The Griffith Transformation Experiment: The Mice That Changed Genetics

In 1928, a British medical officer named Frederick Griffith injected mice with a mixture of dead bacteria and live harmless bacteria. The mice died — killed by living versions of the bacteria he had deliberately destroyed before the injection. Something from the dead cells had crossed into the living ones and turned them lethal. Griffith did not know what that something was. Naming it "the transforming principle," he left behind the single most consequential unsolved puzzle in twentieth-century biology, one that would take sixteen more years and two further experiments to resolve.

The Problem Griffith Was Actually Trying to Solve

Griffith was not hunting for the genetic material. He was a public health researcher at the Ministry of Health in London, and his question was practical: why does Streptococcus pneumoniae exist in multiple forms, and could bacteria of one form be converted to another during an infection?

Pneumococcal pneumonia was a major killer in the 1920s. Vaccines were being tested, but different serological types (type I, type II, type III, and so on) of S. pneumoniae existed, and a vaccine against one type offered no protection against the others. Griffith needed to understand whether the types could interconvert in the body — a question with direct implications for vaccine design.

Source: NIH National Library of Medicine, Profiles in Science. "Shifting Focus: Early Work on Bacterial Transformation, 1928–1940." https://profiles.nlm.nih.gov/spotlight/cc/feature/shifting

His experiments were therefore cast in the framework of immunology and infectious disease, not genetics. The genetic implication of his results was recognized by others, not immediately by Griffith himself.

The Setup: Smooth Bacteria and Rough Bacteria

Griffith worked with two forms of Streptococcus pneumoniae (at the time called Pneumococcus). The distinction between them was visible even to the naked eye on culture plates.

The smooth (S) strain grows in colonies with a glistening, smooth appearance. This form produces a polysaccharide capsule that surrounds each bacterial cell. The capsule is the source of its virulence: it prevents the host immune system from identifying and engulfing the bacterium through phagocytosis. Mice injected with live S-strain bacteria die of pneumonia.

The rough (R) strain grows in dull, rough-edged colonies. It lacks the polysaccharide capsule — a result of mutations in the genes responsible for capsule synthesis. Without the capsule, the immune system can destroy it. Mice injected with live R-strain bacteria survive.

Source: Griffith F. "The Significance of Pneumococcal Types." Journal of Hygiene 27(2): 113–159 (1928). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2167760/

Griffith conducted four experimental conditions with two strains of type III S and a type II R derivative:

Injection	Result
Live S-strain (type III)	Mice die; live S bacteria recovered
Live R-strain (type II)	Mice survive
Heat-killed S-strain (type III)	Mice survive
Heat-killed S-strain (type III) + Live R-strain (type II)	Mice die; live S bacteria (type III) recovered

The fourth result was the one nobody expected. The heat-killed S bacteria should have been biologically inert. The live R bacteria should have been harmless. Combining them should have produced a harmless mixture. Instead, mice died — and from their blood, Griffith recovered living, encapsulated type III S bacteria.

The R bacteria had acquired the properties of the S bacteria Griffith had deliberately killed. And the conversion was heritable: the transformed bacteria produced offspring that were also S-type. This was not a temporary change. Something permanent had been transferred.

Source: Britannica. "Frederick Griffith." https://www.britannica.com/biography/Frederick-Griffith

The Protocol: What Griffith Actually Did

Griffith's experimental technique was straightforward for its era. He grew type III S bacteria in culture, harvested them, and heat-killed them at 60°C for one hour — conditions that reliably destroyed all viability (confirmed by culture tests). He grew type II R bacteria separately and confirmed they were avirulent.

For the critical combination experiment, he mixed the two preparations and injected the mixture subcutaneously into white mice. After the mice died (typically within five to ten days), he drew blood from the heart and cultured it. The resulting colonies showed smooth morphology and were confirmed immunologically as type III — the type of the dead bacteria, not type II, which was the type of the live R bacteria used.

