Selective media are designed to support the growth of specific microorganisms while inhibiting the growth of others. This is usually achieved by adding antibiotics, dyes, or specific salts to the agar.
Two common examples used in microbiology and clinical diagnostics are:
### 1. MacConkey Agar
This medium is selective for **Gram-negative bacteria**. It contains bile salts and crystal violet, which inhibit the growth of most Gram-positive organisms. It also doubles as a differential medium because it contains lactose and a pH indicator (neutral red), allowing researchers to distinguish between lactose fermenters (which turn pink) and non-fermenters.
### 2. Mannitol Salt Agar (MSA)
MSA is selective for bacteria that can withstand high salt concentrations, specifically members of the **Staphylococcus** genus. It contains a 7.5% concentration of sodium chloride ($NaCl$), which is inhibitory to most other bacteria. Like MacConkey agar, it is also differential; it uses phenol red to identify organisms that can ferment mannitol, turning the medium from red to yellow.
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