Replica plating test
The best known experiments that tested whether mutations are random or environmentally directed consist in replica plating tests (Ledeberg & Ledeberg 1952)and also various variants of the fluctuation test (Luria & Delbruck 1943). In the replica plating test (Fig. III.6), a bacteria suspension is seeded on a Petri dish and, after small colonies are formed, they are imprinted using a large round stamp on a dish whose agar contains a suitable selection agent (e.g. a certain antiobiotic) and also on a dish without the antibiotic. Only colonies of mutated cells grow in the dish with the antibiotic. In the next phase, samples of the colonies are taken corresponding to the positions of the colonies of mutated cells in the dish without the antibiotic, i.e. samples of bacteria that never came into contact with this antibiotic, and their resistance is tested. If the mutations occurred only as a consequence of the action of the selection agent, the bacteria from these colonies should not be resistant. In contrast, if the mutations occurred randomly, bacteria from the original colonies should be resistant. The results of replica plating tests demonstrated that bacteria on the original dish are already resistant and thus that they mutated randomly.