Citrate Utilization

Citrate agar slants contain sodium citrate, bromthymol blue (pH indicator), sodium, and water. If an organism is able to utilize citrate, an enzyme called citrase will catabolize the citrate into oxaloacetic acid and acetic acid. The oxaloacetic acid is then broken down into pyruvate and the acetic acid is converted to CO₂. The CO₂ reacts with the water and sodium in the media to produce alkaline sodium carbonate. The sodium carbonate reacts with the pH indicator to produce a “Caribbean Blue” color. 

Tube 1:  Negative for citrate utilization.  Note that there is no color change, and there is no growth on the slant itself.

Tube 2:  Positive for citrate utilization.  Note that the color has changed to "Caribbean blue", and there is also growth on the surface of the slant.

Back to Tutorial