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 betweenEscherichiacoliandEnterobacteraerogenesand to assist in differentiating members of theSalmonellagenus. Organisms able to metabolize the citrate (mostSalmonellaspp., someEnterobacteriaceae, mostKlebsiella 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.