Frederick Griffith's Experiment on Bacterial Transformation

Pneumonia-Causing Bacterium

• The bacterium that causes pneumonia has a smooth coating made of carbohydrates, known as the S bacteria.

Griffith's Experiment

• When Griffith injected mice with only heat-killed S bacteria, nothing happened, and the mice did not die.
• However, when Griffith injected mice with a combination of heat-killed S bacteria and live R bacteria, the mice died.
• Griffith discovered that the live R bacteria had transformed into the deadly S bacteria, causing the mice to die.

Significance of Griffith's Discovery

• The finding of live S bacteria in blood samples from the dead mice injected with a combination of heat-killed S bacteria and live R bacteria showed that the R bacteria had transformed into the S bacteria.
• This transformation implied that some material was transferred from the heat-killed S bacteria to the live R bacteria.

Griffith's Conclusion

• Griffith concluded that the material transferred from heat-killed S bacteria to live R bacteria was a "transforming principle" that allowed the R bacteria to change into the deadly S bacteria.

Avery's Research

• Oswald Avery and his fellow biologists worked on isolating the transforming principle for over ten years after Griffith's discovery.
• Avery's group observed that the transforming principle was able to transform R bacteria into S bacteria in a petri dish.

Properties of the Transforming Principle

• Standard chemical tests revealed that the transforming principle was resistant to protein-destroying enzymes and was not affected by lipid solvents.
• The presence of phosphorus in the extract supported the hypothesis that DNA was the transforming principle.
• When enzymes known to break down proteins were added to the extract, the transforming principle was not affected.

Skepticism of Avery's Findings

• Some scientists questioned Avery's evidence, despite the strong results, suggesting that the extract must have contained protein.
• This skepticism was due to the prevailing belief at the time that proteins were the genetic material, rather than DNA.

Learn about Frederick Griffith's groundbreaking experiment in 1928 involving two forms of bacteria that led to the discovery of a 'transforming principle'. Understand the significance of S and R forms of the bacterium in causing pneumonia.