The Luria–Delbrück experiment, often called the "fluctuation test," was a key 20th‑century investigation into how genetic change arises in microbes. Its central question was whether resistance to a selective agent appears because organisms adapt in response to that agent, or because rare heritable changes already exist prior to selection. The results supported the view that mutations occur spontaneously and independently of the selecting pressure.

Experimental approach

Instead of assuming every cell in a population had the same chance to become resistant only after exposure, the investigators grew many independent small cultures of bacteria and then exposed each culture to a lethal selective agent. If resistance were induced by exposure, the number of resistant colonies on treated plates would be similar across cultures. If resistance arose at random earlier, the counts would fluctuate greatly, sometimes producing "jackpots" with many resistant descendants from a single early mutant.

Typical protocol steps

  • Inoculate multiple independent bacterial cultures from a small common source.
  • Allow them to grow without the selective agent so spontaneous mutations can occur.
  • Plate each culture onto medium containing the selective agent and count resistant colonies.
  • Compare the distribution of resistant colony numbers across cultures to statistical expectations.

Findings and interpretation

The observed large variance in resistant colony counts among parallel cultures matched the prediction for spontaneous, preexisting mutations. The pattern—rare cultures with many resistant colonies and many cultures with few or none—contradicted the idea that exposure itself directed mutations. That conclusion reinforced a Darwinian model in which natural selection favors variants that already exist within a population.

Importance and legacy

The experiment helped establish fundamental concepts in bacterial genetics and evolutionary theory. It influenced how scientists estimate mutation rates and inspired further methods such as replica plating and molecular assays. The work brought prominence to its authors and is often cited in discussions of the relationship between random genetic variation and selection in both microbes and multicellular organisms. For more context on related concepts see mutation, natural selection, Max Delbrück, Salvador Luria, bacteria, DNA, randomly, and Darwin.