Lederberg and Lederberg exposed a large bacterial population to an antibiotic and recovered resistant colonies. They had earlier replicated colonies from a drug-free master plate onto drug-containing plates and obtained resistant colonies at positions matching the master plate. This experiment demonstrated:

• A Antibiotic exposure induces resistance mutations in bacteria
• B Resistance is acquired by transduction from resistant neighbors
• C Resistant mutants arise spontaneously before exposure to the antibiotic ✓
• D Resistance genes transfer only by conjugation between sister colonies

Explanation

The replica plating experiment showed resistant colonies appearing at predictable positions on drug plates copied from an untouched master plate, proving mutants existed prior to any contact with the drug. This supports the random mutation hypothesis over directed or adaptive mutation. Induction (option A), transduction and conjugation do not explain position-matched colonies arising without exposure, which is the discriminating feature of this classic experiment.

Reference: Jawetz, Melnick & Adelberg's Medical Microbiology, 28th ed.

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