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. [1] It contains a high concentration (about 7.5–10%) of salt (NaCl) which is inhibitory to most bacteria - making MSA selective against most Gram-negative and selective for some Gram-positive bacteria ( Staphylococcus, Enterococcus and Micrococcaceae) that tolerate high salt concentrations. [2] It is also a differential medium for mannitol-fermenting staphylococci, containing the sugar alcohol mannitol and the indicator phenol red, a pH indicator for detecting acid produced by mannitol-fermenting staphylococci. [3] 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. [4] If an organism can ferment mannitol, an acidic byproduct is formed that causes the phenol red in the agar to turn yellow. [1] It is used for the selective isolation of presumptive pathogenic (pp) Staphylococcus species. [5]
MSA typically contains: [6]
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. [1] It contains a high concentration (about 7.5–10%) of salt (NaCl) which is inhibitory to most bacteria - making MSA selective against most Gram-negative and selective for some Gram-positive bacteria ( Staphylococcus, Enterococcus and Micrococcaceae) that tolerate high salt concentrations. [2] It is also a differential medium for mannitol-fermenting staphylococci, containing the sugar alcohol mannitol and the indicator phenol red, a pH indicator for detecting acid produced by mannitol-fermenting staphylococci. [3] 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. [4] If an organism can ferment mannitol, an acidic byproduct is formed that causes the phenol red in the agar to turn yellow. [1] It is used for the selective isolation of presumptive pathogenic (pp) Staphylococcus species. [5]
MSA typically contains: [6]