Simmons Agar Base

Simmons Agar Base is a synthetic medium recommended for differentiation between faecal coliform and members of the aerogenes group on the basis of citrate utilization. Bromothymol blue is the pH indicator. Ammonium dihydrogen phosphate and sodium citrate serve as the sole nitrogen and carbon source respectively. Microorganisms also use inorganic ammonium salts as their sole nitrogen source. Metabolism of these salts causes the medium to become alkaline, indicated by a change in colour of the pH indicator from green to blue. Enterobacter aerogenes, Salmonella Typhimurium, Salmonella Enteritidis are citrate positive cultures while Escherichia coli, Salmonella Typhi, Shigella dysenteriae do not utilize citrate.