Often the simplest experiments lead to the most remarkable insights. So it was with the famous fluctuation experiments of Luria and Delbrück and the Waring blender experiments of Hershey and Chase for which they were awarded the Nobel Prize in Physiology or Medicine in 1969. While the results of these experiments are permanently etched into every first year biology student's brain, it is worth recalling what was known at the time these experiments were conducted and the conclusions the authors drew.
In the early 20th century, the role of chance mutations in the genetic variation of higher organisms was generally accepted. However, bacterial cultures seemed to be plastic. Exposed to adverse conditions, cultures of bacteria could give rise to resistant variants that remained resistant “to the action of the virus even if subcultured through many generations in the absence of the virus”1. At the time, many scientists believed that the “virus by direct action induced the resistant variants”1. Others believed that the “resistant bacterial variants are produced by mutation in the culture prior to the addition of virus. The virus merely brings the variants into prominence by eliminating all sensitive bacteria”1. These two alternative theories — induced immunity and selection of spontaneous mutations — were rigorously tested both theoretically and experimentally by Max Delbrück and Salvadore E. Luria, respectively, in 1943.
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