Which observation indicates a positive result in a casein hydrolysis test?

• A. Bacterial colonies turn black on the agar
• B. The medium turns pink around the colonies
• C. A clear zone appears around bacterial growth on milk agar
• D. No change is observed in the appearance of the medium

Answer: (C)

Verified step by step guidance

1. Understand the purpose of the casein hydrolysis test: it is used to detect the ability of bacteria to produce the enzyme casease, which breaks down casein, the protein that gives milk its white color.
2. Recognize that casein is present in milk agar, and when casease is produced by bacteria, it hydrolyzes casein, leading to a visible change in the medium.
3. Identify that a positive result is indicated by the breakdown of casein, which causes the normally opaque milk agar to become clear around the bacterial colonies.
4. Note that other observations such as black colonies or pink medium are not related to casein hydrolysis but may be indicative of other biochemical reactions or tests.
5. Conclude that the presence of a clear zone around bacterial growth on milk agar is the hallmark of a positive casein hydrolysis test.