Frederick Griffith, a British bacteriologist, conducted a landmark series of experiments in 1928 that fundamentally changed the understanding of genetics. His research provided the first clear demonstration that bacteria possess the ability to acquire new traits from their environment. This process, which he named transformation, indicated that some form of hereditary material could be transferred between organisms, permanently altering their characteristics. Griffith’s work ultimately laid the groundwork for the later discovery that deoxyribonucleic acid, or DNA, is the carrier of genetic information.
The Purpose of the Study
Griffith’s initial motivation was not to uncover the nature of genetic material, but rather to understand the pathology of the bacterium Streptococcus pneumoniae, which causes pneumonia. This was a serious public health concern at the time, and his work was partly aimed at developing an effective vaccine against the disease. To study the infection, he utilized different strains of the pneumococcus bacteria, which could be visually distinguished in a laboratory setting.
He focused on two primary strains, known as the S and R types, based on the appearance of their colonies when grown on a culture plate. The S strain bacteria were virulent, meaning they caused disease, and appeared smooth because they were encased in a protective polysaccharide capsule. This capsule shields the bacteria from the host’s immune system, allowing the infection to become lethal.
In contrast, the R strain bacteria were non-virulent and harmless, producing colonies with a rough, irregular surface. They lacked the protective polysaccharide capsule, which made them vulnerable to attack and clearance by the host organism’s immune system. By using these two distinct forms, Griffith established a clear system for tracking the relationship between a bacterial trait (capsule presence) and its ability to cause a fatal infection.
The Four Experimental Conditions
Griffith designed four distinct experiments using mice to determine how the two bacterial strains affected the host. In the first condition, mice injected with the live, non-virulent R strain bacteria remained healthy and survived, confirming the R strain was harmless.
The second condition involved injecting mice with the live, virulent S strain bacteria. The mice quickly developed pneumonia and died, confirming the lethal nature of the encapsulated strain.
For the third test, Griffith killed the virulent S strain bacteria using high heat before injection. When the heat-killed S bacteria were injected into mice, the animals survived, demonstrating that the dead bacteria could no longer cause disease.
The final and most revealing experiment involved mixing the two non-lethal components: the live R strain and the heat-killed S strain. When this mixture was injected into mice, the animals unexpectedly died, contradicting the previous results. Analysis of blood samples showed the presence of live S strain bacteria, which was surprising since only dead S cells had been introduced.
This outcome proved that the living R strain bacteria had acquired the capsule-forming trait from the dead S strain and become virulent. This newly acquired characteristic was stable and heritable, as the recovered S bacteria could subsequently kill new mice upon injection. This result pointed to the existence of a substance capable of transmitting a genetic trait.
Identifying the Transforming Principle
Griffith concluded that some material, which he termed the “Transforming Principle,” passed from the heat-killed S cells into the living R cells. This transfer converted the harmless R bacteria into the deadly S form. The R strain acquired the genetic instructions necessary to synthesize the protective polysaccharide capsule, permanently changing their phenotype and genotype.
The experiment demonstrated that hereditary information could be transferred horizontally between bacterial cells. This discovery challenged the prevailing belief that an organism’s traits were fixed and unchangeable. Although Griffith did not know the chemical identity of the transforming principle, his work established the concept that some molecule carried the blueprint for biological traits.
This finding provided foundational evidence for the field of molecular biology. It set the stage for later research by Oswald Avery, Colin MacLeod, and Maclyn McCarty, who, in 1944, identified the transforming principle as deoxyribonucleic acid. Griffith’s experiment was the first step toward proving that DNA is the molecule of heredity.