Simmons' citrate agar
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Simmons' Citrate Agar is a defined, enrichment medium that tests for an organism's ability to use citrate as a sole carbon source and ammonium ions as the sole nitrogen source.
Although it could be used in other formats (e.g., Petri plates), Simmons Citrate Agar is often used in slants.
Organisms growing on Simmons Citrate Agar are capable of using citrate as the sole carbon source and they can metabolize the ammonium salt in the medium.
Use of citrate increases the pH of the medium. The increase in pH then causes color change in the bromothymol blue indicator, turning it blue. Under acidic condition it changes to yellow color. This color change is useful because growth on Simmons Citrate Agar is often limited and would be hard to observe if it were not for the color change.
Sometimes, it is possible to detect growth on the Simmons Citrate Agar without the accompanying color change to blue. This is most likely due to insufficient incubation. Either a combination of blue color and growth or growth alone without the blue color should be scored as a positive for the citrate use test.
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