What indicates a positive test result in Milk Agar?

A positive test result in Milk Agar is indicated by a clear zone around bacterial growth. This occurs due to the action of proteolytic enzymes, such as caseinase, which are produced by certain bacteria. These enzymes break down casein, the protein found in milk, leading to a clear zone around the colonies where the protein has been digested. In contrast, other answers do not accurately reflect the characteristic changes in the medium that indicate a positive reaction.

For instance, a yellow coloration typically signifies the presence of acid produced during fermentation, which is more common in carbohydrate fermentation tests rather than in Milk Agar. The purple color of the medium indicates an alkaline condition, which is unrelated to casein breakdown. Lastly, while a thick layer of curd might suggest lactose fermentation or coagulation in some tests, it does not specifically signify the proteolytic activity assessed in Milk Agar. Therefore, the clear zone around bacterial growth is the definitive indicator of a positive result in this test.