Mannitol salt agar

Mannitol Salt Agar (MSA) is a selective and differential growth medium used in microbiology for the isolation of Staphylococcus species. It combines a high concentration of salt (approximately 7.5-10%), which inhibits the growth of most bacteria, with mannitol, a sugar alcohol, and the pH indicator phenol red. This medium is particularly useful in identifying pathogenic Staphylococcus species, notably Staphylococcus aureus, which can ferment mannitol, from other non-pathogenic species that cannot.

Composition[edit | edit source]

Mannitol Salt Agar consists of:

Peptone: Provides necessary nutrients for bacterial growth.
Mannitol: Serves as a fermentable carbohydrate source.
7.5% NaCl (sodium chloride): Inhibits the growth of most bacteria other than halotolerant species.
Phenol red: A pH indicator that changes color in response to acid production from mannitol fermentation.
Agar: A solidifying agent.

Mechanism[edit | edit source]

The high salt concentration in MSA selects for members of the genus Staphylococcus, as these bacteria are halotolerant. When a Staphylococcus species is capable of fermenting mannitol, the acid produced lowers the pH of the medium, and the phenol red indicator shifts from red to yellow, signaling mannitol fermentation. This change is particularly indicative of Staphylococcus aureus, which is known for its ability to ferment mannitol, whereas other Staphylococcus species typically do not ferment mannitol and thus do not change the color of the medium.

Applications[edit | edit source]

Mannitol Salt Agar is widely used in clinical laboratories to:

Isolate and identify Staphylococcus aureus from clinical specimens.
Differentiate Staphylococcus aureus from other coagulase-negative Staphylococcus species based on mannitol fermentation.
Screen for Staphylococcus species in food, water, and environmental samples.

Limitations[edit | edit source]

While MSA is effective for isolating Staphylococcus species, it is not definitive. Some strains of Staphylococcus aureus may not ferment mannitol, and rare non-Staphylococcus species that are halotolerant and can ferment mannitol may grow on MSA and potentially be misidentified. Further biochemical or molecular testing is necessary for accurate identification.