Citrate Utilisation Test

 

Citrate Utilisation Test

Aim

To differentiate bacteria on the basis of their ability to utilise citrate as the sole carbon source.

Principle

Citrate utilisation test identifies the ability of an organism to utilize citrate as a source of energy. Certain bacteria possess citrate permease and citrate lyase enzymes, enabling them to utilize sodium citrate as the sole carbon source and ammonium salts as the nitrogen source for growth, with production of CO₂. Simmons citrate agar is formulated with sodium citrate (carbon source), ammonium dihydrogen phosphate (nitrogen source), buffers, and bromothymol blue (pH indicator; green at pH ≈6.9). When the bacteria metabolize citrate, the ammonium salts are broken down to ammonia, which increases alkalinity.  The shift in pH turns the bromthymol blue indicator in the medium from green to Prussian blue above pH 7.6

 

Materials required

Simmon citrate agar

24-48 hour old bacterial culture

Inoculating loop

Bunsen burner

Procedure

Simmon citrate agar slants were prepared.

The test organism was inoculated onto the slant surface using an inoculating loop.

The slant was incubated at 37oC for 24 to 48 hours.

 

Result

The positive result indicate the change of colour from green to prussian blue. The negative result indicates no colour change.

Examples

Positive: Peudomonas sp

Negative: E coli, Staphylococcus sp, Proteus sp, Bacillus sp

 

 

 

 

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