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Brilliant Green (BG) Agar

BG agar plates

The figure shows two BG agar plates. Bacteria were not cultivated on the plate in image A. On the plate in image B, Salmonella sp. has been cultivated. Note that the colour of the pate has changed from yellowish-red to dark red and that the colonies are pink.

Applications

Brilliant Green (BG) Agar is used for selective isolation of Salmonella spp.

Content of the medium:

  • Yeast extract
  • Enzymatic digest of casein
  • Enzymatic digest of animal tissue
  • NaCl
  • Lactose
  • Sucrose
  • Brilliant green
  • Phenol red
  • Agar

The enzymatic digests provide sources of nitrogen, amino acids and carbon. The yeast extract supplies vitamins required for growth of bacteria. NaCl maintains the osmotic balance of the medium. Lactose and sucrose are the carbohydrates in the medium. Brilliant green (BG) inhibits gram-positive bacteria and most gram-negative rods other than Salmonella spp. The pH is 6.9. Phenol red is the pH indicator, which turns the yellow upon acidification due to fermentation of lactose and/or sucrose. Agar is the solidifying agent.

Summary:

  • Salmonella colonies can vary in colour from red to pink-white, depeding upon incubation time and strain. The agar around the colonies must be red.
  • Lactose- or sucrose fermenters, which are not inhibited by BG, form yellow or greenish-yellow colonies surrounded by a yellow-green zone.
  • Colonies of other non-lactose and non-sucrose fermenters than salmonella, may be similar to salmonella colonies.
  • Salmonella enterica subsp. enterica, serovars Typhi and Paratyphi do not grew well on BG agar.

Updated: 2017-02-24.

 

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