MacConkey agar
MacConkey agar is a culture medium designed to grow Gram-negative bacteria and stain them for lactose fermentation.[1]
Contents |
[edit] Contents
It contains bile salts (to inhibit most Gram-positive bacteria, except Enterococcus and some species of Staphylococcus i.e. Staphylococcus aureus), crystal violet dye (which also inhibits certain Gram-positive bacteria), neutral red dye (which stains microbes fermenting lactose), lactose and peptone.
Composition: [2]
- 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 we need to check if Gram-positive bacteria are inhibited.
[edit] History
The medium was developed by Alfred Theodore MacConkey while working as a bacteriologist for the Royal Commission on Sewage Disposal.
[edit] 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.
[edit] 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.[3][4]
[edit] Lac-
Non-Lactose fermenting bacteria such as Salmonella, Proteus species, Pseudomonas aeruginosa and Shigella cannot utilize lactose, and will use peptone instead, with the exception of Shigella sonnei, which is a late lactose fermentor.[5] This forms ammonia, which raises the pH of the agar, and leads to the formation of white/colorless colonies formed in the plate. But they can also look golden to brown with dark centers. They are circular colonies and arranged randomly.
[edit] Slow
Some organisms ferment lactose slowly or weakly, and are sometimes put in their own category. These include Serratia[6] and Citrobacter.[7]
[edit] 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.
[edit] See also
[edit] References
- ^ "tmc.edu". http://medic.med.uth.tmc.edu/path/macconk.htm.
- ^ http://www.microbelibrary.org/index.php/component/resource/laboratory-test/2855-macconkey-agar-plates-protocols
- ^ MacConkey AT (1905). "Lactose-Fermenting Bacteria in Faeces.". J Hyg (Lond) 5 (3): 333–79. PMID 20474229.
- ^ MacConkey AT (1908). "Bile Salt Media and their advantages in some Bacteriological Examinations.". J Hyg (Lond) 8 (3): 322–34. PMID 20474363.
- ^ http://microbewiki.kenyon.edu/index.php/Shigella_sonnei
- ^ Luis M. De LA Maza; Pezzlo, Marie T.; Janet T. Shigei; Peterson, Ellena M. (2004). Color Atlas of Medical Bacteriology. Washington, D.C: ASM Press. pp. 103. ISBN 1-55581-206-6.
- ^ "Medmicro Chapter 26". Archived from the original on 2008-07-06. http://web.archive.org/web/20080706194735/http://gsbs.utmb.edu/microbook/ch026.htm. Retrieved 2008-12-11.
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