MacConkey's Agar

It is selective for gram-negative bacteria.
Grow of gram positive bacteria is inhibited by the crystal violet dye and bile salts in the media.

It is differential for lactose fermentation. It contains neutral red pH indicator.

When the bacterial waste products generated, organic acid metabolites, change the pH indicator in MSA from red to yellow. Pathogenic staph, such as Staphylococcus aureus, are mannitol fermenters, and when growing on Mannitol Salt Agar, their wastes turn the MSA a bright yellow color.

In contrast, nonpathogenic staph such as Staphylococcus epidermidis (aka Staph epi), the normal flora that grows on human skin, does not ferment mannitol. When Staph epi grows on Mannitol Salt, the naturally orange-pink color of the agar doesn’t change, since S. epidermidis doesn’t ferment mannitol or produce the resulting organic acid wastes.

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