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Violet red bile agar (VRB agar)

Violet red bile agar (VRB agar) [under revision]

The figure shows a VRG agar plate on which coliform bacteria have been cultivated. - Click on the image to enlarge it.

Image: Ingrid Hansson (SLU), Lise-Lotte Fernström (SLU) & Karl-Erik Johansson (SLU).


Violet red bile agar (VRB agar) is a selective medium, which is used for enumeration of coliform bacteria in water, milk and other dairy produts.

The medium contains:

  • peptone (contains amino acids and peptides)
  • yeast extract (contains peptides, sugars, minerals and B vitamins)
  • NaCl (contributes to the osmotic balance)
  • bile salts (inhibit gram positive bakteria)
  • lactose (carbon source)
  • neutral red (pH-indicator)
  • crystal violet (inhibit gram positive bakteria)
  • agar (gelifying agent)
  • water

The final pH of the medium should be 7.4.

Other comments:

VRB agar is often used in combination with TSA (Tryptone Soya Agar) for the detection and enumeration of thermotolerant coliform bacteria (including E. coli). A thin layer of TSA (containing the sample with bacteria) is then molded in the bottom of the plate, which is then incubated for 1-2 h. at ambient temperature. Another layer consisting of VRG-agar is then molded on top on the TSA and the plate is incubated further.

Updated: 2023-05-31.


The revised taxonomy of the mollicutes (mycoplasmas)

About 2 years ago, the taxonomy of mycoplasmas was revised at VetBact, because a research group had proposed that 5 new genera should be included together with the genus Mycoplasma. This also had consequences for higher taxa within the phylum, which was then called Tenericutes. Now another group of researchers has suggested that the new names should be rejected.

Published 2023-10-03. Read more...

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