☰ Summary :
Chapman Agar or Mannitol Salt Agar is a selective medium used for the isolation, enumeration and differentiation of Staphylococcus from clinical, food, antiseptic and cosmetic samples.
This medium is both selective and differential agar. The medium will select organisms that can live in areas with a high concentration of salt (sodium chloride) and the fermentation of mannitol, demonstrated by the yellow turn of the pH indicator (phenol red), makes it possible to guide the diagnosis.
Mannitol Salt Agar (@lablife)
◈ The selectivity of this medium is based on the presence of sodium chloride (7.5%) which inhibits most Gram negative and Gram positive bacteria.
◈ The differentiation is based on the ability or not to ferment themannitol (the only sugar in the medium). If there is fermentation, this induces acidification which leads, at pH levels below 6.9, to a yellow coloration of the medium in the presence of phenol red (pH indicator).
S. coagulase negative (left) and S. aureus (right) were inoculated on the agar and were then incubated overnight.(8)
Note :
☰ For 1 liter of medium:
Composition Mannitol Salt Agar |
|||
|---|---|---|---|
| Ingredients | gram / liter | ||
| Peptone | 10 g | ||
| Beef meat extract | 1 g | ||
| Sodium chloride | 75g | ||
| Mannitol | 10g | ||
| Phenol red | 0,025g | ||
| Agar | 15g | ||
| pH final | 7,4 ± 0,2 | ||
Dehydrated Mannitol Salt Agar
Suspend 111 g of the powder in 1 liter of distilled or deionized water. Mix well. Heat to boil for 1 minute shaking frequently until completely dissolved. Sterilize in autoclave at 121°C for 15 minutes.
Melt the content of the bottle in a water bath at 100°C (loosing the cap partially removed) until completely dissolved. Then screw the cap and check the homogeneity of the dissolved medium, if it is the case turning the bottle upside down. Cool at 45-50°C, mix well avoiding foam formation and aseptically distribute into Petri dishes.
Mannitol Salt Agar in bottles
- Staphylococci aureus form lush, pigmented colonies surrounded by a yellow halo due to the fermentation of mannitol. Non-pathogenic staphylococci usually form small red colonies which do not change the color of the medium.
Note :The change in color of the medium demonstrates the fermentation of mannitol, NOT the color of the colony. This is particularly important because many micrococci are pigmented.
- Several species of Staphylococcus other than S. aureus are positive for mannitol and produce yellow colonies surrounded by yellow areas (eg S. capitis, S. xylosus, S. cohnii, S. sciuri, S. simulans ..). Therefore, further biochemical testing is needed to identify S. aureus or other species.
MicrobeLibrery: (A).Staphylococcus aureus: large yellow halo around growth indicates fermentation of mannitol.
(B). Staphylococcus epidermidis: Growth but not color change to the media indicating no fermentation of mannitol.
(C). Staphylococcus saprophyticus: small yellow halo around growth indicates fermentation of mannitol. (10% of S. saprophyticus ferment mannitol)
(D). E. coli: no growth. Inhibited by the 7.5% NaCl.
- Chapman with Oxacillin can be used to screen nasal specimens for MRSA because the 7.5% salt and 6 g of oxacillin in this medium inhibits most other organisms that normally colonize the nostrils.
- The addition of 5% v / v of egg yolk emulsion allows the detection of the lipase activity of staphylococci as well as the fermentation of mannitol. The salt cleans the egg yolk emulsion and the production of lipase is detected as a yellow opaque area around the colonies.
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