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Principle
- M-CP agar base is described by Armon and Payment (1988). The media is composed of tryptose, yeast extract, sucrose, L-cysteine, magnesium sulphate, bromocresol purple, ferric chloride, indoxyl-β-D-glucoside and agar.
- Tryptose, provide nitrogen, long-chain amino acids and other essential minerals. Yeast extract provides nitrogenous, carbonaceous compounds, long-chain amino acids, vitamins, minerals and other essential growth factors. Sucrose is a fermentable carbohydrate. L-cystine lowers the oxidation-reduction potential of the medium by removing oxygen to maintain a low Eh.
- Magnesium sulphate and ferric chloride are essential ions and help to maintain osmotic balance. Bromocresol purple serves as a pH indicator. Indoxyl-ß-D-glucoside is a chromogenic substrate for ß-D-glucosidase. Agar is the solidifying agent. For more selectivity, the media can be fortified with D-cycloserine, polymyxin B sulphate and phenolphthalein diphosphate.
- The addition of D-cycloserine and polymyxin B makes the medium inhibitory to associated non-clostridial microflora. The phenolphthalein diphosphate is an indicator of acid phosphatase. The selectivity can be increased by incubation under anaerobic conditions. After incubation the colonies are exposed to ammonia fumes for 30 seconds, if the colonies remain yellow, then they are ß-D-glucosidase negative. If the colonies becoming old rose to pink-red they are ß-D-glucosidase positive and may be C. perfringens.
- The green-coloured colonies need to be confirmed by sulphite reduction, gram-positive, sporulating rods, non-motile, reduction of nitrate, gelatine liquefaction, lactose fermentation and other biochemical tests.
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