mice and they survived.
4. A mixture of living R-strain and heat-killed S-strain were injected into mice, the mice developed pneumonia and died. Large numbers of live strain bacteria were recovered from the blood of the dead mice.

• He concluded that some genetic factor (transforming principle) from heat-killed S-strain cells transformed live R-strain cells into live S-strain and produced the disease.
• Thus, Griffith’s experiment helped to identify the transformation of genetic material.