Griffith’s experiment was a landmark study published in 1928 by Frederick Griffith, a British bacteriologist, which demonstrated that bacteria can transfer genetic information through a process known as transformation. This finding was a major breakthrough in the field of genetics and helped to lay the groundwork for modern molecular biology.

In the experiment, Griffith studied two strains of the bacterium Streptococcus pneumoniae, a type of bacteria that can cause pneumonia. One strain was virulent (harmful) and had a polysaccharide capsule surrounding it, which made it resistant to phagocytosis (a type of immune response). The other strain was avirulent (non-harmful) and lacked the polysaccharide capsule.

Griffith injected mice with a mixture of the two strains and observed that the mice died from pneumonia. He then hypothesized that the virulent strain had somehow transferred its genetic information to the avirulent strain, allowing it to become virulent.

To test this hypothesis, Griffith repeated the experiment, but this time he heated the mixture of the two strains before injecting it into the mice. He found that the heated mixture was no longer able to cause pneumonia, indicating that the genetic information had been destroyed by the heat.

From these results, Griffith concluded that the virulent strain had transferred its genetic information to the avirulent strain, a process he termed transformation. This process was later found to involve the uptake of naked DNA by the recipient cell.

Griffith’s experiment was an important milestone in the history of genetics, as it demonstrated that genetic information could be transferred from one organism to another, paving the way for further research into the mechanisms of genetic inheritance.

Griffith’s experiment

Experiment demonstrating transfer of genetic information

Griffith’s experiment discovering the “transforming principle” in pneumococcus bacteria.

Griffith’s experiment, reported in 1928 by Frederick Griffith, was the first experiment suggesting that bacteria are capable of transferring genetic information through a process known as transformation. Griffith’s findings were followed by research in the late 1930s and early 40s that isolated DNA as the material that communicated this genetic information.

Pneumonia was a serious cause of death in the wake of the post-WWI Spanish influenza pandemic, and Griffith was studying the possibility of creating a vaccine. Griffith used two strains of pneumococcus (Diplococcus pneumoniae) bacteria which infect mice – a type III-S (smooth) which was virulent, and a type II-R (rough) strain which was nonvirulent. The III-S strain synthesized a polysaccharide capsule that protected itself from the host’s immune system, resulting in the death of the host, while the II-R strain did not have that protective capsule and was defeated by the host’s immune system. A German bacteriologist, Fred Neufeld, had discovered the three pneumococcal types (Types I, II, and III) and discovered the quellung reaction to identify them in vitro. Until Griffith’s experiment, bacteriologists believed that the types were fixed and unchangeable, from one generation to another.

In this experiment, bacteria from the III-S strain were killed by heat, and their remains were added to II-R strain bacteria. While neither alone harmed the mice, the combination was able to kill its host. Griffith was also able to isolate both live II-R and live III-S strains of pneumococcus from the blood of these dead mice. Griffith concluded that the type II-R had been “transformed” into the lethal III-S strain by a “transforming principle” that was somehow part of the dead III-S strain bacteria.

Today, we know that the “transforming principle” Griffith observed was the DNA of the III-s strain bacteria. While the bacteria had been killed, the DNA had survived the heating process and was taken up by the II-R strain bacteria. The III-S strain DNA contains the genes that form the smooth protective polysaccharide capsule. Equipped with this gene, the former II-R strain bacteria were now protected from the host’s immune system and could kill the host. The exact nature of the transforming principle (DNA) was verified in the experiments done by Avery, McLeod and McCarty and by Hershey and Chase.

Notes

References

(References the original experiment by Griffith. Original article and 35th anniversary reprint available.)

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