Decarboxylase

Decarboxylase broth is used to test for the ability of bacteria to cleave the carboxyl (-CO2H) group off of free amino acids. The media contains Glucose, Peptone, Bromeresol purple (purple when alkaline, yellow when acidic), and Pyridoxal Phosphate. This is the base media, and is used as a control when any of the other media are used. To this media is added one of the amino acids, typically arginine, lysine, and ornithine. After the broth is inoculated a layer of sterile mineral oil is added to the top to separate the system from oxygen interference. When the organism ferments glucose, the media becomes acidic and turns yellow. This will happen within 24 hours and indicates the media is working properly. This acidic state also will cause any bacteria growing in the media to start producing decarboxylase enzymes. The reaction to remove carboxyl groups also raises the pH to the point the media returns to a purple coloration. Yellow in 24 hours indicates functioning media, purple again within 96 hours means the organism is positive for a decarboxylase for the particular amino acid tested.