Citrate Test

Citrate is an organic molecule that can be utilized by some bacteria but not by others.

The Simmon's citrate agar plates or slants:

- Determine whether an organism produces citrase
- Content: citrate as the only carbon source and bromthymol blue (pH indicator)

Positive test:

Bacteria with the enzyme citrase metabolize citrate to produce alkaline end-products that raise the pH of the medium to 7.6, causing the bromthymol blue to turn blue.

Citrate(green) -------Citrase------> oxaloacetic acid + acetic acid

Oxaloacetic acid --------------------> Pyruvic acid + Carbon dioxide
Carbon dioxide + Sodium ions ----------> Sodium Carbonate(pH increases)

[Blue and growth]

Negative test:

Citrate(green) -------No citrase-----> Citrate(pH unchanged)
[Green and no growth]

http://www.medschool.lsuhsc.edu/Microbiology/DMIP/citrate.jpg