Mannitol salt agar

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

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 by distinguishing pathogenic microbes in a short period of time.[1] It contains a high concentration (~7.5%-10%) of salt (NaCl), making it selective for gram positive bacterium Staphylococci (and Micrococcaceae) since this level of NaCl is inhibitory to most other bacteria.[2] It is also a differential medium for mannitol-fermenting staphylococci, containing carbohydrate mannitol and the indicators phenol red and a pH indicator for detecting acid produced by mannitol-fermenting staphylococci. [3] Staphylococcus aureus produce yellow colonies with yellow zones, whereas other 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 will cause the phenol red in the agar to turn yellow.[1] It is used for the selective isolation of presumptive pathogen (pp) Staphylococci.

Expected results[edit]

  • Gram + staphylococcus : fermenting mannitol: Media turns yellow (ex. S. aureus)
  • Gram + staphylococci : not fermenting mannitol. Media does not change color (ex. S. epidermidis)
  • Gram + streptococci : inhibited growth
  • Gram - : inhibited growth [1]

Typical composition[edit]

MSA typically contains:[5]


  1. ^ a b c Bachoon, Dave S.; Dustman, Wendy A. (2008). "Exercise 8: Selective and Differential Media for Isolation". In Michael Stranz. Microbiology Laboratory Manual. Mason, OH: Cengage Learning. 
  2. ^ "Mannitol salt agar". 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)". Neogen Corp. 2008. 
  5. ^ The United States Pharmacopeia (23rd ed.). Rockville, MD: The United States Pharmacopeial Convention. 1995. 

External links[edit]