Discussion
An auxotrophic mutant arises spontaneously in a wild type E.coli culture growing in a rich medium. Which ONE of the following techniques will ensure the isolation of the auxotrophic mutant?
Answer: A
The technique is Replica plating.
An auxotroph grows on rich medium like the wild type, so it cannot be told apart on the rich plate. Colonies are transferred with a velvet pad from a master plate onto a complete medium and onto a minimal medium in the same positions. A colony that grows on the complete medium but is missing from the minimal plate is an auxotroph, and it can be picked from the master plate.
Streaking and pour plating isolate single colonies but give no way to tell which colony is the mutant, and a microscope cannot see a nutritional requirement.
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