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Principle
- Mannitol salt agar is a medium used for the detection and enumeration of Staphylococci. It was described by Chapman and prepared in accordance with the harmonized principles of USP/IP/JP as a medium for microbial limit testing of pharmaceutical products and raw materials used in pharmaceutical industries.
- Mannitol salt agar contains pancreatic digest of casein, peptic digest of animal tissue, and meat extract (equivalent to beef extract) as sources of carbon, nitrogen, vitamins, and minerals. Sodium chloride, in high concentration (7.5%), acts as a selective agent, allowing only to the growth of the Staphylococci and halophilic Enterobacteria and inhibiting other bacteria.
- Phenol red is the pH indicator. D-Mannitol is the carbohydrate source. Mannitol fermentation with an accumulation of acid products, detected by the phenol red indicator which turns yellow and produces a yellow halo surrounding the presumptive pathogen colonies.
- Characteristic non-pathogenic Staphylococci do not ferment mannitol and form red colonies. Since, this medium exploits the correlation between the pathogenic and fermentative capacity of mannitol of staphylococci, for a presumptive diagnosis.
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