MacConkey-Agar

MacConkey agar is a selective medium for the isolation of Gram-negative bacteria such as Salmonella, Shigella and coliform bacteria in faeces, food, waste water and other habitats. The colonies of organisms such as Escherichia coli, which can degrade lactose fermentation ( lactose positive) turn red; other Gram-negative organisms remain colorless.

The original (1901 ) published the recipe of the British microbiologist Alfred Theodore MacConkey ( 1861-1931 ) is modified today.

Operation

Bile salts and crystal violet contained in MacConkey agar largely inhibit Gram-positive bacteria. Lactose also serves, together with the pH indicator neutral red to detect the degradation of lactose. Here, lactose - positive bacteria show as red colonies, particularly E. coli colonies additionally show a dull yard, which is caused by bile acids that break down due to the lowering of the pH ( acid formation from lactose).

Typical Composition

The soil mostly consists of ( in grams per liter):

  • Peptone from gelatin 17.0
  • Peptone from casein 1.5
  • Peptone from meat 1.5
  • Saline 5.0
  • Lactose 10.0
  • Bile salt mixture 1.5
  • Neutral 0.03
  • Crystal violet 0.001
  • Agar- agar 13.5
Wastewater Cetrimide agar Cysteine lactose electrolyte deficient agar Endo-Agar Eosin methylene blue
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