Mannitol salt agar

Mannitol salt agar or MSA is a commonly used 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 fermentors, containing mannitol and the indicator phenol red. Coagulase-positive Staphylococci produce yellow colonies with yellow zones, whereas coagulase-negative Staphylococci produce small pink or red colonies with no color change to the medium.^[3] If an organism can ferment mannitol, an acidic byproduct is formed that will cause the phenol red in the agar to turn yellow. ^[4] It is used for the selective isolation of presumptive pathogen (pp) Staphylococci.

Expected Results

• Gram + staphylococci fermenting mannitol: Media turns yellow
• Gram + staphylococci not fermenting mannitol: Media does not change color
• Gram + streptococci: inhibited growth
• Gram - : inhibited growth ^[5]

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

References

Bachoon, Dave S., and Wendy A. Dustman. Microbiology Laboratory Manual. Ed. Michael Stranz. Mason, OH: Cengage Learning, 2008. Exercise 8, "Selective and Differential Media for Isolation" Print.

External links

• Mannitol Salt Agar Description & Formulation