The Citrate Test is part of a group of tests called IMViC (Indole, Methyl-Red, Voges-Proskauer, and Citrate) used to tell the difference between different types of Gram-Negative bacilli in the Enterobacteriaceae family.
The Citrate Test can only be done if the bacteria can ferment citrate. In these tests, the most common citrate medium used is the Simmons formula.
The Citrate Test is used to see if organisms can use citrate as an energy source. It’s also used to see if organisms can use citrate as a carbon source. Organisms that can use citrate as their only carbon source may also use ammonium salts as their only nitrogen source. It’s also used to tell the difference between faecal coli and members of the aerogenes group based on how they use citrate.
The basic idea of this test is to see if organisms can use citrate as their only carbon source for their own metabolism, and then make the environment more alkaline. There’s another test medium for citrate that doesn’t need organisms to use citrate as their only carbon source. This is a specific way to see if organisms can use citrate as their only carbon source.
Alkaline carbonates and bicarbonates formed as by-products of citrate catabolism raise the pH of the medium are above 7.6, causing the bromothymol blue indicator to change colour from its original green to blue.
When an organic acid such as citrate (think of the Krebs cycle) is used as a source of carbon and energy, alkaline carbonates and bicarbonates are formed, which raise the pH of the medium, resulting in a colour change.
When bacteria metabolize citrate, the inorganic ammonium salt is broken down into ammonia, which increases alkalinity and changes the blue bromothymol indicator from green to blue. Sodium citrate combines with CO2 and water to form an alkaline product (sodium carbonate).
Bacteria capable of growing on this medium produce citrate permease, which converts citric acid to pyruvate.
Similarly, citrate-positive organisms first grow on glucose and then switch to citrate metabolism to form cherry (alkaline) colonies through the same reaction pathway as described for Simmons citrate agar. A positive result was obtained (the right tube was positive for citrate).
To assess the overall gene expression response of C. glutamic to citrate, avoiding comparison with glucose or other carbon sources in minimal medium, use complex LB medium for growth.
Recent studies have shown that glutamic growth in the presence of citrate as the sole carbon source if the medium contains high concentrations of Mg2+ ions, possibly due to the co-transport of Mg2+ and citrate.
Recent studies have shown that glutamic is able to grow on citrate as the sole source of carbon and energy and induces two citrate uptake systems in the presence of citrate, CitM and TctABC.
Only bacteria capable of using citrate as their sole source of carbon and energy can grow on Simmons’ citrate medium, so citrate-negative test cultures are nearly indistinguishable from uninoculated slants. Some citrate-negative microorganisms may grow weakly on sloped surfaces without causing discolouration.
Some organisms may require incubation times of up to 7 days due to limited growth rates on citrate medium. As shown, bacterial growth correlated with citrate consumption in the medium.
Large colonies formed after 2-3 days of incubation on longitudinal redundancy check (LRC) agar were also tested on Simmons and Christensen citrate agar. Large blue colonies on Simmons citrate agar from M9C medium were not clonal due to the deposition of predominantly elongated pre-dividing cells.
Citrate positive organisms are Klebsiella pneumonia and Proteus mirabilis. Escherichia coli and Shigella dysenteries are citrate negative.
Citrate enzyme is detected by the citrate utilization test. The basic principle is to detect the ability of an organism that can utilize citrate as a sole source of carbon for their metabolism with resulting alkalinity. The enzyme hydrolyses the citrate to form oxaloacetic acid and acetic acid.
The given test demonstrates the ability of certain bacteria to decompose the amino acid tryptophan to indole, which accumulates in the medium. The Indole production test is important in the identification of Enterobacteriaceae.