Mannitol salt agar or MSA is a commonly used selective and differential

growth medium A growth medium or culture medium is a solid, liquid, or semi-solid designed to support the growth of a population of microorganisms or cells via the process of cell proliferation or small plants like the moss ''Physcomitrella patens''. Differen ...

microbiology Microbiology () is the scientific study of microorganisms, those being unicellular (single cell), multicellular (cell colony), or acellular (lacking cells). Microbiology encompasses numerous sub-disciplines including virology, bacteriology, prot ...

. 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 quantitie ...

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

Staphylococcus ''Staphylococcus'' is a genus of Gram-positive bacteria in the family Staphylococcaceae from the order Bacillales. Under the microscope, they appear spherical (cocci), and form in grape-like clusters. ''Staphylococcus'' species are facultative ...

'', ''

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 charact ...

'' 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 al ...

) 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 lower ...

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 ''Staphylococcus aureus'' is a Gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often positive ...

'' produces yellow colonies with yellow zones, whereas other

coagulase-negative staphylococci ''Staphylococcus'' is a genus of Gram-positive bacteria in the family Staphylococcaceae from the order Bacillales. Under the microscope, they appear spherical (cocci), and form in grape-like clusters. ''Staphylococcus'' species are facultative ...

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) ''

'' 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 (CSN1S1, αS1, aS2, CSN2, β, K-casein, κ) that are commonly found in mammalian milk, comprising about 80% of the proteins in cow's milk and between 20% and 60% of ...

* 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 Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...

* 0.025 g/L

* 15.0 g/L

agar Agar ( or ), or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from ogonori (''Gracilaria'') and "tengusa" (''Gelidiaceae''). As found in nature, agar is ...

* pH 7.4 ± 0.2 at 25 °C

References

Biochemistry detection reactions Microbiological media {{Microbiology-stub