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Description:These four test tubes illustrate both a positive (+), and three negative (-) decarboxylase reactions used to identify a bacterial microorganism through the identification of its life-sustaining metabolic processes. After each tube had been inoculated with a bacterial culture, mineral oil had been added in order to cover the medium, thereby, preventing atmospheric oxygen from coming into contact with the newly inoculated medium. This provided an anaerobic growth environment devoid of oxygen. Tubes 1, 3 and 4 revealed a negative reaction (yellow-orange), while organisms in tube 2 provided a positive decarboxylation reaction (purple).

”Various decarboxylase-base media are used in the biochemical differentiation of Gram-negative enteric bacilli based on the production of arginine dihydrolase, and lysine and ornithine decarboxylase.”
“When the medium is inoculated with a bacterium that is able to ferment dextrose (glucose), acids are produced that lower the pH of the medium and change the color of the indicator from purple to yellow. The acidic condition also stimulates decarboxylase activity. If the organism produces the appropriate enzyme, the amino acid in the medium is degraded, yielding a corresponding amine. Decarboxylation of lysine yields cadaverine, while decarboxylation of ornithine yields putrescine. Arginine is first hydrolyzed to form ornithine, which is then decarboxylated to form putrescine. The production of these amines elevates the pH of the medium, changing the color of the indicator from yellow to purple or violet. If the organism does not produce the appropriate enzyme, the medium remains acidic (yellow).” See the link below.

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Content Providers(s):CDC/ Dr. R. Weaver
Creation Date:1971
Photo Credit:
Links:Becton Dickinson and Company (BD) - Difco™ & BBL™ Manual, 2nd Edition; from ”Biochemical tests for identification of medical bacteria, 3rd ed.”, by MacFaddin, 2000, Lippincott Williams & Wilkins, Baltimore, Md.
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Copyright Restrictions:None - This image is in the public domain and thus free of any copyright restrictions. As a matter of courtesy we request that the content provider be credited and notified in any public or private usage of this image.