Simmons' citrate agar

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Simmons' Citrate Agar (not planted yet)


Simmons' Citrate Agar is a defined, enrichment medium that tests for an organism's ability to use citrate as a sole carbon source and ammonium ions as the sole nitrogen source.

The medium contains citrate, ammonium ions, and other inorganic ions needed for growth.

It also contains bromothymol blue, a pH indicator. Bromothymol blue is green at pH below 6.9, and then turns blue at a pH of 7.6 or greater.

Physical Format[edit]

Although it could be used in other formats (e.g., Petri plates), Simmons Citrate Agar is often used in slants.


Organisms growing on Simmons Citrate Agar are capable of using citrate as the sole carbon source and they can metabolize the ammonium salt in the medium.

Use of citrate results in the creation of carbonates and bicarbonates as byproducts, thus increasing the pH of the medium.[1] The increase in pH then causes color change in the bromothymol blue indicator, turning it blue. Under acidic condition it changes to yellow color. This color change is useful because growth on Simmons Citrate Agar is often limited and would be hard to observe if it were not for the color change.

Sometimes, it is possible to detect growth on the Simmons Citrate Agar without the accompanying color change to blue. This is most likely due to insufficient incubation. Either a combination of blue color and growth or growth alone without the blue color should be scored as a positive for the citrate use test.


  1. ^ Koser, S.A. (1923). "Utilization of the salts of organic acids by the colon-aerogenes group.". Journal of Bacteriology 8: 493–520. 

External Links[edit]

ASM MicrobeLibrary article "Citrate Test Protocol"