The Avery-McLeod-McCarthy Experiment

Griffith Experiment

Oswald Avery and his colleagues showed that DNA was the key component of Griffith’s experiment, in which mice are injected with dead bacteria of one strain and live bacteria of another, and develop an infection of the dead strain’s type

On February 1, 1944, physician and medical researcher Oswald Avery together with his colleagues Colin MacLeod and Maclyn McCarty announced that DNA is the hereditary agent in a virus that would transform a virus from a harmless to a pathogenic version. This study was a key work in modern bacteriology.

The achievement by the scientists Avery, MacLeod, and McCarty were based on Frederick Griffith’s studies on bacteria, believing that bacteria types were not changeable from one to another generation. His also famous attempt is called the Griffith experiment, and was published in 1928. In it, the medical officer Griffith identified a principle in pneumococcal bacteria, in which they could transform from one to another type. After several years of research on the disease pneumonia, he found out that types changed into another rather than multiple types being present at the same time. His later research proved, that the transformation occurred, when dead bacteria of a virulent and live bacteria of a non-virulent type were injected in mice, they would suffer an infection and die shortly after. The other case proved, that the injected virulent bacteria was to be isolated from an infected mouse, depicted in the picture above. The German bacteriologist Fred Neufeld was the first to prove Griffith’s findings right and soon, renowned institutes, like the Koch Institute or the Rockefeller Institute took over the case, doing further research on Griffith’s great accomplishments.

Oswald Avery (1877-1955)

Oswald Avery (1877-1955)

During further experiments it was found, that pneumococcus consists of so called smooth colonies with the ability to induce antibody formation. During the experiment, all bacteria were killed through heat, also with the help of chloroform, protein was precipitated and the capsules located in pneumococcus were hydrolyzed. After the complete destruction of all capsules was confirmed, fibrous strands were developed, which were able to be removed. It was then proven that proportions of carbon, hydrogen, nitrogen, and phosphorus were consistent with chemical compositions of DNA. Avery and his colleagues found out, that DNA was responsible for transformation.

This experiment shows that the genetic information must lie on the DNA, since the R-cells needed information from the S-cells to form a mucus capsule, i. e. to become S-cells. And only the DNA made it possible to transform R cells into S cells. In the counterexample with an enzyme, it became even clearer that the genetic information must lie in the DNA, since only R cells develop when a DNAse is added, because the DNA was broken down by the enzyme.

The achievements of the Avery-MacLeod-McCarty experiment quickly spread out into the scientific community and it was proven right just as fast. However, only very few scientists would accept the thought that genetics were to be applied to bacteria, but as Joshua Lederberg, himself an American molecular biologist suggested, the three scientists paved the early way for molecular genetics. The experiment opened up new possibilities and research fields for following biologists. Avery was awarded the Copley Medal for his bacterial transformations, but neglected by many scientists and organizations for his work.

At yovisto, you may enjoy the TED talk ‘Discovering bacteria‘ by Dr. Bonnie Bassler explaining how bacteria “talk” to each other, using a chemical language that lets them coordinate defense and mount attacks.

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