Why is Mannitol salt agar selective for Staphylococcus aureus?

Mannitol salt agar is selective for Staphylococcus aureus primarily due to its high salt concentration, which can tolerate and support the growth of certain bacteria, specifically those that are adapted to such environments. The agar contains approximately 7.5% sodium chloride, which is sufficient to inhibit the growth of most other bacteria while allowing Staphylococcus species, particularly Staphylococcus aureus, to thrive.

This characteristic is crucial because it distinguishes Staphylococcus aureus from other bacteria that cannot survive in such a saline environment. Additionally, the medium contains mannitol, which Staphylococcus aureus can ferment, leading to a color change in the agar due to the pH indicators present. Thus, the high salt concentration is key to its selectivity, enabling the isolation and identification of Staphylococcus aureus in clinical and laboratory settings. Understanding this selectivity is important for laboratory diagnostics and microbiological studies.