Lederbergs' experiment of the prevalence of mutations before selective culture was proved by replica plating. They spread bacteria on the plate and allowed them to grow. Next, they stamped the growth on the new plate which was already containing penicillin. The bacteria were able to grow on this plate, even when they never exposed to the antibiotic before. This proved that some of the bacteria were already mutated and were not as a result of exposure to selective culture conditions.
