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Enteric Rods: Citrate test

Bacteria that utilize citrate as their sole source of carbon for growth and metabolism also utilize ammonium salts as their sole nitrogen source. The ammonium salts are broken down to ammonia, an alkaline product that turns the pH indicator a deep blue. The results of the citrate test shown here are as follows:

The right tube is positive (blue).
The left tube is negative (green).