After streak plating, what is the recommended method to verify that a colony is pure?

• A. Gram stain the first colony.
• B. Sequence the genome of the colony.
• C. Assume purity if only one colony is visible.
• D. Re-streak to confirm purity.

Answer: (D)

The ability to confirm a pure culture comes from isolating colonies that originate from a single progenitor cell. After streak plating, you verify purity by restreaking onto a fresh plate and then checking that the resulting colonies are uniform and derived from that same single cell line. If multiple colony types appear, it indicates contamination. Gram staining the first colony only tells you cell structure and isn’t a test of purity, and sequencing the genome is unnecessary for routine purity checks. Relying on a single visible colony to claim purity isn’t safe, because hidden contaminants may still be present. Re-streaking to obtain isolated, uniform colonies provides the practical confirmation that the culture is pure.