Mannitol Salt Agar

Mannitol Salt Agar or MSA is a commonly used growth medium in microbiology. It contains a high concentration (~7.5%-10%) of salt NaCl, making it selective for members of the genera Micrococcaceae and Staphylococcus since this level of NaCl is inhibitory to most other bacteria. It is also a differential medium, containing mannitol and the indicator phenol red. Acid production as a result of mannitol fermentation, a feature of several clinically significant species such as Staphylococcus aureus, will result in the agar's normal red color changing to yellow. Mannitol fermenters produce a yellow colony while non-mannitol fermenters will produce a redish/purple colony. used for the selective isolation of presumptive pathogen (pp) Staphylococcus.