If you observe colonies with different colors and edges on the same plate, what does this most likely indicate?

• A. A single species with variable expression.
• B. A mixed culture containing more than one species.
• C. Overgrowth by a single large colony.
• D. No growth on the plate.

Answer: (B)

Seeing colonies that differ in color and edge on the same plate usually means there are multiple organisms present. Different species often produce distinct pigments and colony shapes, so each colony type reflects a different organism. In a pure culture of one species, colonies tend to look similar across the plate because they share the same genetic traits and expression under those conditions. An overgrown single large colony would dominate and wouldn’t create multiple distinctly colored colonies. No growth would leave nothing to compare. So the variation in color and edge is best explained by a mixed culture containing more than one species.