The transforming principle is the substance responsible for permanently changing one type of bacterium into another, a mystery that took 16 years to solve and ultimately revealed that DNA carries genetic information. First observed in 1928 by Frederick Griffith and chemically identified in 1944 by Oswald Avery, Colin MacLeod, and Maclyn McCarty, the transforming principle turned out to be DNA itself. Before these experiments, most scientists assumed proteins carried hereditary instructions. The discovery upended that assumption and laid the groundwork for modern genetics.
Griffith’s 1928 Experiment
Frederick Griffith, a British bacteriologist, was studying pneumonia-causing bacteria (pneumococci) when he stumbled onto something he couldn’t explain. Pneumococci came in two forms: a smooth-coated “S” strain that was deadly to mice, and a rough “R” strain that was harmless. Griffith ran four combinations of injections in mice to understand how the bacteria caused disease.
- Live S bacteria alone: The mice died of pneumonia.
- Live R bacteria alone: The mice survived with no signs of disease.
- Heat-killed S bacteria alone: The mice survived. No living S bacteria could be recovered from them.
- Heat-killed S bacteria mixed with live R bacteria: The mice died, and live S bacteria were recovered from their blood.
That fourth result was the puzzle. Dead S cells couldn’t infect anything on their own, and live R cells were harmless. Yet when combined, something from the dead S cells converted the harmless R cells into deadly S cells. Griffith called whatever was responsible the “transforming principle.” He couldn’t identify the substance, but he proved it existed. The transformation was permanent: the newly converted S bacteria remained virulent through subsequent generations, meaning the change was heritable.
Identifying the Substance: Avery, MacLeod, and McCarty
For over a decade after Griffith’s experiment, no one could pin down what the transforming principle actually was. Starting in 1935, Oswald Avery and his colleagues at the Rockefeller Institute set out to isolate and identify it. Their work, published in 1944, remains one of the most elegant experiments in biology.
The team first reproduced transformation outside of living mice, working entirely in test tubes. They dissolved smooth-strain bacteria using an ionic detergent, filtered out the cellular debris, and precipitated the active material with alcohol. What they collected was a thick, syrupy substance that remained fairly stable and could still transform R bacteria into the S type. This meant the transforming principle was a specific molecule that could be extracted and purified, not something that required intact cells to work.
The critical step was figuring out what kind of molecule it was. At the time, the leading candidates were protein, RNA, or DNA. Avery’s team systematically eliminated each possibility. When they treated the extract with proteases (enzymes that destroy proteins), the substance still transformed bacteria. When they treated it with ribonuclease (an enzyme that destroys RNA), it still worked. But when they exposed it to an enzyme that breaks down DNA, the transforming ability vanished. The transforming principle was DNA.
Chemical analysis backed this up. The purified substance matched the known properties of DNA in its ratio of nitrogen to phosphorus and in its behavior under ultraviolet light. Despite the clarity of these results, many scientists remained skeptical. Proteins were far more complex and diverse than DNA, and it seemed hard to believe that a relatively simple molecule like DNA could encode all the information needed to build a living organism.
Confirmation by Hershey and Chase
Skepticism lingered until 1952, when Alfred Hershey and Martha Chase provided independent confirmation using a completely different experimental system. Instead of bacteria transforming other bacteria, they studied viruses that infect bacteria (called bacteriophages). These viruses are structurally simple: a protein shell surrounding a core of DNA.
Hershey and Chase grew bacteria in radioactive sulfur, which gets incorporated into proteins but not DNA, and separately in radioactive phosphorus, which gets incorporated into DNA but not proteins. They then let phages infect these bacteria, producing viruses labeled in either their protein coat or their DNA core. When labeled phages infected fresh bacteria, only the radioactive phosphorus (DNA) entered the bacterial cells. The radioactive sulfur (protein) stayed outside. Since the phages successfully reproduced inside the bacteria using only the injected DNA, genetic information had to reside in DNA, not protein. This experiment, coming eight years after Avery’s work, finally convinced the broader scientific community.
How Transformation Works at the Molecular Level
We now understand exactly how bacteria pick up free DNA from their environment, the process Griffith observed without knowing its mechanism. Bacteria can enter a temporary physiological state called “competence,” during which they actively take up DNA fragments from their surroundings. The process unfolds in four steps.
First, free-floating DNA in the environment binds to protein structures on the bacterial surface, often hair-like appendages called pili. Second, the pili retract and pull the DNA closer to the cell, where dedicated binding proteins grab hold of it. Third, the double-stranded DNA is cleaved into single strands at the cell membrane, and one strand is transported into the cell’s interior while the other is degraded. Fourth, once inside, the single-stranded DNA can be integrated into the bacterium’s own chromosome through a process called homologous recombination, permanently altering the cell’s genetic makeup.
This is exactly what happened in Griffith’s experiment. When heat-killed S bacteria broke apart, their DNA spilled into the surrounding environment. Live R bacteria, in a state of competence, took up fragments of that DNA, including the genes responsible for producing the smooth protective coat. Once those genes integrated into the R bacteria’s chromosome, the cells began producing the coat, becoming virulent S-type bacteria that could evade the mouse’s immune system.
Why the Transforming Principle Still Matters
Natural transformation isn’t just a historical curiosity. It remains one of the primary ways bacteria acquire new traits in the wild, including resistance to antibiotics. When bacteria die and release their DNA, nearby competent bacteria can pick up genes for drug resistance and incorporate them into their own genomes. This process works even between distantly related bacterial species, meaning resistance genes can jump across taxonomic boundaries that would otherwise keep them contained.
A range of human pathogens use natural transformation to acquire antibiotic resistance, making it a significant concern in clinical settings. Understanding the mechanism that Griffith first glimpsed in 1928 now informs efforts to track and slow the spread of drug-resistant infections.
The discovery of the transforming principle also fundamentally changed how scientists think about heredity. Before Avery’s 1944 paper, genetics was largely an abstract discipline, tracking inheritance patterns without knowing what physical substance carried the instructions. Identifying DNA as that substance opened the door to molecular biology, the genetic code, and eventually the ability to read and edit genomes directly.