Agar is incorporated as the solidifying agent Principle of MacConkey Agar Sodium chloride provides osmotic balance and supplies essential electrolytes for transport. Other ingredients such as enzymatic digest of gelatin, casein, and animal tissue provide nitrogen, vitamins, minerals, and amino acids essential for growth. Lactose present in the medium is fermented by bacteria to form lactic acid that decreases the pH of the agar, and turns the indicator (neutral red) pink, thus differentiating lactose fermenters from non-lactose fermenters. Since Gram-negative enteric bacteria possess a bile-resistant outer membrane, they remain unaffected by bile salts. Key components of the MacConkey medium include crystal violet dye, bile salts, lactose, and neutral red (pH indicator).Ĭrystal violet dye and bile salts prevent the growth of Gram-positive bacteria and fastidious Gram-negative bacteria (such as Neisseria and Pasteurella) making it favorable for the growth of gram-negative bacteria. Modifications of MacConkey Agar Composition of MacConkey Agar Ingredient MacConkey Agar (g/L) Peptone 17 g Polypeptone 3 g Lactose 10 g Bile salts 1.5 g Sodium chloride 5 g Agar 13.5 g Neutral red 0.03 g Crystal violet 0.001 g Distilled water 1 L Final pH 7.1.
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