On bile-esculin agar, what indicates a positive result for enterococci?

On bile-esculin agar, what indicates a positive result for enterococci?

• A. Clear zone around growth
• B. No growth
• C. Growth with blackening of the agar due to esculetin-iron reaction
• D. Color change to yellow

Answer: (C)

On bile-esculin agar, the test looks for the organism’s ability to hydrolyze esculin in the presence of bile. When esculin is hydrolyzed, it’s split into esculetin, which then reacts with ferric ions in the medium to form a dark brown to black compound around the colonies. This darkening, seen as a black halo around growth, is the positive indicator for enterococci. If there’s growth without any dark zone, or no growth at all, it’s not positive. A yellow color change isn’t associated with a positive esculin hydrolysis on this medium.

On bile-esculin agar, the test looks for the organism’s ability to hydrolyze esculin in the presence of bile. When esculin is hydrolyzed, it’s split into esculetin, which then reacts with ferric ions in the medium to form a dark brown to black compound around the colonies. This darkening, seen as a black halo around growth, is the positive indicator for enterococci. If there’s growth without any dark zone, or no growth at all, it’s not positive. A yellow color change isn’t associated with a positive esculin hydrolysis on this medium.