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