Experiment of Luria and Delbrück

the experiment of El Salvador Luria and max Delbrück or test of fluctuation (1943) shows that in the bacteria, the Mutation S appear before the change of the medium, and are not an answer to the Natural selection. Thus the theory of the natural selection of Darwin, applies as well to the Bactérie S, as at the complex organizations.

Principles and results

In their Experiment, El Salvador Luria and max Delbrück inoculates a small number of bacteria in several different tube S of cultures.

After one period of growth, they spread out equal quantities of these cultures separated over gel from Agar containing Phage S. If resistance to the virus in the bacteria are caused by an answer of the bacterium, then the number of resistant colonies should be equal in each box. However it is not what observed Luria and Delbrück. In fact, the number of resistant colonies varies in an important way.

Luria and Delbrück proposed that these results could be explained by a constant rate of change S random with each generation of bacteria in the first tube of culture. Delbrück developed a Mathematical model sophisticated which returned this Hypothèse compatible with the results. The conclusion thus that the changes in the bacteria, as at the more complex organizations, are Aléatoire S and not is directed.

The results of Luria and Delbrück were confirmed in a less quantitative way by Newcombe (1949).

Confirmation

Newcombe put in culture of the bacteria in boxes of $petri during a few hours, then it made counterparts of each box in two others boxes of $petri different containing Phage S. One D-slack the colony S appeared in one from the boxes. The other box is left such as it is.

The 2 boxes are incubated with 37°c during the night. It results 50 times from them more colonies in the boxes which D-was spread out. This shows that the Mutation S allowing resistance to the Virus at the Bactérie S appeared during the first incubation. The changes appear before the selection.

Random links: