Mannitol salt agar explained

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]

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]

• 5.0 g/L enzymatic digest of casein
• 5.0 g/L enzymatic digest of animal tissue
• 1.0 g/L beef extract
• 10.0 g/L D-mannitol
• 75.0 g/L sodium chloride
• 0.025 g/L phenol red
• 15.0 g/L agar
• pH 7.4 ± 0.2 at 25 °C

Notes and References

1. Book: Bachoon. Dave S. . Wendy A.. Dustman . Microbiology Laboratory Manual . Michael Stranz . Mason, OH . Cengage Learning . 2008 . Exercise 8: Selective and Differential Media for Isolation.
2. Web site: Becton, Dickinson and Company . Mannitol salt agar . 2005 . https://web.archive.org/web/20090126144117/http://bd.com:80/ds/technicalCenter/inserts/Mannitol_Salt_Agar.pdf . 2009-01-26.
3. Book: Anderson. Cindy. Great Adventures in the Microbiology Laboratory. 2013. Pearson. 978-1-269-39068-2. 175–176.
4. Web site: Neogen Corp. . Mannitol salt agar (7143) . 2011 . https://web.archive.org/web/20160303173323/http://www.neogen.com/acumedia/pdf/prodinfo/7143_pi.pdf . 2016-03-03.
5. Identification of Staphylococcus aureus: DNase and Mannitol salt agar improve the efficiency of the tube coagulase test
6. Book: 1995 . The United States Pharmacopeia . 23rd . The United States Pharmacopeial Convention . Rockville, MD.