In Joshua Lederberg’s 1952 replica-plating experiment, what did bacterial colonies show about antibiotic resistance?

Joshua Lederberg’s 1952 replica-plating experiment showed that resistance mutations pre-existed before antibiotic exposure.

Lederberg and Esther Lederberg grew bacterial colonies on a nutrient plate, then pressed a velvet surface onto the colonies and transferred matching patterns to plates containing an antibiotic. Resistant colonies appeared in corresponding positions, showing that they had already existed in the original population.

The method demonstrated that the antibiotic selected resistant bacteria rather than directing each bacterium to mutate in response. This distinction became fundamental to understanding natural selection in microbes and the evolution of antibiotic resistance.

Replica plating was especially powerful because it preserved the spatial arrangement of colonies, allowing researchers to compare growth on different media. The experiment complemented, but did not duplicate, the earlier Luria–Delbrück fluctuation test.