Luria-Delbrück Experiment

The Luria-Delbrück experiment of 1943 showed that mutations in bacteria arise at random, before any selection, rather than being caused by the conditions that favor them. Salvador Luria and Max Delbrück designed it, and it helped to earn Luria and Delbrück, with Alfred Hershey, the 1969 Nobel Prize in Physiology or Medicine.

Bacteria such as Escherichia coli can become resistant to a virus, the bacteriophage T1. One idea was that exposure to the virus induces resistance in a few cells. The alternative was that resistant mutants arise spontaneously at random, and the virus merely selects the ones that are there. The two views predict different patterns of numbers.

Luria, inspired by watching a slot machine, grew many separate small cultures from a few cells and spread each one on a plate with phage, counting survivors. If resistance was induced, every culture would give a similar number of colonies. If mutations arise at random during growth, a mutation occurring early would produce a "jackpot" of many resistant cells in one culture, while most cultures would have none. The counts varied enormously, matching random mutation.

The result showed that bacteria undergo genetic mutation like other organisms and that natural selection acts on pre-existing variation. The statistical method, the fluctuation test, is still used to measure mutation rates, including in studies of antimicrobial resistance. It is a landmark in the history of microbiology and evolution.

• Genetic Mutation — what the experiment measured
• Bacteriophage — the virus used as the selective agent
• Natural Selection — the process acting on random variation