MacConkey Medium

MacConkey agar (MAC) is a bacterial culture medium named after bacteriologist Alfred T. MacConkey (1861-1931). MacConkey agar is a selective and differentiating agar that only grows gram-negative bacterial species; it can further differentiate the gram-negative organisms based on their lactose metabolism. The selective and differentiating properties of MacConkey agar enable utilization for both research and clinical applications. The fermentation of lactose produces organic acids, particularly lactic acid, which decreases the pH of the agar. MAC contains a pH indicator that turns pink under acidic conditions. Therefore, lactose-fermenting-gram-negatives (lactose-fermenters) will form pink colonies, while non-lactose fermenters will form off-white opaque colonies. Even within lactose-fermenters, species will show a varying rate of growth. The rate of growth is also a way to further differentiate organisms in the MAC medium. Lastly, some species that form a capsule appear differently. Altogether, MacConkey agar only grows gram-negative bacteria, and those bacteria will appear differently based on their lactose fermenting ability as well as the rate of fermentation and the presence of a capsule or not. This makes MAC a powerful tool for differentiating and isolating bacterial species from the sample source. MAC is one of the many bacterial cultures clinical microbiologists utilize for diagnostic testing; this is still widely used in the clinical laboratory to identify causal agents from a patient (ie, stool sample).