Simmons’ citrate agar was developed by James S. Simmons in 1926 by adding 1.5% agar and bromothymol blue as a pH indicator to Koser’s citrate agar to observe changes in pH as a result of oxidative reactions from citrate metabolism.
The development of Simmons' citrate agar was driven by the need to observe pH changes associated with citrate metabolism in bacteria. This agar utilizes bromothymol blue as a pH indicator to visualize these changes.