Microbiology Practice Exam 2025 - Free Microbiology Practice Questions and Study Guide

Mannitol salt agar is specifically designed to select for and isolate Gram-positive bacteria, particularly those in the Staphylococcus genus, such as Staphylococcus aureus. The high salt concentration in the medium (7.5% NaCl) inhibits the growth of most Gram-negative bacteria and many other types of bacteria that cannot tolerate such an osmotic pressure. This selective feature makes Mannitol salt agar particularly useful for isolating and differentiating Staphylococcus species from other bacteria.

In addition to being selective, Mannitol salt agar contains mannitol, a carbohydrate that some Staphylococcus species can ferment. When mannitol is fermented, acidic by-products are produced, which cause a color change in the pH indicator (phenol red) present in the agar, indicating the presence of mannitol-fermenting bacteria. This dual function of being both selective and differential makes it a valuable tool in microbiology for identifying specific bacteria in clinical or laboratory settings.