Simmons Citrate agar is a medium containing citrate as the sole carbon source and ammonium salts as the sole nitrogen source. Organisms that metabolize citrate utilize the ammonium salts releasing ammonia and increasing the pH of the medium. Brom thymol blue is present in the medium as the indicator dye. It is green at neutral pH and deep blue above pH 7.6. Koser's citrate broth is another medium used to test for citrate utilization.
Growth is evidence of a positive reaction.
Using a sterile inoculating needle, streak one organism over the surface of the agar slant, then stab the butt. Repeat with the second organism. Examine for growth. Several media are designed to yield more than one biochemical reaction. Check for motility by observing migration of the inoculum from the stab line through the semisolid medium.