MacConkey agar


MacConkey agar is an indicator, a selective and differential culture medium for bacteria designed to selectively isolate Gram-negative and enteric bacilli and differentiate them based on lactose fermentation. The crystal violet and bile salts inhibit the growth of Gram-positive organisms which allows for the selection and isolation of gram-negative bacteria. Enteric bacteria that have the ability to ferment lactose can be detected using the carbohydrate lactose, and the pH indicator neutral red.

Contents

It contains bile salts, crystal violet dye, neutral red dye.
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 is omitted. Crystal violet at a concentration of 0.0001% 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.

History

The medium was developed by Alfred Theodore MacConkey while working as a bacteriologist for the Royal Commission on Sewage Disposal.

Uses

Using neutral red pH indicator, the agar distinguishes those Gram-negative bacteria that can ferment the sugar lactose from those that cannot.
This medium is also known as an "indicator medium" and a "low selective medium". Presence of bile salts inhibits swarming by Proteus species.

Lac positive

By utilizing the lactose available in the medium, Lac+ bacteria such as Escherichia coli, Enterobacter and Klebsiella will produce acid, which lowers the pH of the agar below 6.8 and results in the appearance of pink colonies. The bile salts precipitate in the immediate neighbourhood of the colony, causing the medium surrounding the colony to become hazy.

Lac negative

Organisms unable to ferment lactose will form normal-colored colonies. Examples of non-lactose fermenting bacteria are Salmonella, Proteus species, Yersinia, Pseudomonas aeruginosa and Shigella.

Slow

Some organisms ferment lactose slowly or weakly, and are sometimes put in their own category. These include Serratia and Citrobacter.

Mucoid colonies

Some organisms, especially Klebsiella and Enterobacter, produce mucoid colonies which appear very moist and sticky. This phenomenon happens because the organism is producing a capsule, which is predominantly made from the lactose sugar in the agar.

Variant

A variant, sorbitol-MacConkey agar, can assist in the isolation and differentiation of enterohemorrhagic E. coli serotype E. coli O157:H7, by the presence of colorless circular colonies that are non-sorbitol fermenting.