What is mannitol salt agar?

Mannitol salt agar is a gelatinous growth medium that is used in microbiological laboratory settings. MSA is typically comprised of animal proteins, mannitol sugar, agar, table salt and phenol red.

The high salt content of MSA promotes the growth of saline-tolerant bacteria such as Staphylococcus aureus. It also weakens the growth of other bacteria. Additionally, Staphylococcus aureus ferments mannitol, creating a by-product that turns the phenol red component of MSA to yellow. Because Staphylococcus aureus can cause a variety of harmful infections to humans, growing it on MSA, where it can be clearly identified, allows microbiologists to experiment and create ways to control the bacteria.