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Veterinary bacteriology: information about important bacteria
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Violet Red Bile Glucose (VRBG) agar

VRBG agar plates

The figure shows three VRBG agar plates. Bacteria have not been cultivated on the plate in image A. Two different members of the family Enterobacteriaceae have been cultivated at 37°C during 24 h on the plates B and C. A halo can be discerned around the colonies because of precipitated bile salts, which also makes the colonies look unsharp. This is particularly evident on the plate in image C.

Image: Karl-Erik Johansson (BVF, SLU) and Lise-Lotte Fernström (BVF, SLU).


Violet Red Bile Glucose (VRBG) agar is a selective medium, which is used for isolation and enumeration of members of the family Enterobacteriaceae in food products.

Content of the medium:

  • peptone (contains amino acids and peptides)
  • yeast extrakt (contains peptides, sugars, minerals and B vitamins)
  • NaCl (contributes to the osmotic balance)
  • bile salts (inhibit gram positive bacteria, except Enterococcus faecalis)
  • glucose (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:

Members of the family Enterobacteriaceae produce small (1-2 mm in diameter), round and violet to pink colonies with a halo on VRBG agar. There are some other genera, which also grow on VRBG agar, e.g. Aeromonas. The identity of the colonies must, therefore, be confirmed by an independent method, e.g. the oxidase test.

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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