Here's how it works:
* Mannitol: This sugar is a key ingredient in MSA. Bacteria that can ferment mannitol will produce acids as byproducts.
* Phenol red: This pH indicator is also included in MSA. It turns yellow in the presence of acids, indicating mannitol fermentation.
* High salt concentration (7.5% NaCl): This makes MSA selective, inhibiting the growth of most bacteria except staphylococci, which are salt-tolerant.
Therefore, MSA helps distinguish between:
* Mannitol-fermenting staphylococci: These bacteria will grow on the MSA and turn the medium yellow due to acid production. An example is *Staphylococcus aureus*.
* Non-mannitol-fermenting staphylococci: These bacteria will also grow on the MSA, but the medium will remain its original red color. An example is *Staphylococcus epidermidis*.
In summary: MSA is a differential medium because it allows you to differentiate between staphylococcal species based on their ability to ferment mannitol, resulting in a visible color change in the medium.
