Mannitol salt agar is selective for Staphylococcus aureus because this organism...

Mannitol salt agar is designed specifically to isolate and differentiate Staphylococcus species, particularly Staphylococcus aureus, due to its unique formulation. The medium has a high concentration of salt, which selects for organisms that can tolerate this osmotic pressure. Staphylococcus aureus is capable of surviving and growing in this high-salt environment, whereas many other bacteria cannot, making the high salt concentration a crucial selective component.

While Staphylococcus aureus does ferment mannitol, which can lead to a color change in the medium, the primary reason for selectivity is its ability to tolerate and thrive in the presence of elevated salt levels. Other features, such as pigmentation or hydrogen sulfide production, are not key to the selectivity in mannitol salt agar. Thus, the high salt concentration serves as the primary criterion for isolating Staphylococcus aureus from other less tolerant organisms.