Final answer:
Hektoen Enteric Agar is both selective, as it inhibits the growth of Gram-positive bacteria, and differential because it differentiates bacteria based on their ability to ferment carbohydrates and produce hydrogen sulfide.
Step-by-step explanation:
The Hektoen Enteric Agar (HE) is both selective and differential because it is designed to selectively allow the growth of certain bacteria while inhibiting others, and it differentiates between bacteria that are capable of specific biochemical reactions. The media is selective due to the presence of bile salts and dyes such as bromothymol blue and acid fuchsin, which inhibit the growth of Gram-positive organisms, allowing mainly Gram-negative enteric bacilli to grow. It is differential because it contains lactose, sucrose, and salicin; bacteria that can ferment these carbohydrates produce acid, leading to a change in the color of the colonies. Particular compounds like iron salts are incorporated so that hydrogen sulfide-producing organisms will produce colonies with black centers. These features are important in isolating and identifying pathogens from gastrointestinal specimens.