Mannitol Salt Agar (MSA) Overview

Staphylococcus aureus is isolated and identified from clinical and non-clinical materials using Mannitol Salt Agar (MSA), a selective and differentiating medium. It promotes the development of some bacteria while inhibiting the growth of others, and it is advised for the identification and enumeration of coagulase-positive Staphylococci in milk, food, and other materials.

Principle of Mannitol Salt Agar (MSA)

Because they include vital growth factors and trace elements, including nitrogen, vitamins, minerals, and amino acids that are necessary for development, beef extract and proteose peptone, which are found in mannitol salt agar, make it exceptionally nutrient-dense. Other bacterial species, except staphylococci, are partially or completely inhibited by the medium’s sodium chloride content of 7.5%. The source of fermentable carbohydrates is mannitol, which when fermented produces acid.

On this medium, Staphylococcus aureus develops and ferments mannitol to form yellow colonies. The majority of staphylococcal and micrococcal species that lack coagulase do not ferment mannitol and develop as little red colonies. The phenol red (indicator) reaction to the medium’s pH is what gives the colonies and medium their colour. Phenol red becomes red at a pH of 8.4 and turns yellow at a pH of 6.8. The solidifying agent is agar. Additionally, mannitol fermentation and staphylococci lipase activity may both be detected in a medium by adding 5% v/v egg yolk emulsion. As the salt clarifies the egg yolk emulsion, the lipase synthesis appears as a yellow, opaque zone encircling the colonies.

Composition of Mannitol Salt Agar (MSA)

Ingredients                                                         Gms/Litre

Proteose peptone                                                   10.0

Sodium chloride                                                      75.0

D- mannitol                                                              10.0

Beef extract                                                               1.0

Phenol red                                                                 0.025

Agar                                                                         15.0

Final pH 7.4 +/- 0.2 at 25ºC.

Preparation of Mannitol Salt Agar (MSA)

1. Pour 1000 cc of distilled water over 111 grams of Mannitol Salt Agar.
2. To thoroughly dissolve the medium, boil.
3. By autoclaving for 15 minutes at 121 °C and 15 lbs of pressure, sterilise.
4. After autoclaving, sterile egg yolk emulsion (E7899) can optionally be added to a final concentration of 5% v/v.
5. Mannitol Salt Agar should be poured onto clean petri dishes and allowed to cool to room temperature.

Results on Mannitol Salt Agar (MSA)

Uses of Mannitol Salt Agar (MSA)

• Using this method, Staphylococcus aureus is isolated and differentiated from clinical samples.
• Additionally, it is used to count staphylococci in dairy and food items.
• The Bacteriological Analytical Manual also contains this method for evaluating cosmetics.
• Additionally, it is employed in the membrane-based bacteriological testing of drinking water, spa water, and swimming pool water.

Limitations

• On this medium, several Staphylococcus species other than aureus develop yellow colonies surrounded by yellow zones (e.g. S. capitis, S. xylosus, S. cohnii, S. sciuri, S. simulans, and other species).
• As a result, further biochemical tests are necessary to identify S. aureus or other species.
• Many organisms, apart from staphylococci, are inhibited by the high salinity present in Mannitol Salt Agar, with the exception of halophilic marine species.
• A few Staphylococcus aureus strains may show mannitol fermentation that is delayed. Before discarding, negative plates should be re-incubated for an additional night.
• Coagulation test results are required to confirm suspected Staphylococcus aureus.

References

• Mannitol Salt Agar (MSA) (Chapman Medium) European Pharmacopoeia, Usp.
• Hardy Diagnostics.
• Mannitol Salt Agar. Dehydrated Culture Media. Thermo Fisher Scientific Inc.
• https://en.wikipedia.org/wiki/Mannitol_salt_agar
• http://microbeonline.com/mannitol-salt-agar-msa-composition-uses-and-colony-characteristics/
• http://www.vumicro.com/vumie/help/VUMICRO/Mannitol_Salt_Agar.htm
• https://microbiologyinfo.com/mannitol-salt-agar-for-the-isolation-of-staphylococcus-aureus/
• https://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Sigma/Datasheet/4/m9052dat.pdf