Final answer:
In positive esculin hydrolysis, a black precipitate forms when esculetin from hydrolyzed esculin reacts with ferric ions in Bile Esculin Agar, indicating enterococci presence.
Step-by-step explanation:
The type of precipitate that forms as a result of positive esculin hydrolysis is black. This is seen in microbial testing, specifically with Bile Esculin Agar, which is a medium that is used to isolate and identify enterococci. In this agar, esculin is hydrolyzed by certain enterococci to produce glucose and esculetin. This esculetin reacts with ferric ions present in the medium to form a dark brown to black complex, which indicates a positive esculin hydrolysis test result and, therefore, signifies the presence of enterococci.