citrate test

(Simmons citrate test)

A test to determine the ability of an organism to use citrate as its sole carbon source of energy. For example, it is performed as a part of IMViC test (Indole, Methyl Red, Voges Proskauer, and Citrate) to differentiate members of Enterobacteriaceae.

It uses Simmons' citrate agar, that contains citrate and ammonium dihydrogen phosphate (NH4)(H2PO4) and bromothymol blue as an indicator. The agar is green before inoculation, and turns blue (at pH 7.5 or above) as a positive test indicator, meaning citrate is used.

Organisms that give a positive result are capable of converting citrate to pyruvate with the enzyme citrate-permease (citrase). Pyruvate can then enter the organism's metabolic cycle (Krebs cycle) for the production of energy. During the metabolism, ammonium dihydrogen phosphate is broken down to ammonia (NH3), increasing the pH.

Positive:
- Klebsiella pneumoniae
- Enterobacter species (majority of strains)
- Salmonella (except Typhi and Paratyphi A)
- Citrobacter freundii
- Serratia marcescens
- Proteus mirabilis (majority of strains)
- Edwardsiella
- Providencia

Negative:
- Escherichia coli
- Shigella
- Salmonella Typhi
- Salmonella Paratyphi A
- Morganella morganii
- Yersinia enterocolitica

Variable:
- Proteus vulgaris
- Vibrio cholerae
- Vibrio parahaemolyticus