Mannitol salt agar

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 (about 7.5%-10%) of salt (NaCl), making it selective for Gram-positive bacteria (Staphylococcus and Micrococcaceae) since this level of salt is inhibitory to most other bacteria.^[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.

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:^[5]

• 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

References

1. 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" (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. The United States Pharmacopeia (23rd ed.). Rockville, MD: The United States Pharmacopeial Convention. 1995.

External links