Mannitol Salt Agar (Selective and different culture media) for Halophilic bacteria

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

Growth of Halophillic bacteria on Mannitol Salt Agar
Growth of Halophillic bacteria on 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 Casein5.0 g/L
Peptic Digest of Animal Tissue5.0 g/L
beef extract1.0 g/L
D-mannitol 10.0 g/L 
Sodium chloride75.0 g/L   
Phenol red0.025 g/L   
agar15.0 g/L   
pH 7.4 ± 0.2 at 25°C 
Also read  Stool examination (Wet mount vs Lugol iodine staining) Method

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.

  1. Suspend 111.025 g of MSA media in 1000 ml of distilled water.
  2. Boil to dissolve the media completely (Pay attention while boiling to avoid over boiling which can explode or pour off
  3. Autoclave at 121°C for 15-20 minutes.
  4. Cool to 45-50°C and pour into petri dishes.

Colonies Growth appearance on Mannitol salt agar

OrganismsResults
Gram Positive Staphylococcus aureusYellow  colonies with yellow zones.
Gram positive Staphylococcus: not fermenting mannitol (e.g. Staphylococcus epidermidis )Colorless or Red colonies with red zones.
StreptococciNo growth to trace growth.
MicrococciLarge white to orange.
Gram-negative bacteriaNo growth to trace growth.
Mannitol non fermenters(Pink)-mannitol fermenter (yellow)
Mannitol non fermenters(Pink)-mannitol fermenter (yellow)
Staphylococcus aureus on MSA
Staphylococcus aureus on MSA
Staphylococcus aureus on MSA
Staphylococcus aureus on MSA

References :

  • 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

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About the Author: Arthur Westmann

DEFFE ARTHUR (AMOEBAMANN) is the founder and author of MLTGEEKS and MLTEXPO.He’s from Cameroon and is currently a Final year State Medical Laboratory Technician (MLT MA). Beyond lab works, he’s a passionate internet user with a keen interest in web design and blogging. Furthermore He likes traveling, hanging around with friends and social networking to do in his spare time.

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