In Lederberg's replica plating experiment, what shall be used to obtain a streptomycin-resistant strain?
- Only minimal medium
- Complete medium and streptomycin
- Only complete medium
- Minimal medium and streptomycin
Answer
(4) Minimal medium and streptomycin
Solution
In replica plating, the colonies of a master plate are transferred with a sterile velvet pad, in the same pattern, onto a selective plate.
∵ To select streptomycin-resistant bacteria, the selective plate must contain streptomycin; the sensitive cells are killed and only resistant mutants form colonies.
⇒ Hence minimal medium with streptomycin is used; the resistant colonies appear at places matching colonies of the master plate, which had never met the antibiotic.
∴ The mutations were already present before exposure (pre-adaptive), and the antibiotic only selects them — as per NCERT, the use of antibiotics has resulted in selection of resistant microbes.
Answer: (4)