Final answer:
EMB agar uses eosin and methylene blue, while MacConkey agar contains bile salts and crystal violet to inhibit gram-positive bacteria, making both mediums selective for gram-negative bacteria.
Step-by-step explanation:
The ingredients in EMB agar that inhibit the growth of gram-positive bacteria are eosin and methylene blue, which act as indicator dyes. These dyes are selective agents that not only inhibit gram-positive bacteria but also allow differentiation between lactose fermenters and non-lactose fermenters based on colony coloration.
MacConkey agar contains bile salts and crystal violet as its selective agents. These substances are particularly effective at inhibiting the growth of most gram-positive, non-enteric bacteria, thereby favoring the growth of gram-negative bacteria from the Enterobacteriaceae family. This makes MacConkey agar a selective medium suited for identifying and isolating enteric microorganisms.
Uses of Selective Media
Selective media like EMB agar and MacConkey agar are used to isolate and identify pathogenic bacteria by promoting the growth of specific bacteria while suppressing others. This is especially useful in medical microbiology to identify causative agents of infections.