MacConkey agar is aselective anddifferential culture medium forbacteria. It is designed to selectively isolategram-negative and enteric (normally found in the intestinal tract) bacteria and differentiate them based onlactose fermentation.[1] Lactose fermenters turn red or pink on MacConkey agar, and nonfermenters do not change color. The media inhibits growth ofgram-positive organisms withcrystal violet andbile salts, allowing for the selection and isolation ofgram-negative bacteria. The media detects lactose fermentation by enteric bacteria with the pH indicatorneutral red.[2]
It containsbile salts (to inhibit mostgram-positive bacteria),crystal violet dye (which also inhibits certain gram-positive bacteria), andneutral red dye (which turns pink if the microbes are fermentinglactose).
Composition:[3]
There are many variations of MacConkey agar depending on the need. If the spreading or swarming ofProteus species isnot required, sodium chloride 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 isolateE. coli O157, an enteric pathogen.[4]
The medium was developed byAlfred Theodore MacConkey while working as abacteriologist for theRoyal Commission on Sewage Disposal.[5]
Using neutral redpH indicator, 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 byProteus species.
By utilizing the lactose available in the medium, Lac+ bacteria such asEscherichia coli,Enterobacter andKlebsiella will produceacid, which lowers the pH of the agar below 6.8 and results in the appearance of pinkcolonies. The bile salts precipitate in the immediate neighborhood of the colony, causing the medium surrounding the colony to become hazy.[6][7]
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 includeSalmonella,Proteus, andShigella spp.[4]
Some organisms ferment lactose slowly or weakly, and are sometimes put in their own category. These includeSerratia[8] andCitrobacter.[9]
Some organisms, especiallyKlebsiella andEnterobacter, produce mucoid colonies which appear very moist and sticky and slimy. This phenomenon happens because 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 enterohemorrhagicE. coliserotype O157:H7, by the presence of colorless circular colonies that are non-sorbitol fermenting.[4]