Simmons Citrate Agar is used for the differentiation of gram-negative bacteria on the basis of citrate utilization.
SUMMARY AND EXPLANATION
Koser, in 1923, developed a liquid medium consisting of inorganic salts in which an ammonium salt was the only source of nitrogen and citrate was the sole carbon source in order to differentiate between what are now known as Escherichia coli and Enterobacter aerogenes as part of the IMViC (Indole- Methyl Red-Voges Proskauer-Citrate) reactions. Simmons, in 1926, modified Koser’s formulation with the addition of 1.5% agar and bromthymol blue. Organisms capable of metabolizing citrate grow well on this medium.
Organisms able to utilize ammonium dihydrogen phosphate and sodium citrate as the sole sources of nitrogen and carbon, respectively, will grow on this medium and produce an alkaline reaction as evidenced by a change in the color of the bromthymol blue indicator from green (neutral) to blue (alkaline).