MacConkey agar

A MacConkey agar plate with an active bacterial culture.

Lactose

MacConkey agar is a culture medium designed to grow Gram-negative bacteria and differentiate them for lactose fermentation.

Contents

It contains bile salts (to inhibit most Gram-positive bacteria), crystal violet dye (which also inhibits certain Gram-positive bacteria), neutral red dye (which stains microbes fermenting lactose), lactose and peptone.

Composition:

• Peptone - 17 g
• Proteose peptone - 3 g
• Lactose - 10 g
• Bile salts - 1.5 g
• Sodium chloride - 5 g
• Neutral red - 0.03 g
• Agar - 13.5 g
• Water - add to make 1 litre; adjust pH to 7.1 +/- 0.2

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% (0.001 g per litre) is included when needing to check if Gram-positive bacteria are inhibited.

History

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

Uses

Acting as a visual pH 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". Absence of electrolytes serves to inhibit swarming by Proteus species.

Lac+

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 red/pink colonies. The bile salts precipitate in the immediate neighborhood of the colony, causing the medium surrounding the colony to become hazy.

Lac-

Non-Lactose fermenting bacteria such as Salmonella, Proteus species, Pseudomonas aeruginosa and Shigella cannot utilize lactose, and will use peptone instead. This forms ammonia, which raises the pH of the agar, and leads to the formation of white/colorless colonies formed in the plate. They can also look golden to brown with dark centers. They are circular colonies and arranged randomly.

Slow

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

Variant

A variant, Sorbitol-MacConkey agar, (with the addition of additional selective agents) can assist in the isolation and differentiation of enteropathogenic E. coli serotypes such as E. coli O157:H7, by the presence of white circular colonies that are non-sorbitol fermenting.

See also

• R2a agar