12.6.1: MacConkey Agar

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Note

Video Review How to Interpret the Results of 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. See Fig. \(\PageIndex{1}\)).

Photograph of an uninoculated plate of MacConkey agar. — Figure \(\PageIndex{1}\): **An Uninoculated Plate of 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. ( Photograph from From MicrobeLibrary.org
Courtesy of David Miller and Patrick Hanley, Hartwick College, Oneonta, NY. )

If the Gram-negative bacterium ferments the sugar lactose in the medium, the acid end products lower the pH of the medium. The neutral red in the agar turns red in color once the pH drops below 6.8. As the pH drops, the neutral red is absorbed by the bacteria, causing the colonies to appear bright pink to red.

Strong fermentation of lactose with high levels of acid production by the bacteria causes the colonies and confluent growth to appear bright pink to red. The resulting acid, at high enough concentrations, can also causes the bile salts in the medium to precipitate out of solution causing a pink precipitate (cloudiness) to appear in the agar surrounding the growth. See Fig. \(\PageIndex{2}\)).

Weak fermentation of lactose by the bacteria causes the colonies and confluent growth to appear pink to red, but without the precipitation of bile salts there is no pink halo around the growth. See Fig. \(\PageIndex{3}\).

If the bacteria do not ferment lactose, the colonies and confluent growth appear colorless and the agar surrounding the bacteria remains relatively transparent. See Fig. \(\PageIndex{15}\) and Fig. \(\PageIndex{5}\)

Fig \(\PageIndex{4}\): Proteus mirabilis Growing on MacConkey Agar	Fig. \(\PageIndex{5}\): Pseudomonas aeruginosa Growing on MacConkey Agar

If the bacteria do not ferment lactose, the colonies and confluent growth appear colorless and the agar surrounding the bacteria remains relatively transparent (arrow).	If the bacteria do not ferment lactose, the colonies and confluent growth appear relatively colorless and the agar surrounding the bacteria remains relatively transparent (black arrows). The green, water-soluble pigment produced by Pseudomonas aeruginosa (white arrows) is also evident here
Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0

Typical results for our strains of Enterobacteriaceae and Pseudomonas aeruginosa on MacConkey agar is as follows:

A. Strong fermentation of lactose

Escherichia coli

B. Weak fermentation of lactose

Klebsiella pneumoniae, Klebsiella aerogenes (formerly known as Enterobacter aerogenes), Enterobacter cloacae.

C. No fermentation of lactose

Proteus mirabilis, Proteus vulgaris, Serratia marcescens, Salmonella enterica, Pseudomonas aeruginosa.

Contributors and Attributions

Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)