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.[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 carbohydrate 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]

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

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[1]

Typical composition

MSA typically contains:[6]

References
  1. Bachoon, Dave S.; Dustman, Wendy A. (2008). "Exercise 8: Selective and Differential Media for Isolation". In Michael Stranz (ed.). Microbiology Laboratory Manual. Mason, OH: Cengage Learning.
  2. "Mannitol salt agar" (PDF). Becton, Dickinson and Company. 2005.
  3. Anderson, Cindy (2013). Great Adventures in the Microbiology Laboratory. Pearson. pp. 175–176. ISBN 978-1-269-39068-2.
  4. "Mannitol salt agar (7143)" (PDF). Neogen Corp. 2008.
  5. Identification of Staphylococcus aureus: DNase and Mannitol salt agar improve the efficiency of the tube coagulase test
  6. The United States Pharmacopeia (23rd ed.). Rockville, MD: The United States Pharmacopeial Convention. 1995.
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