In the Lederbergs' experiment, colonies grown overnight on a drug-free master plate were pressed with sterile velvet onto agar containing streptomycin. Resistant colonies appeared at positions corresponding exactly to certain colonies on the original plate, although those bacteria had never encountered the drug. This experiment established which principle?

• A Streptomycin induces resistance mutations in exposed bacteria
• B Resistance is acquired only through horizontal transfer of R plasmids
• C Resistance mutations arise spontaneously before exposure to the drug, and the drug merely selects them ✓
• D Replica plating induces stress responses that create resistant variants

Explanation

Replica plating showed that streptomycin-resistant mutants were present on the drug-free master plate before any contact with the antibiotic, since their position on the selective plate matched the position on the master plate. This demonstrated that mutations occur spontaneously and randomly at a low rate, independent of the selecting agent, and that exposure to the drug only enriches pre-existing mutants. If the drug induced resistance, resistant colonies could not show positional correspondence to colonies never exposed to it.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

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