Citrate test

The citrate test detects the ability of an organism to use citrate as the sole source of carbon and energy.

Principle

Bacteria are inoculated on a medium containing sodium citrate and a pH indicator such as bromothymol blue. The medium also contains inorganic ammonium salts, which are utilized as sole source of nitrogen. Use of citrate involves the enzyme citrate lyase, which breaks down citrate to oxaloacetate and acetate. Oxaloacetate is further broken down to pyruvate and carbon dioxide (CO₂). Production of sodium bicarbonate (NaHCO₃) as well as ammonia (NH₃) from the use of sodium citrate and ammonium salts results in alkaline pH. This results in a change of the medium's color from green (neutral) to blue (alkaline).

Bacterial colonies are picked up from a straight wire and inoculated into slope of Simmons citrate agar and incubated overnight at 37 °C. Inoculating from a broth culture is not recommended because the inoculum would be too heavy. If the organism has the ability to use citrate, the medium usually changes its color from green to blue, though growth on the medium even without colour change is considered a positive result.^[1] An observation of no growth is a negative result.

Raoultella planticola

Proteus myxofaciens

Two bacteria (both Enterobacteriaceae) on citrate agar, showing a positive result (left side) and a negative result (right side) for the citrate test.

Examples:

Escherichia coli: Negative
Klebsiella pneumoniae: Positive
Frateuria aurantia: Positive

References

^ A. Forbes, Betty; Daniel F. Sahm; Alice S.Weissfeld (1998). BAILEY & SCOTT'S Diagnostic Microbiology (tenth ed.). Don Ladig. p. 430. ISBN 0-8151-2535-6.

Techniques in clinical microbiology

Isolation
and culture

Isolation techniques	Asepsis Streak plate Selective media
Cultures by body site	Blood culture Genital culture Sputum culture Throat culture Urine culture Wound culture
Cultures by organism	Bacterial culture Fungal culture Viral culture

Identification
and testing

Manual testing: basic techniques	Colonial morphology Hemolysis Staining Gram stain Acid-fast stain Giemsa stain India ink stain Ziehl–Neelsen stain Wet prep Rapid tests Oxidase Catalase Indole PYR
Manual testing: biochemical and immunologic tests	ALA test Amino acid decarboxylase test Bile solubility test CAMP test Citrate test Coagulase test DNAse test IMViC KOH test Methyl red test Nitrite test ONPG test Oxidative/fermentation glucose test Phenylalanine deaminase test Reverse CAMP test Salt tolerance test Sulfide indole motility test Triple sugar iron test Urease test rapid Voges–Proskauer test X and V factor test Bacitracin susceptibility test Optochin susceptibility test Novobiocin susceptibility test Lancefield grouping RPR test
Automated and point-of-care testing	Analytical profile index MALDI-TOF Polymerase chain reaction VITEK system Rapid strep test Monospot test
Antibiotic susceptibility testing	Beta-lactamase test Disk diffusion test Etest McFarland standards Minimum inhibitory concentration