MacConkey agar

Source: Wikipedia, the free encyclopedia.
Plate with an active bacterial culture
With an active bacterial culture
Lactose
The organism on the left is a lactose fermenter, as evidenced by the pink color. The organism on the right produced no color, so it does not appear to be a lactose fermenter.

MacConkey agar is a

bile salts, allowing for the selection and isolation of gram-negative bacteria. The media detects lactose fermentation by enteric bacteria with the pH indicator neutral red.[2]

Contents

It contains

Gram-positive bacteria), crystal violet dye (which also inhibits certain Gram-positive bacteria), and neutral red dye (which turns pink if the microbes are fermenting lactose
).

Composition:[3]

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. MacConkey with sorbitol is used to isolate E. coli O157, an enteric pathogen.[4]

History

The medium was developed by Alfred Theodore MacConkey while working as a bacteriologist for the Royal Commission on Sewage Disposal. [5]

Uses

Using neutral red 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". Presence of bile salts inhibits swarming by Proteus species.

Lac positive

By utilizing the lactose available in the medium, Lac+ bacteria such as

colonies. The bile salts precipitate in the immediate neighborhood of the colony, causing the medium surrounding the colony to become hazy.[6][7]

Lac negative

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 include Salmonella, Proteus, and Shigella spp..[4]

Slow

Some organisms ferment lactose slowly or weakly, and are sometimes put in their own category. These include Serratia[8] and Citrobacter.[9]

Mucoid colonies

Some organisms, especially Klebsiella and Enterobacter, 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.

Variant

A variant, sorbitol-MacConkey agar, (with the addition of additional selective agents) 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.[4]

See also

  • R2a agar
  • MRS agar (culture medium designed to grow Gram-positive bacteria and differentiate them for lactose fermentation).

References

  1. ^ "MacConkey Agar". Texas Medical Center. Archived from the original on 2008-11-04.
  2. ISBN 978-1-269-39068-2
    .
  3. ^ "MacConkey Agar Plates Protocols". Archived from the original on 2010-12-03. Retrieved 2011-03-20.
  4. ^
    ISBN 9781683670063
    .
  5. ^ Smith, Kenneth (2019-10-14). "The Origin of MacConkey Agar". American Society for Microbiology. Retrieved 2023-12-01.
  6. PMID 20474229
    .
  7. PMID 20474363
    .
  8. ISBN 1-55581-206-6
    .
  9. ^ "Medmicro Chapter 26". Archived from the original on 2008-07-06. Retrieved 2008-12-11.
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