MacConkey agar is used for the selective isolation and identification of members of the family Enterobacteriaceae from feces, urine, wastewater, and foods.It differentiates between lactose-fermenting and nonfermenting gram-negative rods by the color of colonial growth. MacConkey is a commonly used media to differentiate members of Enterobacteriaceae.MacConkey agar is commonly used for the isolation of Gram-negative enteric bacteria.Pale, non-lactose fermenting colonies of Salmonella in MacConkey Agar Uses of MacConkey Agar Medium This gives sticky, wet-appearing colonies on MacConkey medium. Since, non-fermenters can’t utilize lactose, colonies appear colorless or transparent.Įncapsulated bacteria such as Klebsiella and Enterobacter produce capsules using lactose. Lactose-fermenting bacteria produce pink-red colonies, after fermenting the lactose to acids and dropping the pH of the indicator ( neutral red) present in the medium. Lactose acts as a source of carbohydrates. Bile salts and crystal violet act as selective agents that inhibit the growth of Gram-positive organisms, and aid in the selective growth of non-fastidious gram-negative bacteria. MacConkey agar contains four key ingredients (lactose, bile salts, crystal violet, and neutral red) that make it a selective and differential media. 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.

Lactose negative gram negative rods