Source: NCBI PMC. "The Transformation Experiment of Frederick Griffith I: Its Narrowing and Potential for the Creation of Novel Microorganisms." MDPI Bioengineering 12(3): 324 (2025). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11939280/

Griffith also showed that transformation could occur in vitro — in culture, outside a mouse — though the in vitro result was less clean. He made it clear that the transforming substance survived heat treatment, was not a live contaminant, and could confer type-specific properties (the specific polysaccharide capsule) on organisms that had not previously produced them.

He named the unknown agent "the transforming principle" and resisted speculating further about its chemical nature. That restraint was methodologically sound: he had no tools to identify the agent chemically. Guessing would have been premature.

The Results: What Griffith Saw and What He Concluded

Griffith's central finding was that heritable biological characteristics — specifically, the ability to produce a type III polysaccharide capsule — could be transferred from dead bacteria to living bacteria of a different type. The transfer was not merely phenotypic (a temporary change in appearance); the offspring of the transformed bacteria retained the new type, generation after generation.

Source: Genome.gov. "1944: DNA is 'Transforming Principle.'" National Human Genome Research Institute. https://www.genome.gov/25520250/online-education-kit-1944-dna-is-transforming-principle

He proposed two possible explanations. Either the R bacteria were using material from the dead S bacteria as a "food" source to construct their own capsule (an interpretation that would not require a heritable change), or a specific substance from the dead S bacteria was being incorporated into the R bacteria and stably altering their hereditary constitution. He leaned toward the second explanation but could not prove it.

What he did not conclude — and what would have been unjustifiable at the time — was that the transforming principle was DNA. The very idea that nucleic acids could carry hereditary information was not part of mainstream scientific thinking in 1928. Proteins, with their twenty amino-acid alphabet and complex three-dimensional structures, were the favored candidates for encoding biological information.

Source: Nature Scitable. "Discovery of DNA as the Hereditary Material." https://www.nature.com/scitable/topicpage/discovery-of-dna-as-the-hereditary-material-340/

The Controversy: An Implausible Result That Had to Be True

Griffith's 1928 paper initially met resistance. The finding challenged a firmly held view in bacteriology: that pneumococcal serotypes were fixed and stable — that type III bacteria always produced type III offspring and could not be converted to another type. The idea of bacterial transformation contradicted what bacteriologists understood about the stability of microbial types.

Several labs initially failed to reproduce Griffith's results, which deepened skepticism. The transformation was real but finicky: it depended on bacterial culture conditions, the ratio of dead S to live R bacteria, and technical details that Griffith's paper did not fully specify. Later investigators who were careful about these conditions could reproduce the result reliably.

Source: PMC. "Rambling and Scrambling in Bacterial Transformation — a Historical and Personal Memoir." https://pmc.ncbi.nlm.nih.gov/articles/PMC141969/

The result was eventually confirmed by Martin Henry Dawson, working at the Rockefeller Institute, who in 1928–1930 replicated the mouse experiment and extended it to in vitro transformation. Dawson's confirmation began the slow process of converting skeptics.

Source: Embryo Project Encyclopedia, Arizona State University. https://embryo.asu.edu/

The deeper controversy — one that persisted until 1944 — was about the nature of the transforming principle itself. Even after Oswald Avery, Colin MacLeod, and Maclyn McCarty identified the principle as DNA, many prominent scientists remained unconvinced. Alfred Mirsky at the Rockefeller Institute argued that the transforming preparations might be contaminated with trace amounts of protein, and that protein rather than DNA was the active agent. This argument persisted for years, even as the biochemical evidence accumulated.

Avery, MacLeod, and McCarty: What the Transforming Principle Was

The identity of Griffith's transforming principle was resolved in 1944 by a team at the Rockefeller Hospital in New York. Oswald Avery, Colin MacLeod, and Maclyn McCarty spent nearly a decade systematically fractionating S-strain bacterial extracts, testing each fraction for transforming activity.

