Citrate Test

General Information About the Citrate Test

The citrate test is part of the IMViC panel of tests and is helpful in differentiating the Enterobacteriaceae. Simmons citrate agar tests the ability of organisms to utilize citrate as a carbon source. Organisms that can utilize citrate as their sole carbon source use the enzyme citrase or citrate-permease to transport the citrate into the cell.

The Basic Chemistry of Simmons Citrate Agar

Simmons citrate agar contains sodium citrate as the sole source of carbon, ammonium dihydrogen phosphate as the sole source of nitrogen, other nutrients, and the pH indicator bromthymol blue. At pH 7.5 or above, bromthymol blue turns royal blue. At a neutral pH, bromthymol blue is green

Organisms that can utilize citrate as their sole carbon source convert the ammonium dihydrogen phosphate to ammonia and ammonium hydroxide, which creates an alkaline environment in the medium. 

Reading the Results

If the medium turns blue, the organism is citrate positive.  If there is no color change, the organism is citrate negative.  Some citrate negative organisms may grow weakly on the surface of the slant, but they will not produce a color change.

Application of the Citrate Test

You will use the citrate test when you are identifying your Gram negative unknown. Because it is part of the IMViC panel, you will use it at the start of your flowchart or dichotomous key, directly after isolating your organism from your MAC plate.