Answer: Mannitol Salt Agar, and MacConkey Agar
Step-by-step explanation:
A solid media that is only selective means that it only allows certain types of microorganisms to grow while inhibiting the growth of others. Here are two examples of solid media that are only selective:
1. MacConkey Agar: MacConkey agar is a selective medium used to isolate and differentiate gram-negative bacteria, particularly those belonging to the family Enterobacteriaceae. It contains bile salts and crystal violet, which inhibit the growth of gram-positive bacteria, while allowing the growth of gram-negative bacteria. In addition, it also contains lactose and pH indicator called neutral red. If a bacterium can ferment lactose, it produces acid, which causes the pH indicator to turn pink or red. This allows for the differentiation of lactose-fermenting bacteria (pink/red colonies) from non-lactose fermenting bacteria (colorless colonies).
2. Mannitol Salt Agar: Mannitol salt agar is a selective medium used to isolate and differentiate staphylococci, particularly Staphylococcus aureus. It contains high concentrations of salt, which inhibits the growth of most bacteria except for staphylococci. In addition, it also contains the sugar mannitol and a pH indicator called phenol red. Staphylococcus aureus can ferment mannitol, producing acid that causes the pH indicator to turn yellow. This allows for the differentiation of Staphylococcus aureus (yellow colonies) from other staphylococci species (red colonies).
These examples illustrate how selective media can be designed to favor the growth of specific microorganisms while suppressing the growth of others. By incorporating specific ingredients and indicators, these media provide a way to selectively isolate and identify particular bacterial species based on their ability to ferment specific substrates or tolerate specific inhibitory factors.