Source: Avery OT, MacLeod CM, McCarty M. "Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types." Journal of Experimental Medicine 79(2): 137–158 (1944). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2135445/

Their approach was a process of chemical elimination. They prepared large quantities of S-strain extract — sometimes 75 liters of bacterial culture at a time — and subjected the extract to treatments that selectively destroyed one class of molecules at a time:

• Protease enzymes (which degrade proteins): did not destroy transforming activity.
• Lipase enzymes (which degrade fats): did not destroy transforming activity.
• RNase (which degrades RNA): did not destroy transforming activity.
• DNase (which degrades DNA): destroyed transforming activity completely.

The conclusion was unambiguous: the transforming principle was DNA. Not protein, not RNA, not any other molecular class — specifically DNA.

Source: NIH Profiles in Science. "DNA as the 'Stuff of Genes': The Discovery of the Transforming Principle, 1940–1944." https://profiles.nlm.nih.gov/spotlight/cc/feature/dna

Avery himself was characteristically cautious in his published conclusions, writing that the data were "consistent with the belief" that DNA was the transforming principle while acknowledging that he could not completely rule out some other factor. But the experimental evidence was overwhelming, and the paper marked the beginning of the end for the protein hypothesis.

Even after 1944, skeptics held on. The definitive nail in the coffin came in 1952, with the Hershey-Chase experiment, which showed that when bacteriophage viruses infect bacteria, it is the viral DNA — not the viral protein — that enters the host cell and directs the production of new viruses.

Replication and Reanalysis: Transformation Today

The phenomenon Griffith discovered is now called bacterial transformation, and it is understood to be one of three mechanisms by which bacteria exchange genetic material horizontally (the other two being conjugation and transduction). Natural transformation occurs when bacteria take up free DNA from their environment — a process that requires specific proteins on the bacterial surface and is regulated by environmental signals.

Source: Lorenz MG, Wackernagel W. "Bacterial gene transfer by natural genetic transformation in the environment." Microbiological Reviews 58(3): 563–602 (1994). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC372978/

The ability of bacteria to take up foreign DNA and incorporate it stably into their genome is exploited in essentially every molecular biology laboratory today. When researchers want to introduce a gene into bacteria — to produce insulin, for example, or to express a fluorescent protein — they use artificial transformation. Bacteria are treated to make their membranes temporarily permeable (either by chemical treatment with calcium chloride or by a brief electrical pulse called electroporation), then mixed with a plasmid carrying the gene of interest. Competent bacteria take up the plasmid and express the gene.

Source: Green MR, Sambrook J. Molecular Cloning: A Laboratory Manual, 4th ed. Cold Spring Harbor Laboratory Press, 2012.

Griffith's mice are the ancestor of this procedure. Every recombinant protein produced in bacteria — including the synthetic insulin taken by over 9 million people in the United States — traces its production method to the principle he discovered.

The link to horizontal gene transfer and antibiotic resistance

The same mechanism of natural transformation contributes to the spread of antibiotic resistance genes between bacteria. When resistant bacteria die and lyse, they release their DNA into the environment. Susceptible bacteria can take up this DNA and acquire the resistance genes. This is one reason antibiotic resistance spreads even between bacteria that never come into direct contact.

Source: Johnsborg O, Eldholm V, Håvarstein LS. "Natural genetic transformation: prevalence, mechanisms and function." Research in Microbiology 158(10): 767–778 (2007). https://doi.org/10.1016/j.resmic.2007.09.004

This was not something Griffith could have anticipated, but it is a direct consequence of the biological reality he uncovered: bacterial genomes are not closed systems. They can receive information from their environment.

Where It Stands Today

Griffith died in the German Blitz on London in April 1941, killed during an air raid on the Ministry of Health. He did not live to see Avery, MacLeod, and McCarty's 1944 paper identifying the transforming principle as DNA, let alone the Watson-Crick structure of 1953 or the complete molecular understanding of DNA replication and gene expression that followed.

His experimental result stood, however. Every subsequent investigation confirmed what he found: that heritable information could be transferred between bacteria through a chemical substance. The substance was DNA. The mechanism was transformation.

