MSA contains 7.5% sodium chloride, making it a selective medium for halophilic (salt-loving) bacteria. It also contains mannitol, a sugar alcohol, and phenol red, a pH indicator. When organisms ferment mannitol, they produce acidic by-products, leading to a color change in the medium from red to yellow. This property makes MSA a differential medium, allowing for the distinction between mannitol-fermenting and non-fermenting organisms.
