MacConkey agar


MacConkey agar
Macconkey agar with lactose (left) and non-lactose (right) fermenters
A MacConkey agar plate with an active bacterial culture.

MacConkey agar is a culture medium designed to grow Gram-negative bacteria and stain them for lactose fermentation.[1]

Contents

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]

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.

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.[3][4]

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.

Slow

Some organisms ferment lactose slowly or weakly, and are sometimes put in their own category. These include Serratia[6] and Citrobacter.[7]

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

References

  1. ^ "tmc.edu". http://medic.med.uth.tmc.edu/path/macconk.htm. 
  2. ^ http://www.microbelibrary.org/index.php/component/resource/laboratory-test/2855-macconkey-agar-plates-protocols
  3. ^ MacConkey AT (1905). "Lactose-Fermenting Bacteria in Faeces.". J Hyg (Lond) 5 (3): 333–79. PMID 20474229. 
  4. ^ MacConkey AT (1908). "Bile Salt Media and their advantages in some Bacteriological Examinations.". J Hyg (Lond) 8 (3): 322–34. PMID 20474363. 
  5. ^ http://microbewiki.kenyon.edu/index.php/Shigella_sonnei
  6. ^ 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. 
  7. ^ "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. 



Wikimedia Foundation. 2010.

Look at other dictionaries:

  • MacConkey-Agar — Kolonien von Escherichia coli auf MacConkey Agar. MacConkey Agar ist ein Selektivnährboden zur Isolierung von gramnegativen Bakterien wie Salmonellen, Shigellen und coliformen Bakterien in Fäkalien, Nahrungsmitteln, Abwasser un …   Deutsch Wikipedia

  • MacConkey agar — Mac·Con·key agar (mə kongґke) [Alfred Theodore MacConkey, English bacteriologist, 1861–1931] see under culture medium …   Medical dictionary

  • MacConkey agar — an agar medium containing peptone, lactose bile salts, sodium chloride, neutral red, and crystal violet, used to differentiate lactose fermenters (coliforms) from non–lactose fermenters among the enteric bacilli. Called also MC a …   Medical dictionary

  • Sorbitol-MacConkey agar — MacConkey Sorbitol Agar is a variant of traditional MacConkey agar used in the detection of O157:H7. Traditionally, MacConkey agar has been used to distinguish those bacteria that ferment lactose from those that do not. This is important because… …   Wikipedia

  • Agar plate — Infobox Laboratory equipment name = Agar plate caption = An agar plate streaked with microorganisms isolated from a deep water sponge. Individual colonies may be seen center right. acronym = other names = uses = Culturing inventor = manufacturer …   Wikipedia

  • agar de MacConkey — un medio de cultivo utilizado para la identificación de patógenos entéricos que fermentan la lactosa, inhibiendo el crecimiento de los gérmenes Gram positivos. Las colonias de los gérmenes que fermentan la lactasa colorean el medio de rojo. El… …   Diccionario médico

  • MacConkey&’s agar — Agar based medium used for isolation of bacteria from faeces etc. Contains lactose and neutral red as an indicator so that lactose fermenting bacteria will produce red pink colonies …   Dictionary of molecular biology

  • Agar McConkey — Una placa de cultivo de McConkey incubada con E. coli …   Wikipedia Español

  • MacConkey's agar — Mac·Con·key s agar or Mac·Con·key agar mə käŋ kē(z) n an agar culture medium containing bile salts and lactose that is used esp. to isolate coliform bacteria MacConkey Alfred Theodore (1861 1931) British bacteriologist. MacConkey introduced the… …   Medical dictionary

  • agar — A complex polysaccharide (a sulfated galactan) derived from seaweed (various red algae); used as a solidifying agent in culture media; it has the valuable property of melting at 100°C, but not solidifying until 49°C. [Bengalese] bile salt a. an a …   Medical dictionary