During a sterility check, what would indicate contamination?

• A. No growth in either broth or plate.
• B. Turbidity in broth but no colonies on plate.
• C. Growth only in the plate.
• D. Turbidity in broth and colonies on the plate.

Answer: (D)

Contamination in sterility testing is shown when you observe growth in both media. Turbidity in the broth means microorganisms are growing in the liquid, while colonies on the agar plate show organisms forming discrete colonies on solid medium. If you see both signs together, it confirms that a contaminant is present in the sample and is able to grow under both conditions. Turbidity alone could occur if the organism grows in liquid but doesn’t form colonies under the plating conditions, and plate growth alone could happen due to surface contamination or plating artifacts, but growth in both media provides the clearest, strongest indication of contamination.