The Griffith's experiment

Griffith's experiment

In 1928, Griffith conducted experiments with the transformation of two different strains of a bacterium i.e. Diplococcus pneumonia (which is now called Streptococcus pneumonia) - virulent strains that cause pneumonia in some vertebrate species (like mice, people) and avirulent strains that cause don’t pneumonia.

The difference in virulence is associated with the bacterial polysaccharide capsule. This capsule is present in virulent strains, while avirulent strains don’t have any capsule. The non-encapsulated bacteria are readily engulfed and destroyed by phagocytic cells.

Virulent bacteria, which possess the polysaccharide capsule, are not easily engulfed. Hence, they are able to multiply and cause pneumonia. Encapsulated bacteria form smooth colonies (S) when grown on an agar culture plate; whereas non-encapsulated strains produce rough colonies (R). Each Diplococcus strain may have various serotypes. Immunological techniques are used to identify serotypes. Two different strains were carried out by Griffith — IIR, and IIIS.

 

Griffith injected the different strains of bacteria into mice. The IIIS strain killed the mice; the IIR strain did not. He further noted that if the heat-killed IIIS strain was injected into a mouse, it did not cause pneumonia. When he combined heat-killed IIIS with live IIR and injected the mixture into a mouse (remember neither alone will kill the   mouse) that the mouse developed pneumonia and died.

Griffith concluded that the heat-killed IIIS bacteria were responsible for converting live avirulent IIR cells into virulent IIIS ones and called the phenomenon transformation.  He called the genetic information which could be passed from the dead IIIS cells to IIR cells, the transforming principle.

Avery-MacLeod-McCarty experiment

In 1944, Oswald Avery, Collin MacLeod, and Maclyn McCarty revisited Griffith's experiment and concluded that the transforming material was pure DNA, not protein or RNA. These investigators found that DNA extracted from a virulent strain the bacterium Streptococcus pneumonia, also called pneumococcus, genetically transformed an avirulent strain of this organism into a virulent form.

Figure 1.56 Avery, MacLeod, and McCarty had shown that only DNA extracts from S cells caused the transformation of R cells to S cells. To demonstrate that contaminating molecules in the DNA extract was not responsible for transformation, the DNA extract from S cells was treated with RNase, DNase, and protease and then mixed with R cells.