Fig. 9: Proteus mirabilis Growing on MacConkey Agar


MacConkey agar is a selective medium used for the isolation of non-fastidious Gram-negative rods, particularly members of the family Enterobacteriaceae and the genus Pseudomonas, and the differentiation of lactose fermenting from lactose non-fermenting Gram-negative bacilli. MacConkey agar contains the dye crystal violet well as bile salts that inhibit the growth of most Gram-positive bacteria but do not affect the growth of most Gram-negatives.

If the bacteria do not ferment lactose, the colonies and confluent growth appear colorless and the agar surrounding the bacteria remains relatively transparent (arrow).

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Microbiology Laboratory Manual by Gary E. Kaiser, PhD, Professor of Microbiology
is licensed under a Creative Commons Attribution 4.0 International License.
Last updated: August, 2017