Mannitol Salt Agar (MSA) is a selective and differential culture medium used primarily for the isolation of Staphylococcus species. It contains a high concentration of salt, mannitol, and a pH indicator to differentiate between mannitol fermenters and non-fermenters.
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Mannitol Salt Agar is selective due to its high salt concentration, which inhibits the growth of most bacteria except for halophiles like Staphylococcus species.
The differential aspect of MSA comes from mannitol and phenol red; organisms that ferment mannitol produce acid, turning the medium yellow.
Staphylococcus aureus ferments mannitol, whereas Staphylococcus epidermidis does not, allowing for differentiation on MSA plates.
The phenol red in MSA serves as a pH indicator: it turns yellow in acidic conditions and remains red in neutral to basic conditions.
MSA is frequently used in clinical settings to identify potential pathogenic Staphylococcus strains from skin and eye infections.
Review Questions
What component of Mannitol Salt Agar makes it selective?
How does Mannitol Salt Agar differentiate between different types of Staphylococcus species?
What color change would indicate mannitol fermentation on an MSA plate?
Related terms
Selective Medium: A type of culture medium designed to suppress the growth of unwanted microorganisms while encouraging the growth of desired ones.
Differential Medium: A culture medium with specific ingredients that allow one to distinguish between different types or species based on their biological characteristics.
Phenol Red: A pH indicator commonly used in microbiological media that changes color based on acidity or alkalinity.