A citrate test determines if a microbe can grow by using citrate as its only source of energy and carbon. If the microbe grows on Simmons citrate agar, it passes the test.
Using citrate leads to an increase in pH as alkaline byproducts accumulate in the microbe, according to the producers of VirtualUnknown Microbiology. Simmons citrate agar slant is used as the medium. It contains sodium citrate and mineral salts that supply carbon, and it contains ammonium phosphate as a nitrogen source. A pH indicator used for the test is brom thymol blue. When the pH is neutral, the color turns green, yellow means the pH is acidic and blue equals an alkaline pH.