The citrate test detects the ability of an organism to use
citrate as the sole source of carbon and energy.
Principle
Bacteria are inoculated on a medium containing
sodium citrate and a
pH indicator such as
bromothymol 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 to
oxaloacetate and
acetate
An acetate is a salt (chemistry), salt formed by the combination of acetic acid with a base (e.g. Alkali metal, alkaline, Alkaline earth metal, earthy, Transition metal, metallic, nonmetallic or radical Radical (chemistry), base). "Acetate" als ...
. Oxaloacetate is further broken down to
pyruvate
Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell.
Pyruvic aci ...
and carbon dioxide (CO
2). Production of
sodium bicarbonate
Sodium bicarbonate (IUPAC name: sodium hydrogencarbonate), commonly known as baking soda or bicarbonate of soda, is a chemical compound with the formula NaHCO3. It is a salt composed of a sodium cation ( Na+) and a bicarbonate anion ( HCO3−) ...
(NaHCO
3) as well as
ammonia (NH
3) 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 citrate
agar
Agar ( or ), or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from ogonori (''Gracilaria'') and "tengusa" (''Gelidiaceae''). As found in nature, agar is ...
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.
An observation of no growth is a negative result.
Examples:
* ''
Escherichia coli'': Negative
* ''
Klebsiella pneumoniae'': Positive
* ''
Frateuria aurantia
''Frateuria aurantia'' is a species of bacteria. It is named after the Belgian microbiologist Joseph Frateur. The cells are mostly straight rods.Garrity, George M. (2005). Bergey's manual of systematic bacteriology, Volume Two: The Proteobacteri ...
'': Positive
References
Chemical tests
Microbiology techniques
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