Chemical Composition of Mannitol Salt Agar

In this chemical composition of mannitol salt agar post we have briefly explained about Mannitol salt agar media principle, Mannitol salt agar media composition, appearances, storage, preparation, result, applications, and limitations of Mannitol salt agar media.

For isolating pathogenic staphylococci from samples, cosmetics, and microbiological limit tests, Mannitol salt agar media is recommended. Bacteria that live in a high salt environment and ferment mannitol create acid compounds, changing the colour of the phenol red pH indicator from red to yellow.

Chapman first described mannitol salt agar media (MSA) in 1945, which is both a selective and differential medium for cultivating staphylococci. Because the presence of a high salt concentration (7.5%) limits the development of most bacteria, the medium is selective.

Staphylococci that cause disease, such as Staphylococcus aureus, will ferment mannitol. Mannitol is not fermented by most non-pathogenic staphylococci. Mannitol is fermented by Staphylococcus aureus, which makes the medium yellow. Because of the high salt concentration, Serratia marcescens does not grow.

Chemical composition of mannitol salt agar

Item

g/L

Proteose Peptone               

10.00

Beef Extract                         

1.00

Sodium Chloride                   

75.00

D-Mannitol                           

10.00

Phenol Red                           

0.025

Agar

15.00

Final pH (at 25°C) 7.4 ± 0.2

 

Principle

Amino acids, nitrogen, carbon, vitamins, and minerals are provided via pancreatic digest of casein, peptic digest of animal tissue, and beef extract for the growth of organisms. The fermentable carbohydrate is mannitol. Except for staphylococci, most microorganisms are inhibited by the high salt level of 7.5 percent. The pH indicator is phenol red. The solidifying agent in Mannitol salt agar media is agar.

Medium Preparation

Dehydrated medium

In 1 litre of distilled or deionized water, dissolve 111 g of the powder. Mix thoroughly the Mannitol salt agar media. Bring to a boil for 1 minute, stirring constantly until the sugar is completely dissolved. Sterilize for 15 minutes in an autoclave at 121°C.

Medium in bottles

Melt the contents of the bottle in a water bath at 100°C until entirely dissolved (leaving the cap partially off). Then screw on the cap and turn the bottle upside down to check for homogeneity of the dissolved medium. Cool to 45-50°C, thoroughly mix to avoid foam formation, then distribute the Mannitol salt agar media aseptically into Petri dishes.

Procedure

Inoculate plates by strewing the substance to be investigated directly onto the agar surface. Incubate aerobically for 24-48 hours at 35 2°C. The inoculation of the sample in Tryptic Soy Broth is recommended by the Harmonized USP/EP/JP technique for microbiological evaluation of non-sterile items. Subculture on Mannitol salt agar media and incubate for 18-72 hours at 30-35°C.

Results

S.aureus grows in colonies that are yellow or white and encircled by a yellow zone. Identification tests should be used to confirm. Coagulase-negative Staphylococci produce small, colourless to red colonies that do not alter the medium’s colour.

Mannitol Salt Agar

Mannitol salt agar media is fermented by Staphylococcus aureus, which makes the medium yellow. Because of the high salt concentration, Serratia marcescens does not grow on Mannitol salt agar media.

Limitations

1. On this medium, mannitol-positive Staphylococcus species other than aureus develop yellow colonies surrounded by yellow zones. As a result, additional biochemical tests are required to identify S. aureus or other species.

2. Presumptive a coagulase test is required to establish the presence of Staphylococcus aureus.

3. Except for some halophillic marine organisms, most organisms other than staphylococci are inhibited by the high salt concentration observed in Mannitol salt agar media.

4. Mannitol salt agar media fermentation may be delayed in a few strains of Staphylococcus aureus. Before discarding negative plates, they should be re-incubated overnight.

Further Readings

Reference