Mannitolsaltagar

Mannitol Salt Agar (MSA) is a selective and differential culture medium used in microbiology laboratories. Its primary purpose is to isolate and differentiate staphylococci, particularly *Staphylococcus aureus*, from other bacteria. MSA contains several key components that contribute to its functionality. High salt concentration, typically 7.5% sodium chloride, makes the medium inhibitory to most bacteria, allowing salt-tolerant organisms like staphylococci to grow while inhibiting the growth of many Gram-negative and other Gram-positive bacteria.

The differential aspect of MSA comes from the presence of mannitol and phenol red indicator. Mannitol is a sugar alcohol that can be fermented by certain bacteria. *Staphylococcus aureus* is one of the few staphylococcal species that ferments mannitol. When mannitol is fermented, it produces acidic byproducts. The phenol red indicator in the medium changes color in response to pH changes. In the presence of acid, the phenol red turns yellow, indicating a positive mannitol fermentation. Bacteria that do not ferment mannitol will grow on the medium, but the surrounding medium will remain pink or red, the original color of the phenol red at a neutral pH. Therefore, colonies that grow on MSA and cause the surrounding agar to turn yellow are strongly suggestive of *Staphylococcus aureus*. Other staphylococci, such as *Staphylococcus epidermidis*, will grow on MSA but will not ferment mannitol, leaving the agar pink.