LuriaDelbrückmodellen

The Luria-Delbrück model, also known as the fluctuation test, is a foundational concept in the field of molecular biology and genetics that explains the origin of spontaneous mutations. Proposed by Salvador Luria and Max Delbrück in 1943, the model addresses how bacterial resistance to bacteriophages arises. Prior to this model, it was commonly believed that mutations were induced by external factors or were a direct response to environmental pressure.

The Luria-Delbrück model posits that mutations occur randomly and spontaneously, independent of the selective agent. In their experiment, they cultured multiple small batches of bacteria. Each batch was then exposed to bacteriophages. They observed that some batches developed resistance to the phages while others did not. Crucially, the number of resistant bacteria varied significantly between the batches.

The model explains this variation by suggesting that mutations conferring resistance happened at different times in the bacterial growth cycle in each independent culture. Cultures that experienced a mutation early in their growth would have a larger population of resistant bacteria by the time the phage was introduced, as the mutated cells would have had time to divide. Conversely, cultures where mutations occurred late, or not at all, would have fewer or no resistant bacteria.

This understanding was revolutionary because it demonstrated that the environment acts as a selective force on pre-existing variations, rather than inducing mutations directly. The fluctuation test remains a cornerstone for understanding the genetic basis of resistance and evolution in microbial populations.