Which technique is used to obtain isolated colonies by spreading across a surface using a loop?

• A. Drop technique
• B. Spread plate technique
• C. Streak plate technique
• D. Pour plate technique

Answer: (C)

Isolating single colonies by spreading across a surface with a loop is done with the streak plate technique. A sterile inoculating loop carries a small amount of sample and is used to streak across the agar in a deliberate pattern, gradually thinning the inoculum with each pass. Flaming and cooling the loop between segments helps reduce carryover and achieves further dilution. As cells become increasingly sparse, individual cells land separate from one another and grow into discrete colonies after incubation. This method is ideal for obtaining pure cultures because each colony ideally descends from a single cell.

In contrast, other methods spread or mix the sample differently: the spread plate distributes a diluted sample evenly to form a uniform lawn rather than isolated colonies; the pour plate mixes cells into molten agar so colonies form within the gel, not just on the surface; the drop technique uses small drops that may not reliably yield well-separated colonies on the surface.