Final answer:
The selective agents present in the agar inhibit the growth of Gram-positive bacteria in these agar plates.
Step-by-step explanation:
The ingredient in these agar plates that inhibit the growth of Gram-positive bacteria are the selective agents present in the agar.
1. Nutrient Agar does not contain specific ingredients that inhibit the growth of Gram-positive bacteria.
2. MacConkey Agar contains bile salts and crystal violet which inhibit the growth of most Gram-positive, non-enteric bacteria.
3. EMB Agar contains eosin and methylene blue dyes that inhibit the growth of Gram-positive bacteria.
4. XLD Agar contains selective agents like bile salts and phenol red that inhibit the growth of Gram-positive bacteria.
Ingredients such as bile salts and crystal violet in media like MacConkey Agar inhibit the growth of gram-positive bacteria, favoring gram-negative bacteria. These components of selective media create an environment that is hostile to gram-positive bacteria due to the disruption of their cell wall structure. On the other hand, EMB Agar, which contains indicator dyes eosin and methylene blue, also suppresses gram-positive bacteria and differentiates between lactose fermenters and non-lactose fermenters by the color of the colonies formed. EMB agar is an example of a selective medium in microbiology.