In the 1943 Luria–Delbrück experiment, what did bacterial fluctuations show about virus resistance?

The 1943 Luria–Delbrück experiment showed that mutations conferring virus resistance arise before selection.

Salvador Luria and Max Delbrück grew separate bacterial cultures and exposed them to bacteriophages, viruses that infect bacteria. The numbers of resistant colonies varied dramatically from culture to culture. Some cultures had many resistant bacteria, while others had few or none.

That large fluctuation fit a model in which random mutations occurred during earlier bacterial growth. A mutation arising early would be copied into many descendants, producing a large resistant group; a mutation arising late would produce only a few. If exposure itself caused each mutation, the cultures would have shown much less variation.

The experiment did not claim that every mutation is beneficial or that bacteria deliberately adapt. Instead, it established a key evolutionary principle: selection preserves advantageous variants that already arise randomly. Luria and Delbrück received the 1969 Nobel Prize in Physiology or Medicine with Alfred Hershey.