Citrate Utilization

The testing media:

(1) must not contain protein and carbohydrates.

(2) contains ammonium phosphate as the sole source of nitrogen

(3) contains citrate as the sole source of carbon

A pure isolate is inoculated into the media and incubated at 35°C for 24 to 48 hours.

If the organism can use citrate then alkaline products are released, which changes the color indicator bromthymol blue from green to blue.

Interpretation:

• A positive test is indicated by a deep blue color and/or by visible growth of the organism.

Causes of a false positive:

(1) mixed isolate

(2) media not made properly

Causes of a false negative:

(1) reading the test too soon