Frederick Griffith’s 1928 experiments marked a significant turning point in the understanding of genetic information. His groundbreaking work provided clear evidence that biological information could be transferred between organisms. Griffith’s findings laid fundamental groundwork for future discoveries in genetics.
Setting the Stage: The Bacteria
Griffith’s research focused on Streptococcus pneumoniae, a bacterium known to cause pneumonia. This bacterium exists in two primary forms, with distinct characteristics.
One form, the “smooth” (S) strain, possesses a protective polysaccharide capsule that gives its colonies a smooth, shiny appearance. This capsule allows the S strain to evade the host’s immune system, making it virulent and capable of causing severe disease.
In contrast, the “rough” (R) strain lacks this protective capsule, resulting in colonies with a rough, irregular texture. Without the capsule, the R strain is easily recognized and eliminated by the host’s immune defenses, rendering it non-virulent. These characteristics were central to Griffith’s experimental design.
The Experimental Design and Observations
Griffith conducted a series of experiments using these bacterial strains and laboratory mice.
In his first experimental group, mice injected with live R strain bacteria remained healthy, and no bacteria were recovered from their systems. This confirmed the R strain’s non-virulent nature.
The second group received injections of live S strain bacteria. These mice rapidly developed pneumonia and died, with live S strain bacteria recovered from their bodies. This result demonstrated the S strain’s virulence.
For the third group, Griffith injected mice with S strain bacteria that had been killed by heat. These mice survived, and no live bacteria were found in their systems. This indicated the S strain’s disease-causing ability was destroyed by heat.
The most unexpected result came from the fourth experimental group. Griffith injected mice with a mixture of live R strain bacteria and heat-killed S strain bacteria. These mice died, and live S strain bacteria were recovered from their blood. This outcome was surprising, as neither the live R strain nor the heat-killed S strain alone had caused disease.
Unveiling the “Transforming Principle”
The death of mice injected with the mixture of live R and heat-killed S strains led Griffith to a conclusion. He reasoned that some substance from the heat-killed S bacteria had been transferred to the live R bacteria. This transfer caused the harmless R strain to acquire the characteristics of the virulent S strain, including the ability to form a capsule and cause disease.
Griffith termed this unknown substance the “transforming principle.” This principle allowed the R bacteria to undergo a permanent genetic change, becoming virulent S bacteria. At the time, Griffith did not identify the chemical nature of this transforming principle, only that such a transfer of heritable information had occurred.
The Lasting Scientific Legacy
Griffith’s 1928 experiments impacted the field of biology. His work provided the first clear experimental evidence of bacterial transformation, demonstrating that genetic traits could be transferred between bacterial cells.
The concept of a “transforming principle” stimulated scientific inquiry into the nature of genetic material. Griffith’s findings directly paved the way for subsequent research, most notably by Oswald Avery, Colin MacLeod, and Maclyn McCarty in 1944. Their work built upon Griffith’s observations, ultimately identifying deoxyribonucleic acid (DNA) as the transforming principle and the carrier of genetic information.