An MSA plate with Micrococcus sp. (1), Staphylococcus epidermis (2) and S. aureus (3) colonies.

Mannitol salt agar or MSA is a commonly used growth medium in microbiology.

It contains a high concentration (~7.5%-10%) of salt (NaCl), making it selective for Staphylococci (and Micrococcaceae) since this level of NaCl is inhibitory to most other bacteria^[1]. It is also a differential medium, containing mannitol and the indicator phenol red. Coagulase-positive Staphylococci produce yellow colonies with yellow zones, whereas coagulase-negative Staphylococci produce small pink or red colonies with no color change to the medium.^[2]

It is used for the selective isolation of presumptive pathogen (pp) Staphylococci.

Typical composition

MSA typically contains^[3]:

• 5.0 g/L enzymatic digest of casein
• 5.0 g/L enzymatic digest of animal tissue
• 1.0 g/L beef extract
• 10.0 g/L D-mannitol
• 75.0 g/L sodium chloride
• 0.025 g/L phenol red
• 15.0 g/L agar
• pH 7.4 ± 0.2 at 25°C

References

External links

• Mannitol Salt Agar Description & Formulation

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