Mannitol salt agar

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

Mannitol salt agar or MSA is a commonly used growth medium in microbiology. It encourages the growth of a group of certain bacteria while inhibiting the growth of others. This medium is important in medical laboratories by distinguishing pathogenic microbes in a short period of time. It contains a high concentration (~7.5%-10%) of salt (NaCl), making it selective for gram positive bacterium Staphylococci (and Micrococcaceae) since this level of NaCl is inhibitory to most other bacteria. It is also a differential medium for mannitol fermentors, 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. If an organism can ferment mannitol, an acidic byproduct is formed that will cause the phenol red in the agar to turn yellow. It is used for the selective isolation of presumptive pathogen (pp) Staphylococci.

Expected results

• Gram + staphylococci fermenting mannitol: Media turns yellow
• Gram + staphylococci not fermenting mannitol: Media does not change color
• Gram + streptococci: inhibited growth
• Gram - : inhibited growth

Typical composition

MSA typically contains:

• 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

External links

• Mannitol Salt Agar Description & Formulation