Thecitrate test detects the ability of an organism to usecitrate as the sole source of carbon and energy.
Bacteria are inoculated on a medium containingsodium citrate and apH indicator such asbromothymol blue. The medium also contains inorganic ammonium salts, which are utilized as sole source of nitrogen. Use of citrate involves the enzyme citrate lyase, which breaks down citrate tooxaloacetate andacetate. Oxaloacetate is further broken down topyruvate and carbon dioxide (CO2). Production ofsodium bicarbonate (NaHCO3) as well asammonia (NH3) from the use of sodium citrate and ammonium salts results in alkaline pH. This results in a change of the medium's color from green (neutral) to blue (alkaline).
Bacterial colonies are picked up from a straight wire and inoculated into slope of Simmons citrateagar and incubated overnight at 37 °C. Inoculating from a broth culture is not recommended because the inoculum would be too heavy. If the organism has the ability to use citrate, the medium usually changes its color from green to blue, though growth on the medium even without colour change is considered a positive result.[1] An observation of no growth is a negative result.
Examples:
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