Source: Wikipedia. "Griffith's experiment." https://en.wikipedia.org/wiki/Griffith%27s_experiment

Modern sequencing has allowed researchers to observe transformation in real time. When S. pneumoniae acquires antibiotic-resistance genes by natural transformation, the genomic event — which chromosome fragment was taken up and where it integrated — can be read from a sequencing run. The molecular detail Griffith's tools could not access is now routine.

The Griffith experiment also played an indirect role in the development of recombinant DNA technology and genetic engineering. The recognition that DNA from one organism can function when introduced into another — the foundational premise of Griffith's transforming principle — is the same recognition that underlies gene therapy, transgenic crops, and the production of biological medicines. For context on how those technologies work in practice, see CRISPR Gene Editing and mRNA Vaccines.

The Rutherford Gold Foil Experiment offers a parallel story from physics: a result so unexpected that it required abandoning the prevailing model entirely. In biology, Griffith's transformation result was the first such rupture of the twentieth century.

From Griffith to Avery: Identifying the Transforming Principle

Griffith's 1928 paper created a puzzle with no molecular answer. It would take sixteen years, a team of three scientists at the Rockefeller Hospital in New York, 75-liter bacterial cultures, and five distinct lines of biochemical evidence to name what Griffith had only been able to call "the transforming principle."

Oswald Avery had been working on pneumococcal biochemistry since the 1910s. By the late 1930s, Colin MacLeod and then Maclyn McCarty joined him in what became a systematic dismantling of every candidate molecule except one. Their approach was conceptually simple: purify the transforming activity from heat-killed S-strain bacteria, then destroy candidate molecules one class at a time with specific enzymes and test whether transforming activity survived.

Source: Avery OT, MacLeod CM, McCarty M. "Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types." Journal of Experimental Medicine 79(2): 137–158 (1944). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2135445/

Transforming activity — the ability of an extract or purified fraction to convert R-strain pneumococci to the S phenotype at measurable frequency in culture.

The purification process itself was grueling. Avery's team processed enormous volumes of bacterial culture — sometimes 50 to 75 liters per preparation — through repeated precipitation and extraction steps designed to remove polysaccharide, protein, and lipid. What remained after these steps was a viscous, fibrous material that continued to transform bacteria at very high dilutions. Chemical analysis showed that the material had an elementary composition consistent with a nucleic acid, a high molecular weight, and a phosphorus-to-nitrogen ratio characteristic of DNA rather than RNA or protein. The optical absorption spectrum peaked at 260 nm, the signature wavelength of nucleic acids.

Each of the five enzyme classes tested left transforming activity intact: protease (removes proteins), lipase (removes fats), RNase (removes RNA), hyaluronidase (removes certain polysaccharides), and a serum-derived polysaccharidase. Only a single enzyme destroyed activity entirely: deoxyribonuclease, the enzyme that specifically cleaves DNA. One DNase treatment, and the transforming material was inert.

The reaction of the broader scientific community was more cautious than the evidence warranted. Alfred Mirsky, a protein chemist also at Rockefeller, led the skeptical faction. His argument was that Avery's preparations, however carefully made, could not be certified as absolutely free of protein. Trace contaminating protein, he maintained, might be the true active agent. Mirsky continued to make this argument publicly for years after 1944, which slowed the acceptance of the finding in a way that later historians found disproportionate to the actual weakness of the evidence.

Source: Judson HF. The Eighth Day of Creation: The Makers of the Revolution in Biology. Cold Spring Harbor Laboratory Press, 1996.

Avery himself, writing privately to his brother Roy in 1943 before the paper was published, was less ambiguous than his cautious public prose suggested. He wrote: "Nucleic acids of the deoxyribose type" — that is, DNA — "are biologically active and functionally specific." He recognized what he had found. The restraint in the published paper was professional caution, not personal doubt.

