MacConkey agar is a selective and differential culture medium for bacteria . It is designed to selectively isolate gram-negative and enteric (normally found in the intestinal tract) bacteria and differentiate them based on lactose fermentation . Lactose fermenters turn red or pink on MacConkey agar, and nonfermenters do not change color. The media inhibits growth of gram-positive organisms with crystal violet and bile salts , allowing for the selection and isolation of gram-negative bacteria . The media detects lactose fermentation by enteric bacteria with the pH indicator neutral red .
5-397: It contains bile salts (to inhibit most gram-positive bacteria), crystal violet dye (which also inhibits certain gram-positive bacteria), and neutral red dye (which turns pink if the microbes are fermenting lactose ). Composition: There are many variations of MacConkey agar depending on the need. If the spreading or swarming of Proteus species is not required, sodium chloride
10-479: Is omitted. Crystal violet at a concentration of 0.0001% (0.001 g per litre) is included when needing to check if gram-positive bacteria are inhibited. MacConkey with sorbitol is used to isolate E. coli O157 , an enteric pathogen. The medium was developed by Alfred Theodore MacConkey while working as a bacteriologist for the Royal Commission on Sewage Disposal . Using neutral red pH indicator,
15-436: The agar distinguishes those gram-negative bacteria that can ferment the sugar lactose (Lac+) from those that cannot (Lac-). This medium is also known as an "indicator medium" and a "low selective medium". Presence of bile salts inhibits swarming by Proteus species. By utilizing the lactose available in the medium, Lac+ bacteria such as Escherichia coli , Enterobacter and Klebsiella will produce acid , which lowers
20-441: The organism is producing a capsule, which is predominantly made from the lactose sugar in the agar. A variant, sorbitol-MacConkey agar , (with the addition of additional selective agents) can assist in the isolation and differentiation of enterohemorrhagic E. coli serotype O157:H7, by the presence of colorless circular colonies that are non-sorbitol fermenting. Bile salts Too Many Requests If you report this error to
25-742: The pH of the agar below 6.8 and results in the appearance of pink colonies . The bile salts precipitate in the immediate neighborhood of the colony, causing the medium surrounding the colony to become hazy. Organisms unable to ferment lactose will form normal-colored (i.e., un-dyed) colonies. The medium may also turn yellow. Examples of non-lactose fermenting bacteria include Salmonella , Proteus , and Shigella spp. Some organisms ferment lactose slowly or weakly, and are sometimes put in their own category. These include Serratia and Citrobacter . Some organisms, especially Klebsiella and Enterobacter , produce mucoid colonies which appear very moist and sticky and slimy. This phenomenon happens because
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