MSA (Mannitol Salt Agar)

Mannitol salt agarPicture.sources

MSA is selective for gram-positive Staphylococci bacteria.

It contains: 7% salt in the medium which inhibits the growth of most gram-positive and gram-negative bacteria.

MSA is also differential for it’s mannitol fermentation

It contains: phenol red pH indicator

The indicator will turns yellow if there are presence of acid by products of mannitol fermentation.

The bacterial waste products generated, organic acid metabolites change the pH indicator in MSA from red to yellow. Pathogenic staph, such as Staphylococcus aureus, is 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.

                                                                        Figure 1

A Mannitol Salt plate(figure 1) with two separate samples on same plate. Yellow differential reaction showing pathogenic, mannitol-fermenting Staph present. Opposite side of same plate remains pink, with nonpthogenic Staph growing on it.