The resistance to Avery's findings illustrates a pattern common in science: when a result contradicts the prevailing framework without yet fitting into a new one, it tends to wait. In 1944, there was no model of how DNA could encode information. Proteins had twenty amino acid "letters"; DNA was thought to be a simple tetranucleotide repeat with no informational capacity. The result was correct, but it lacked a mechanism to make it believable. That mechanism arrived in 1953 with the Watson-Crick double helix — nine years after Avery's paper — and only then did the evidence from 1944 snap cleanly into place as the logical predecessor.

Source: Dahm R. "Discovering DNA: Friedrich Miescher and the early years of nucleic acid research." Human Genetics 122(6): 565–581 (2008). https://link.springer.com/article/10.1007/s00439-007-0433-0

The Avery-MacLeod-McCarty paper is now recognized as one of the most important publications in twentieth-century biology. Avery was nominated repeatedly for the Nobel Prize and never received it — he died in 1955, and the Prize is not awarded posthumously. Whether the omission was a consequence of lingering skepticism about his finding or simply the committee's pace is debated by historians of science, but the scientific record is unambiguous: the path from Griffith's mice to the double helix runs directly through Avery's culture flasks.

FAQs

Q: Why didn't Griffith conclude that DNA was the transforming principle? A: He had no way to chemically identify it. The analytical tools needed to isolate and characterize the active substance — enzyme digestion panels, nucleic acid purification, ultracentrifugation — were developed at the Rockefeller Institute over the decade following his 1928 paper. Without those tools, any claim about the chemical identity of the transforming principle would have been speculation.

Q: Why did it take from 1928 to 1952 for the scientific community to fully accept that DNA was the genetic material? A: Scientific consensus moves more slowly than individual experiments. Avery's 1944 paper was compelling but left a small opening for skeptics who argued that trace protein contamination might explain the result. The Hershey-Chase experiment in 1952 used radioactive isotopes to track protein and DNA separately during bacteriophage infection, making a protein-contamination argument essentially impossible to sustain. Both lines of evidence together, plus the Watson-Crick structure in 1953, created a conceptual framework that left no room for alternatives.

Q: What is bacterial transformation today? A: Transformation is the uptake of free DNA from the environment by a bacterial cell. It occurs naturally in many bacterial species, including S. pneumoniae, Haemophilus influenzae, and Bacillus subtilis. In laboratory settings, transformation is induced artificially by treating bacteria with calcium chloride or by electroporation. It is the standard method for introducing recombinant plasmids into bacteria for gene expression.

Q: Did Griffith get credit for his discovery? A: Griffith received recognition within the bacteriology community but is not as widely known as Watson, Crick, or even Avery among the general public. His 1928 paper is cited in every major molecular biology textbook, but because he did not identify the chemical nature of the transforming principle, the Nobel Prize that might have recognized the discovery was not awarded to him. He died in 1941 before the significance of his work was fully appreciated.

Q: Could Griffith's result have been an artifact — contamination of the heat-killed S bacteria with live S bacteria? A: This was the first objection raised by skeptics. Griffith addressed it by demonstrating that the heat-killed preparation was completely nonviable in culture and in mice when injected alone. The transformation only occurred in combination with the live R bacteria, and the organisms recovered from the dead mice had the immunological type of the dead S bacteria (type III) rather than the type of the live bacteria (type II R) — a combination that would be inexplicable if the result were contamination.

Q: What happened when Griffith's result was applied to in vitro systems? A: Griffith himself showed that transformation could occur outside a mouse, in culture, though the in vitro result was less reproducible. Later workers — particularly at the Rockefeller Institute — developed reliable in vitro transformation protocols that did not require mice, which allowed them to manipulate the transforming principle chemically and ultimately identify it as DNA.

The Takeaway

Griffith's transformation experiment produced an observation that refused to fit into the biology of its time: dead bacteria transferring a heritable property to live bacteria of a different type. The observation was correct, reproducible, and ultimately explained by the most important discovery in twentieth-century biology — that DNA is the molecule of heredity. Griffith did not know what he had found. That is often how foundational science works: the experimenter identifies the phenomenon and names the unknown; others, equipped with better tools, resolve the name into a specific molecule. In this case, the molecule was a double helix, and the resolution of the mystery took twenty-four years.

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