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Mannitol Salt Agar

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Mannitol Salt Agar for the isolation of Staphylococcus (MSA)
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Mannitol Salt Agar (MSA) is used as a selective and differential medium for the isolation and identification of Staphylococcus aureus from clinical and non-clinical specimens. It encourages the growth of a group of certain bacteria while inhibiting the growth of others. It is a selective medium prepared according to the recommendations of Chapman for the isolation of presumptive pathogenic staphylococci.

Composition of Mannitol Salt Agar 

Sodium Chloride75.0gm
Proteose Peptone10.0gm
Mannitol10.0gm
Beef Extract1.0gm
Phenol Red0.025gm
Agar15.0gm

Distilled Water = 1000 ml
Final pH 7.4 ± 0.2 at 25°C.
Principle of Mannitol Salt Agar
Mannitol Salt Agar contains peptones and beef extract, which supply nitrogen, vitamins, minerals and amino acids essential for growth. The 7.5% concentration of sodium chloride results in the partial or complete inhibition of bacterial organisms other than staphylococci. Sodium chloride also supplies essential electrolytes for transport and osmotic balance. Mannitol is the fermentable carbohydrate, fermentation of which leads to acid production, detected by phenol red indicator, aids in the differentiation of staphylococcal species. Coagulase test positive staphylococci (e.g., Staphylococcus aureus) produce yellow colonies and a surrounding yellow medium while coagulase negative staphylococci produce red colonies and no color change of the phenol red indicator. Agar is the solidifying agent.

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Uses of Mannitol Salt Agar
1.    It is used for the selective isolation and differentiation of Staphylococcus aureus from clinical samples.
2.    It is also used for the enumeration of staphylococci in food and dairy products.
3.    This medium is also included in the Bacteriological Analytical Manual for cosmetics testing.
4.    It is also used in the bacteriological examination of swimming pool water, spas and drinking water using membrane filtration
Preparation of Mannitol Salt Agar
1.    Suspend 111.025 gm of MRS media in 1000 ml of distilled water.
2.    Boil to dissolve the media completely.
3.    Autoclave at 121°C for 15-20 minutes.

4.    Cool to 45-50°C and pour into petri dishes.
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Prepare culture
culture medium 18-24h 
subculture to dividing staphylococci aureus between staphylococcus other and bacterial no grow on culture medium are high salt concentration .
Image result for mannitol salt agar strin

Result Interpretation on Mannitol Salt Agar
Organisms
Results
Staphylococcus aureus
Yellow colonies with yellow zones.
Staphylococci other than S. aureus (e.g. Staphylococcus epidermidis ) and Bacillus anthracis
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.
Quality Control on Mannitol Salt Agar
Positive Control: Staphylococcus aureus ATCC 6538, Medium-sized yellow colonies
Negative Control: Escherichia coli ATCC 25922, Partial to Complete Inhibition.

Limitations of Mannitol Salt Agar
1.    Several Staphylococcus species other than aureus are mannitol positive and produce yellow colonies surrounded by yellow zones on this medium (e.g. S. capitis, S. xylosus, S. cohnii, S. sciuri, S. simulans, and other species). Therefore, further biochemical tests are necessary for the identification of S. aureus or other species.
2.    Most organisms other than staphylococci are inhibited by the high salt concentration found in Mannitol Salt Agar except for some halophillic marine organisms.
3.    A few strains of Staphylococcus aureus may exhibit a delayed fermentation of mannitol. Negative plates should be re-incubated overnight before discarding.
4.    Presumptive Staphylococcus aureus must be confirmed with a coagulase test.

References
www.time2026end.com
https://catalog.hardydiagnostics.com/cp_prod/Content/hugo/MannitolSaltAgar.htm
https://en.wikipedia.org/wiki/Mannitol_salt_agar
Mannitol Salt Agar. PML Microbiologicals, Inc.
Mannitol Salt Agar. Acumedia Manufacturers, Inc.
Mannitol Salt Agar. Difco™ & BBL™ Manual, 2nd Edition
Mannitol Salt Agar. Liofilchem SRL.

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