Bacteria can exist in a variety of different forms. The study of such forms in the bacterium Diplococcus pneumoniae (which causes the lethal pneumonia) has led to the discovery of bacterial transformation. Virulent D. pneumoniae strains are surrounded by a slimy coat. This capsule is made of polysaccharide, and gives colonies of D. pneumoniae a smooth, shiny appearance. This form is called 'S', for smooth form.
In 1923, Griffiths discovered that D. pneumoniae can also exist in a harmless, avirulent form, distinguished by the absence of the polysaccharide capsule, and a rough appearance of the colonies. The avirulent forms are called 'R', for rough form. This is actually not a major discovery as the existence of rough and smooth forms of colonies was known for Streptococcus. What interested Griffiths was the apparent ability of one form to transform into another.
Griffith carried out 4 particularly important experiments. In each experiment, he injected mice with a solution containing virulent or avirulent forms of D. pneumoniae. The observations are as follow.
|Experiment||What was injected?||Observation|
|1||Virulent 'S' form||Mouse dies|
|2||Avirulent 'R' form||Mouse lives|
|3||Heated 'S' form||Mouse lives|
|4||Heated 'S' form + 'R' form||Mouse dies|
By answering all these questions, Griffith and his team made a breakthrough. So try to think about the answer before you find out about the conclusions....
1) Why does the mouse die in experiment 1?
The mouse is injected with the live virulent D. pneumoniae. It will therefore die of pneumonia.
2) Why does the mouse live in experiment 2?
This time the mouse is injected with the avirulent form of the bacterium. This form does not have a capsule, and will be attacked and defeated by the mouse immune system. The mouse will live.
3) Why doesn't the 'S' form kill the mouse in experiment 3?
The mouse is injected with the 'S' form, but it lives. The 'S' form injected was heated prior to injection. Heat kills bacteria. The bacteria injected was dead, and therefore couldn't kill the mouse.
4) What is the difference between an 'S' form bacterium and an 'R' form?
The 'S' form has a capsule that protects it against the immune system. That is what makes it virulent.
5) What will help an 'R' form become an 'S' form?
If the 'R' form could inherit a capsule, it will become an 'S' form, and be virulent.
6) What happened in Experiment 4? Can you explain why the mouse dies?
In experiment 4, the killed 'S' form was mixed with live 'R' form and then injected. The killed bacteria solution therefore contains proteins from the capsule of the smooth bacteria and genetic material. There is not enough protein to cover all the 'R' forms to make them into virulent bacterium. It is therefore the genetic material of the smooth form that transformed the 'R' form into a virulent 'S' form.
In addition, a large number of living 'S' forms were isolated after the experiment.
The conclusion to be drawn from the Griffith experiment is that components of the heat-killed bacteria were able to enter the 'R' bacteria and transform them into the virulent form. The component injected must be able to induce the cell to synthesise the components of the capsule. Introduction of this transforming principle into the 'R' cells results in the acquisition by these bacteria of the new heritable characteristic. Identify the transforming principle, and the nature of the genetic material would become known.