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The Lederberg experiment
In 1952, Esther and Joshua Lederberg performed an experiment that helped to show
that many mutations are random, not directed.
Here is the experimental set-up for the Lederberg experiment. All you really need
to know in terms of background information is that bacteria grow into isolated colonies
on plates, and that you can reproduce the colonies from an original plate to new plates
by "stamping" the original plate with a cloth and then stamping empty plates
with the same cloth. Bacteria from each colony are picked up on the cloth and then
deposited on the new plates by the cloth.
The hypothesis for the experiment is that antibiotic resistant strains of bacteria
surviving an application of antibiotics had the resistance before their exposure to the
antibiotics, not as a result of the exposure.
So the penicillin-resistant bacteria were there in
the population before
they encountered penicillin. They did not evolve resistance in response to exposure
to the antibiotic.