Abbreviate MSA, Mannitol salt agar is a selective and differential culture medium used in microbiology for the growth and isolation of Gram positive Staphylococci species such as Staphylococcus aureus. It was described by Chapman and has been adopted by many official organisations. Several modifications of it have been developed, all formulations resulting in media with similar efficiency. This media encourages the growth of halophillic bacteria while inhibiting the growth of others.
Principle of Mannitol salt Agar
It takes advantage of the high saline tolerance of staphylococci, by using sodium chloride (NACl) as salt source selective agent. Only staphylococci and halophilic enterobacteria (Salt loving bacteria) are able to grow freely at the concentration of salt content in this medium, while other bacteria are inhibited. It also exploits the correlation between the pathogenicity of staphylococci and their ability ferment mannitol (a type of sugar alcohol). Mannitol fermentation results in an accumulation of acid products, indicated by the phenol red indicator turning yellow. A yellow halo surrounds the presumptive pathogenic colonies (Mannitol fermenter), while the rest of the medium remains red/orange in colour. Other coagulase-negative staphylococci produce small pink or red colonies with no colour change to the medium.
Typical composition of Mannitol salt agar
The following ingredient makes up the composition of Mannitol Salt Agar in 1000Ml Distilled water
|Pancreatic Digest of Casein||5.0 g/L|
|Peptic Digest of Animal Tissue||5.0 g/L|
|beef extract||1.0 g/L|
|Sodium chloride||75.0 g/L|
|Phenol red||0.025 g/L|
|pH||7.4 ± 0.2 at 25°C|
Preparation of Mannitol Salt Agar
Depending on the quantity of plates to be prepared,each laboratory should be able to accurately determine the quantity required for a specific number of petri dishes based.
- Suspend 111.025 g of MSA media in 1000 ml of distilled water.
- Boil to dissolve the media completely (Pay attention while boiling to avoid over boiling which can explode or pour off
- Autoclave at 121°C for 15-20 minutes.
- Cool to 45-50°C and pour into petri dishes.
Colonies Growth appearance on Mannitol salt agar
|Gram Positive Staphylococcus aureus||Yellow colonies with yellow zones.|
|Gram positive Staphylococcus: not fermenting mannitol (e.g. Staphylococcus epidermidis )||Colorless or Red colonies with red zones.|
|Streptococci||No growth to trace growth.|
|Micrococci||Large white to orange.|
|Gram-negative bacteria||No growth to trace growth.|
- 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.
- “Mannitol salt agar”(PDF). Becton, Dickinson and Company. 2005.
- Anderson, Cindy (2013). Great Adventures in the Microbiology Laboratory. Pearson. pp. 175–176. ISBN978-1-269-39068-2.
- Identification of Staphylococcus aureus: DNase and Mannitol salt agar improve the efficiency of the tube coagulase test