Griffith’s Experiment: The Discovery of the “Transforming Principle”

Updated: Nov 8, 2020

By Isabel Zhang


You probably know what DNA is, or at least have heard of it. It’s what gives us our specific traits, like our hair color, or our ability to roll our tongue. But not many of us know how it was discovered that DNA determined all this genetic information about us. So we’re going to dive into the first of what’s known as the “classic experiments”: experiments that led to our understanding of DNA as the carrier of genetic information, or the “transforming principle”!

The Experiment

In 1928, Frederick Griffith, a British bacteriologist, conducted an experiment to try to develop a vaccine for pneumonia. During the experiment, he injected mice with Streptococcus pneumoniae, a bacteria that causes the pneumonia sickness. He used two strains of pneumococcus bacteria: the smooth strain (S) and the rough strain (R) (named after their appearances).

When Griffith injected the S bacteria into the mice, the mice died from pneumonia. He concluded that the S bacteria were virulent, or harmful. When he injected the R bacteria into the mice though, the mice survived. So he concluded that the R bacteria were nonvirulent, or not harmful.

Although it wasn’t known then, the reason why the S bacteria were virulent was because each S bacterium had a capsule surrounding it. The capsule was important because it protected the bacteria from the mice’s immune system defenses. This allowed the S bacteria to survive and kill the mice. However, the R bacteria didn’t have the capsules, so they couldn’t survive to kill the mice and were nonvirulent.

An Unexpected Turn for the Experiment

In the next part of the experiment, Griffith injected S bacteria that was heat-killed. This meant that the S bacteria that had been heated to high temperatures, causing them to die. What do you think happened to the mice? You probably guessed correctly: the mice survived because the heat-killed S bacteria was dead and not virulent anymore!

Then, Griffith tried something interesting: he injected R bacteria AND heat-killed S bacteria into a mouse. What do you think happened to the mouse? You probably guessed that the mouse survived because both the R bacteria and heat-killed S bacteria were nonvirulent.

However, that wasn’t the case- the mouse ended up dying! Not only did the mouse die, but when Griffith took a blood sample from the dead mouse, the sample had living S bacteria swimming in it! How on earth could this have happened when the mouse was injected with S bacteria that were heat-killed and R bacteria that were nonvirulent?

Summary of Experiment Results

The Birth of the “Transforming Principle”

Griffith came to the conclusion that the R bacteria must have taken up something special from the heat-killed S bacteria, causing it to transform into a virulent smooth-coated bacteria. So even though the S bacteria was dead, R bacteria was still able to take the part of it that allowed it to have its smooth capsule. Griffith referred to this certain “something” as the "transforming principle".

We know today that DNA is the transforming principle for all organisms, or living things. However, it would take several more experiments (the rest of the classic experiments) to prove this to people who didn’t think it was true. Despite that, we can appreciate Griffith’s work, which began the path to understanding the importance of DNA today!


1. Classic experiments: DNA as the genetic material (article). (n.d.). Retrieved from