Simmons' citrate agar

[3] In a sample, E. coli, which is citrate-negative, can be distinguished from non-fecal, citrate-positive coliforms that are often found in water, soil, and on plants using Simmons’ agar.

Additionally, Simmons’ agar is commonly used as part of the IMViC tests to identify coliforms.

[6] With a colorless agar, misinterpreting negative results as positives is more common, which can be reduced by Simmons’ modification of adding bromothymol blue.

The medium is prepared in 1 litre of deionized water at pH 6.9 ± 0.2 (at 25°C) with the following composition,[7][8] Although it could be used in other formats (e.g., Petri plates), Simmons' citrate agar is often used in slants or slopes in test tubes.

One advantage of using slants over Petri dishes is access to oxygen, which is required by citrate metabolism.

Organisms degrading citrate must also use the ammonium salts, producing ammonia,[10] thus increasing the pH of the medium.

Sometimes, it is possible to detect growth on the Simmons' citrate agar without the accompanying color change to blue.