Final answer:
EMB agar plates are selective because they inhibit certain bacteria and differential because they distinguish bacteria based on lactose fermentation, evidenced by color changes in the colonies.
Step-by-step explanation:
Eosin methylene blue (EMB) agar plates can be classified as both selective and differential. They are considered selective because they contain ingredients like eosin and methylene blue that inhibit the growth of Gram-positive bacteria, thereby selecting only for Gram-negative bacteria. They are differential due to the ability of the media to distinguish between lactose fermenters and non-lactose fermenters based on colony color. Lactose fermenters produce acid that reacts with the dyes, forming metallic green or deep purple colonies, while non-lactose fermenters produce colorless colonies.
The dyes used in EMB agar serve as both inhibitors and indicators, depending on their interaction with the bacteria. This is crucial for microbiologists when they aim to isolate and identify specific types of bacteria, as it simplifies the identification process of pathogenic bacteria in clinical, food, and environmental samples.
Eosin methylene blue agar (EMB) is a selective and differential medium used to isolate fecal coliforms. Eosin Y and methylene blue are pH indicator dyes which combine to form a dark purple precipitate at low pH; they also serve to inhibit the growth of most Gram positive organisms.