History of Bacterial Transformation and Plasmids

Bacterial Transformation was discovered in 1928 by a British medical officer, Frederick Griffith. During a test regarding pneumonia, he discovered that a strain of pneumonia could transform into a virulent strain. One was virulent (IIIS) and could kill mice while the other was nonvirulent (IIR) and could not. He did a lab regarding this discovery. He injected mice with different strains. With the virulent strain, it died. With the nonvirulent strain, it lived. However, when he injected the mixture of heat-killed virulent strain and the nonvirulent strain, the mice died. Also the mice still had the virulent bacteria in its blood. With this he concluded that the virulent strain’s genetic material was getting into the nonvirulent strain somehow. With no explanation, he called this the “transforming principle”. 

Then in 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty discovered this “transforming principle” as being genetic. In other words, they identified that the “transforming principle” was DNA. In an experiment, they removed the cell extract from the cellular debris from the IIIS cells and incubated it with the IIR. They found that the agar plates were growing colonies of IIIS instead of IIR which proved that the IIIS extract contained the “transforming principle”. It showed that one of the four principles: polysaccharides, proteins , RNA , DNA was responsible for transforming the bacteria. In order to find that out, they treated the cell extract with enzymes that degraded one or multiple macromolecules. The results were that any enzyme that did not digest the DNA did not remove the transforming principle. This means that only the DNA is responsible for the transformation of bacteria.

The little information of the history of plasmids was this:

In 1952, J. Lederberg used the term “plasmid” for the generic term of extrachromosomal genetic particle. This term was initially used to clarify the classification of organelles, parasites, genes, or, symbionts. Then, as years gone by, plasmids became a popular study.  It also got confused with the term episome for a while.