Citrate test

Citrate Test is a biochemical test used to determine the ability of an organism to utilize citrate as its sole carbon source. This test is significant in the field of microbiology, particularly in the identification of Gram-negative enteric bacteria, such as those belonging to the genera Escherichia, Klebsiella, and Enterobacter. The citrate test is part of the IMViC series—a group of tests including Indole, Methyl Red, Voges-Proskauer, and Citrate tests—used for differentiating Escherichia coli from other enteric bacteria.

Background

Citrate, the salt or ester of citric acid, serves as a key intermediate in the citric acid cycle (also known as the Krebs cycle or TCA cycle), which is a central metabolic pathway in all aerobic organisms. The citrate test specifically assesses an organism's ability to use citrate as a sole carbon source for energy and to produce bicarbonate ions from citrate, leading to alkaline conditions in the medium. This ability is mediated by the enzyme citrate permease, which facilitates the transport of citrate into the cell.

Procedure

The citrate test is performed using Simmons Citrate Agar, which contains sodium citrate as the sole carbon source and ammonium dihydrogen phosphate as the sole nitrogen source. The medium also contains bromothymol blue, a pH indicator that turns from green to blue at a pH above 7.6, indicating alkaline conditions and a positive test result.

1. A sterile inoculating loop is used to transfer a small amount of the test organism onto the surface of the agar. 2. The inoculated agar is then incubated at 35-37°C for up to 48 hours. 3. A change in the color of the medium from green to blue indicates a positive result, meaning the organism can utilize citrate as its sole carbon source. 4. No color change (remaining green) indicates a negative result, meaning the organism cannot utilize citrate.

Interpretation

A positive citrate test is indicative of an organism's ability to use citrate as its sole carbon source and is characteristic of certain enteric bacteria, such as Klebsiella pneumoniae and Enterobacter aerogenes. A negative test result is typical for organisms like Escherichia coli, which cannot utilize citrate in the presence of oxygen.

Clinical Significance

The citrate test is used in clinical microbiology laboratories as part of the process to identify and differentiate among members of the Enterobacteriaceae family. It helps in diagnosing infections caused by specific pathogens and in determining the appropriate treatment.

Limitations

While the citrate test is useful for differentiating among species of enteric bacteria, it is not definitive on its own. It should be used in conjunction with other biochemical tests for accurate identification of bacterial species.