The citrate test determines whether a bacterium can survive using citrate as its only carbon source. It’s one of the standard biochemical tests used in microbiology labs to identify unknown bacteria, particularly members of the Enterobacteriaceae family. The test is simple: inoculate a special agar slant, incubate it, and watch for a color change from green to blue.
What the Test Actually Measures
Every living organism needs carbon to build its cellular structures and fuel its metabolism. Most bacteria can use a variety of carbon sources, like glucose or lactose. The citrate test strips away those options. The medium, called Simmons citrate agar, contains sodium citrate as the sole carbon source and an ammonium salt as the sole nitrogen source. If the bacterium can grow under these minimal conditions, it tells you something specific about its metabolic capabilities.
When a bacterium breaks down citrate, it produces alkaline byproducts, mainly carbonates and bicarbonates. These compounds absorb free hydrogen ions in the medium, raising the pH. The agar contains a pH indicator called bromothymol blue, which stays deep forest green when the pH is between 6.9 and 7.6. Once the pH rises above 7.6, the indicator shifts to an intense blue, sometimes described as Prussian blue. That color change is your positive result.
How to Read the Results
A positive result means the agar slant turns blue, with or without visible bacterial growth on the surface. The blue color confirms that the organism metabolized citrate and generated enough alkaline byproducts to shift the pH. A negative result means the slant stays green and shows little to no growth. The organism either couldn’t transport citrate into its cells or lacked the enzymes to break it down.
One important detail: even trace growth without a color change still counts as negative. The color shift is the definitive indicator, not just the presence of bacteria on the slant.
Why It Matters for Identifying Bacteria
The citrate test is one of four tests in a classic series called the IMViC battery (Indole, Methyl Red, Voges-Proskauer, and Citrate). Together, these tests help distinguish between closely related species of gram-negative bacteria, especially the rod-shaped members of Enterobacteriaceae. This family includes many of the organisms responsible for urinary tract infections, foodborne illness, and hospital-acquired infections.
Some of the most clinically relevant distinctions the citrate test helps make:
- Citrate-positive species include Klebsiella pneumoniae, Enterobacter aerogenes, and Citrobacter freundii. These organisms grow readily on citrate agar and produce a clear blue color change.
- Citrate-negative species include Escherichia coli and Proteus mirabilis. They cannot use citrate as a sole carbon source, so the slant remains green.
This distinction is especially useful for separating E. coli (citrate-negative) from Klebsiella and Enterobacter (both citrate-positive), organisms that can otherwise look similar on other tests. In water quality testing, for instance, distinguishing between fecal coliforms like E. coli and non-fecal coliforms is critical, and the IMViC battery including the citrate test is a standard tool for making that call.
The Biochemistry Behind the Color Change
Bacteria that test positive possess a citrate permease, a transport protein that pulls citrate across the cell membrane. Once inside the cell, enzymes convert citrate to oxaloacetate, which is then converted to pyruvate. Along the way, the process generates carbonate as a byproduct. Carbonate is strongly alkaline. It reacts with hydrogen ions in the surrounding medium, effectively making the environment more basic and driving the pH upward.
This pH shift does more than trigger a color change in a lab test. Research on Vibrio cholerae has shown that citrate metabolism and the resulting carbonate production can raise the environmental pH enough to actively inhibit the growth of competing bacteria like E. coli. Mutant strains of V. cholerae that lack the enzymes for converting citrate to oxaloacetate lose this competitive advantage entirely. So the same metabolic pathway the citrate test detects has real ecological significance for how bacteria compete in the gut.
How the Test Is Performed
The procedure is straightforward. A single colony of the unknown bacterium is picked from a pure culture using a needle (not a loop, to avoid transferring too much nutrient-rich media). The needle is used to streak the surface of a Simmons citrate agar slant lightly. The tube is then incubated at 35 to 37°C, typically for 24 to 48 hours, though some slow-growing organisms may need up to four days.
A common source of error is using too heavy an inoculum. If you transfer a large clump of bacteria along with residual nutrients from the original growth medium, the organism may grow briefly on those carried-over nutrients rather than on citrate itself. This can produce a misleading color change. Keeping the inoculum light and using a needle rather than a loop helps avoid false positives. It’s also important to use a pure culture, since contamination with a citrate-positive organism will obviously skew the result.
Where the Citrate Test Fits in Lab Workflow
No single biochemical test identifies a bacterium on its own. The citrate test is always used alongside other tests. In a typical microbiology lab workflow, an unknown gram-negative rod would first be characterized by colony appearance, Gram stain, and whether it ferments lactose. From there, a battery of biochemical tests narrows the identification. The IMViC pattern, expressed as a series of plus and minus signs for each of the four tests, points toward a specific genus and species.
For example, the classic IMViC pattern for E. coli is + + − − (indole positive, methyl red positive, VP negative, citrate negative). Enterobacter aerogenes gives − − + + (the mirror image). These patterns, combined with additional tests when needed, allow reliable identification without expensive molecular methods. The citrate test remains a standard part of this toolkit in clinical, environmental, and food microbiology labs worldwide.