Here's why:
* High Salt Concentration: MSA contains a high concentration of sodium chloride (7.5%). This high salt concentration inhibits the growth of most bacteria, making the medium selective for halophiles, which are bacteria that can tolerate high salt concentrations.
* Mannitol: Mannitol is a sugar that is fermented by certain bacteria, such as *Staphylococcus aureus*, producing acid. This acidification of the medium is detected by a pH indicator (phenol red), turning the medium yellow. However, mannitol is not the component that makes the medium selective.
In summary:
* Selectivity: High salt concentration (NaCl)
* Differential: Mannitol fermentation, indicated by a color change
Therefore, MSA is selective for halophiles and differential for mannitol fermentation.
