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.
Read more about Simmons' Citrate Agar: Physical Format, Interpretation