Mannitol salt agar
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. This medium is important in medical laboratories as one method of distinguishing pathogenic microbes in a short period of time.
It contains a high concentration of salt, making it selective for most Gram-negative and some Gram-positive bacteria since this level of salt is inhibitory to most other bacteria. It is also a differential medium for mannitol-fermenting staphylococci, containing carbohydrate mannitol and the indicator phenol red, 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 Staphylococcus species.- Gram + Staphylococcus: fermenting mannitol: medium turns yellow
- Gram + Staphylococcus: not fermenting mannitol, medium does not change color
- Gram + Streptococcus: inhibited growth
- Gram -: inhibited growth
MSA typically contains:
