Discovery of Bacterial Transformation

The phenomenon of bacterial transformation, where genetic material can be transferred from one bacterium to another, was famously discovered by F. Griffith.

Griffith's Landmark Experiment

In 1928, Frederick Griffith conducted experiments using two strains of Streptococcus pneumoniae (pneumococcus), a bacterium that can cause pneumonia:

• S (Smooth) strain: This strain was virulent (disease-causing) and had a polysaccharide capsule. It grew in smooth colonies on culture plates.
• R (Rough) strain: This strain was non-virulent (harmless) and lacked the capsule. It grew in rough colonies on culture plates.

Experimental Procedure and Findings

Griffith performed several experiments:

• Injecting mice with the live S strain killed the mice.
• Injecting mice with the live R strain did not harm the mice.
• Injecting mice with heat-killed S strain bacteria did not harm the mice.
• However, when Griffith injected a mixture of heat-killed S strain and live R strain bacteria into the mice, the mice died.

Upon examining the blood of the dead mice, Griffith found live S strain bacteria. He concluded that something from the dead S strain bacteria had transformed the live R strain bacteria into the virulent S strain. He called this substance the 'transforming principle'.

Significance and Later Work

Griffith's experiment demonstrated that genetic information could be transferred between bacteria, a process crucial for understanding genetic exchange. Although Griffith discovered the phenomenon, the chemical nature of the transforming principle (which was later identified as DNA) was established in experiments by Oswald Avery, Colin MacLeod, and Maclyn McCarty in 1944.

Therefore, the initial discovery of bacterial transformation is credited to F. Griffith.