Written by the Fiveable Content Team • Last updated August 2025
Written by the Fiveable Content Team • Last updated August 2025
Definition
Mannitol Salt Agar (MSA) is a selective and differential growth medium used to isolate and identify Staphylococcus species. It contains high salt concentrations and mannitol to differentiate bacteria based on their ability to ferment mannitol.
MSA selectively promotes the growth of halophilic (salt-loving) bacteria, particularly Staphylococcus species.
The high concentration of NaCl (7.5%) in MSA inhibits the growth of most other bacteria.
Mannitol fermentation by bacteria results in acid production, leading to a color change in the medium due to the pH indicator phenol red turning yellow.
Staphylococcus aureus typically ferments mannitol, while Staphylococcus epidermidis does not, allowing differentiation between these species on MSA.
Non-Staphylococcal bacteria generally do not grow on MSA due to its high salt content.
Growth media that contains specific ingredients to allow distinction between different types of microorganisms based on their biological characteristics.