Mannitol Salt Agar (MSA)
•Important Ingredients: Phenol red, mannitol (sugar), 7.5% NaCl
•Differential: between pathogenic S. aureus and S. epidermidis
•Selective: NaCl inhibits non-halophilic organisms. Organism that can grow are halophilic.
•Mannitol Fermentors: fermentation of acid causes a drop in pH of the media and the indicator (phenol red) turns yellow.
A). Staphlococcus aureus: large yellow halo around growth indicates fermentation of mannitol.
B). Staphlococcus epidermidis: Growth but not color change to the media indicating no fermentation of mannitol.
C). Staphlococcus saprophyticus: small yellow halo around growth indicates fermentation of mannitol. (10% of S. saprophyticus ferment mannitol)
D). E. coli: no growth. Inhibited by the 7.5% NaCl.