Simmons Citrate Agar
Simmons Citrate Agar is used as a differential medium for the identification of enteric bacteria by using Citrate as the sole source of carbon and Ammonium as the sole source of nitrogen. Koser developed a fluid medium to determine citrate utilization, with Simmon further modifying the formula by adding 1.5% agar and Bromthymol Blue as an indicator. The medium can be used to distinguish between Escherichia coli and Enterobacter aerogenes and to assist in differentiating members of the Salmonella genus. Organisms able to metabolize the citrate (most Salmonella spp., some Enterobacteriaceae, most Klebsiella spp.) grow well on the surface of the medium and alkalinize the medium, changing it from green to blue. In turn, non-citrate fermenters (E. coli, Shigella spp.) are unable to shift the color.