MacConkey Agar is use as a selective and differential medium for the isolation of gram-negative bacilli including coliform organisms and enteric pathogens, on the basis of lactose fermentation. MacConkey Agar was one of the earliest culture media for the cultivation and identification of enteric organisms1 and is a modification of Neutral Red Bile Salt Agar developed by MacConkey. It has also been used in the isolation of pathogens from foods and coliforms in water samples2,3. The MacConkey Agar formulation presently in use is a modification where in addition to sodium chloride, the modified formula has a lowered agar content and an adjusted concentration of bile salts and neutral red. The differentiation of enteric microorganisms is achieved by the combination of the neutral red indicator and lactose. Lactose-fermenting organisms form pink colonies due to production of acid surrounded by a zone of bile salt precipitation. Acid production is also responsible for the formation of bile salt precipitation. Non-lactose-fermenters (Salmonella spp. and Shigella spp.) develop into transparent, colorless colonies with no precipitated zone. Bile salts and crystal violet are added to inhibit the growth of most gram-positive organisms. MacConkey's Medium is a selective and differential media; it is selective for Gram negative bacteria and can differentiate those bacteria that have the ability to ferment lactose. Bile salts, one of the components of the medium, inhibits most Gram-positive bacteria, except Enterococcus and some species of Staphylococcus i.e. Staphylococcus aureus. Also crystal violet dye, another component of the medium, inhibits certain Gram-positive bacteria thus selecting for Gram negatives.
By utilizing the available lactose in the medium, Lac+ bacteria such as Escherichia coli, Enterobacter and Klebsiella will produce acid in the medium lowering the pH of the agar below 6.8 and resulting in the appearance of red or pink colonies. The bile salts in the medium precipitate in the immediate neighborhood of the colony, causing the medium surrounding the colony to become hazy appearance. The non-lactose fermenting bacteria such as, Proteus species,Salmonella, Pseudomonas aeruginosa and Shigella that cannot utilize lactose in the medium, but use peptone instead resulting in the formation of ammonia, and raises the pH of the agar leading to the formation of white or colorless colonies. But, in some cases, they can also look golden to brown with dark centers and are usually circular colonies with random arrangement.
|Product Name||Catalog #||Quantity|
|MacConkey Agar mono plate, 90 x 15 mm||CM9360P||10/pk|
|MacConkey Agar w/CV mono plate, 90 x 15 mm||CM9361P||10/pk|
The following organisms are routinely used for testing for this medium.
|Escherichia coli ATCC® 25922||Growth; colonies pink to red with bile salt precipitate surrounding the colonies|
|Escherichia coli ATCC® 8739||Growth; colonies pink to red with bile salt precipitate surrounding the colonies|
|Proteus mirabilis ATCC® 12453||Growth; colonies colorless with no swarming|
|Salmonella enterica ATCC® 14028||Growth; colonies colorless|
|Pseudomonas aeruginosa ATCC® 9027||Growth; colonies colorless|
|Enterococcus faecalis ATCC® 29212||Partial to complete inhibition|
|Staphylococcus aureus ATCC® 6538||Partial to complete inhibition|
User Quality Control
Check for signs of contamination and deterioration. Users of commercially prepared media may be required to perform quality control testing with at least one known organism to demonstrate growth or a positive reaction; and at least one organism to demonstrate inhibition or a negative reaction (where applicable).
Ingredients g/L(Final pH 7.1 +/- 0.2 at 25°C)
3 total citations
- MacConkey, A.T. 1905. Lactose-fermenting bacteria in faeces.J. Hyg.; 5:333-379
- American Public Health Association. 2004.Standard Methods for the Examination of Dairy Products,17th ed. APHA, Washington, D.C.
- American Public Health Association. 2005.Standard Methods for the Examination of Water and Wastewater, 21st ed. APHA, Washington, D.C.