Final answer:
The selective agent for Mannitol Salt Agar, which selects for halotolerant bacteria such as staphylococci, is sodium chloride. It inhibits the growth of bacteria that cannot tolerate high salt concentrations. Mannitol and phenol red in the agar aid in identifying bacteria capable of fermenting mannitol by inducing a color change.
Step-by-step explanation:
The selective agent for Mannitol Salt Agar (MSA) is sodium chloride. MSA is a selective medium that is designed to support the growth of halotolerant bacteria such as staphylococci, while inhibiting the growth of bacteria that are less tolerant to high salt concentrations. The high concentration of salt (7.5%) in the agar applies osmotic stress to bacteria, effectively preventing the growth of those that cannot withstand such conditions. Additionally, MSA contains mannitol, which some bacteria can ferment, and phenol red, which serves as an indicator by changing color when acid is produced from mannitol fermentation.
To use this medium for testing, bacteria are streaked onto the surface of an MSA plate and incubated. If a bacterium can ferment mannitol, the phenol red in the medium will change from red to yellow around the growth, indicating a positive result for mannitol fermentation as can be seen in Figure 6 with cultures both positive and negative for this process.