Name two common signs of contamination in a culture plate.

• A. Mixed colony morphologies and growth in sterile controls or unexpected growth in areas where none should be
• B. Uniform colony morphology and no growth in controls
• C. No growth anywhere
• D. Growth only in the inoculated area

Answer: (A)

Contamination shows up on a culture plate when there are signs of microbes beyond what you intended to grow. The best indicators are: colonies that look different from each other on the same plate, indicating multiple species (mixed colony morphologies), and growth appearing in sterile controls or in areas where no growth should occur, showing that unwanted organisms are present despite sterile conditions. In a clean plate with a single inoculum, you’d expect uniform colony appearance and no growth in control areas. Growth only in the inoculated area is expected and does not point to contamination.