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Veterinary bacteriology: information about important bacteria
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Chrom agar


The figure shows Chromagar plates (Orientation) with cefotaxime, where bacteria have not been cultivated on plate A. Escherichia coli (to the left) and Enterococcus spp. (to the right) have been cultivated on plate B. Faeces samples with a mixed culture of E. coli and Enterococcus spp. from a calf have been cultivated on plates C and D . Note the coulur differences, E. coli colonies have a pink colour and Enterococcus spp. are turquoise blue. Since cefotaxime has been added to these Chromagar plates, the growth on these plates indicate that the bacteria are resistant to carbapenem (ESBL producing). The Images is photographed with lighting from below - Click on the image to enlarge it.

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


Chromogenic media facilitate the isolation and differentation of bacteria as specific bacteria grow with different distinct colony appearance. Chromagar Orientation is often used for isolation and differentiation of urinary tract pathogens. This medium can also be used for identification of ESBL producing bacteria for example E. coli if cefotaxime (cephalosporin) is added to the media. When cefotaxime is added to the media, the growth of bacteria sensitive to carbapenem is inhibited.

Content of the medium:

  • agar (gelifying agent)
  • peptone
  • yeast extract
  • chromogenic mix (according to the manufacurer)

The final pH of this medium should be 7.0.

Other comments:

Colonies of different bacteria will appear in different colours, bellow are some examples.

Bacterial genus or species Appearance of colonies
Citrobacter, Enterobacter, Klebsiella, Serratia metallic blue (+/- reddish halo)
E. coli dark pink to reddish
Enterococcus turquoise blue
Morganella, Proteus, Providencia brown halo
Pseudomonas translucent (+/- natural pigmentation cream to green)
S. aureus golden, small opaque

Updated: 2019-02-07.

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