What is the function of phenol red in MSA?
pH indicator
Phenol red is a pH indicator used in Mannitol Salt Agar (MSA). MSA is a selective and differential medium primarily used to isolate and differentiate pathogenic Staphylococcus species from non-pathogenic ones. MSA contains high concentrations of salt (7.5% NaCl), which serves as a selective agent to inhibit the growth of many organisms.
The function of phenol red in MSA is to differentiate between mannitol fermenters and non-fermenters. Mannitol is a sugar alcohol that is included in the medium as a carbohydrate source. Staphylococcus aureus, a pathogenic Staphylococcus species, ferments mannitol and produces acid, which is detected by phenol red. The pH indicator turns the color of the medium from reddish pink to yellow when the pH drops below 6.8. However, Staphylococcus epidermidis, a non-pathogenic species, cannot ferment mannitol, and thus, does not produce acid, and the medium remains the same color. Therefore, phenol red is a crucial component of MSA as it allows microbiologists to differentiate between pathogenic and non-pathogenic Staphylococcus species based on their ability to ferment mannitol.
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