Mannitol salt agar or MSA is a commonly used selective and differential growth medium in microbiology. It encourages the growth of a group of certain bacteria while inhibiting the growth of others. It contains a high concentration (about 7.5–10%) of

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...

(NaCl) which is inhibitory to most bacteria - making MSA selective against most Gram-negative and selective for some Gram-positive bacteria ('' Staphylococcus'', ''

Enterococcus ''Enterococcus'' is a large genus of lactic acid bacteria of the phylum Bacillota. Enterococci are gram-positive cocci that often occur in pairs (diplococci) or short chains, and are difficult to distinguish from streptococci on physical char ...

'' and

Micrococcaceae The family ''Micrococcaceae'' includes bacterial genera of Gram positive cocci that inhabit the air and skin, such as '' Micrococcus luteus''. Genera The family ''Micrococcaceae'' comprises the following genera: * '' Acaricomes'' Pukall ''et a ...

) that tolerate high salt concentrations. It is also a differential medium for mannitol-fermenting staphylococci, containing carbohydrate

mannitol Mannitol is a type of sugar alcohol used as a sweetener and medication. It is used as a low calorie sweetener as it is poorly absorbed by the intestines. As a medication, it is used to decrease pressure in the eyes, as in glaucoma, and to lo ...

and the indicator

phenol red Phenol red (also known as phenolsulfonphthalein or PSP) is a pH indicator frequently used in cell biology laboratories. Chemical structure and properties Phenol red exists as a red crystal that is stable in air. Its solubility is 0.77 grams per ...

, a pH indicator for detecting acid produced by mannitol-fermenting staphylococci. '' Staphylococcus aureus'' produces yellow colonies with yellow zones, whereas other coagulase-negative staphylococci produce small pink or red colonies with no colour change to the medium. If an organism can ferment mannitol, an acidic byproduct is formed that causes the phenol red in the agar to turn yellow. It is used for the selective isolation of presumptive pathogenic (pp) '' Staphylococcus'' species.

Expected results

* Gram + ''Staphylococcus'': fermenting mannitol: medium turns yellow (e.g. ''S. aureus'') * Gram + ''Staphylococcus'': not fermenting mannitol, medium does not change color (e.g. ''S. epidermidis'') * Gram + ''Streptococcus'': inhibited growth * Gram -: inhibited growth

Typical composition

MSA typically contains: * 5.0 g/L enzymatic digest of

casein Casein ( , from Latin ''caseus'' "cheese") is a family of related phosphoproteins ( αS1, aS2, β, κ) that are commonly found in mammalian milk, comprising about 80% of the proteins in cow's milk and between 20% and 60% of the proteins in hum ...

* 5.0 g/L enzymatic digest of animal tissue * 1.0 g/L beef extract * 10.0 g/L

D-mannitol Mannitol is a type of sugar alcohol used as a sweetener and medication. It is used as a low calorie sweetener as it is poorly absorbed by the intestines. As a medication, it is used to decrease pressure in the eyes, as in glaucoma, and to lower ...

* 75.0 g/L sodium chloride * 0.025 g/L

* 15.0 g/L agar * pH 7.4 ± 0.2 at 25 °C

References

Biochemistry detection reactions Microbiological media {{Microbiology-stub