How can you obtain a single-colony isolate from a mixed culture?

• A. Perform streak plating with successive dilutions to isolate a single colony, then re-streak to confirm purity.
• B. Simply pick any colony and culture it.
• C. Use a blender to mix colonies.
• D. Heat-kill all but one colony.

Answer: (A)

To obtain a single-colony isolate from a mixed culture, you need a method that physically separates cells on a solid surface so each colony arises from a single organism, followed by a check that the colony is pure. Streak plating with successive dilutions spreads the cells across the agar in a way that the numbers thin out enough to yield isolated colonies. Once you see an isolated colony, you pick it and re-streak onto fresh agar to confirm purity—if the same single type appears again, you’ve successfully isolated a pure lineage.

Why this works: spreading cells out reduces their density so that individual cells land spaced apart and grow into distinct colonies. Restreaking from a single isolated colony provides a second verification step: if the growth remains uniform in the same form, you’ve demonstrated that the culture originated from one cell type and is pure.

Why the other ideas don’t fit: simply picking any colony doesn’t guarantee purity because the original culture might contain more than one organism, and that colony could reflect multiple cells growing together. Using a blender would mix the cells rather than separate them, destroying the possibility of isolation. Heat-killing all but one colony would eliminate most cells and leave nothing viable to culture, so you wouldn’t obtain a usable pure isolate.