LuriaDelbrück

Luria-Delbrück refers to a landmark experiment in bacterial genetics conducted by Salvador E. Luria and Max Delbrück in the early 1940s. The work demonstrated that genetic mutations arise spontaneously and in a random, preexisting manner rather than being induced by exposure to a selective agent. This finding supported the random-mutation theory of genetic variation and helped establish the modern understanding of how mutations contribute to evolution.

In the core experiment, multiple independent cultures of Escherichia coli were grown in parallel and then challenged with a bacteriophage to selectively propagate resistant mutants. The number of phage-resistant colonies was counted after plating on phage-containing medium. A key result was the wide variation in resistant colonies among cultures: some produced many resistant colonies, while others produced few or none. This pattern, including occasional large "jackpot" cultures, could not be explained by mutations arising solely in response to the phage.

Interpreting the fluctuations, Luria and Delbrück argued that resistance mutations occurred randomly during growth and were already present in some cells before exposure to the phage. If mutations were induced by the selective agent, the distribution of resistant colonies would be more uniform; instead, the observed variability aligned with a stochastic, preexisting-mutation model.

The experiment introduced the fluctuation test and the concept now known as the Luria-Delbrück distribution. It had a lasting impact on genetics, providing strong evidence for random mutation, influencing subsequent research in mutation rates, antibiotic resistance, and the origins of genetic variation.