Simmons' citrate agar

Simmons' Citrate Agar is a selective medium that tests for an organism's ability to utilize 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 turns blue at a pH of 7.6 or greater. When an organism catabolizes citrate, it produces alkaline waste products, causing the medium to turn blue. Furthermore, only an organism that can utilize citrate will produce visible growth on the citrate slant.