MacConkey Agar EP/USP/ISO
Principles and uses
MacConkey Agar is used for the selective isolation and identification of Enterobacteriaceae from feces, urine, wastewater and foods. It is also a selective and differential medium for the isolation of enteric Gram-negative bacteria.
The specimen can be streaked directly on the medium or inoculated first into an enrichment broth such a Tetrathionate Broth , Selenite Cystine Broth , GN Enrichment Broth or MacConkey Broth .
The European Pharmacopoeia, USP recommends this media in the paragraph 2.6.13: “Microbiological examination of non-sterile products: Test for specified microorganisms” for the growth promoting and indicative properties of the media in the test for E. coli. Also, this medium is recommended for the testing of E. coli in products.
ISO 21567 recommends this medium for the detection of Shigella spp. It is used to obtain isolated colonies. After incubating the plates of Agar McConkey a confirmatory step is necessary.
ISO 21150 recommends this medium for the detection of E. coli in cosmetic products.
Pancreatic digest of gelatin and peptones meat & casein provide nitrogen, vitamins, minerals and amino acids essential for growth. Lactose is the fermentable carbohydrate providing carbon and energy. Sodium chloride supplies essential electrolytes for transport and osmotic balance. Bile salts and crystal violet are the selective agents, and inhibit Gram-positive organisms. Neutral red is the pH indicator. When lactose is fermented, the pH of the medium decreases, changing the color of neutral red to pink. Bacteriological agar is the solidifying agent.
Isolated colonies of Escherichia coli are brick red in color and are surrounded by a zone of precipitated bile. This bile precipitate is due to a local pH drop around the colony due to lactose fermentation.
Preparation
Suspend 60 grams of the medium in one liter of distilled water. Mix well until a homogeneous suspension is obtained. Heat with frequent agitation and boil for one minute until complete disolution. DO NOT AUTOCLAVE. Cool to 45-50 °C and distribute in Petri dishes.
