Common Biochemical Tests in Microbiology: Utilization of Citrate as the Sole Source of Carbon

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BACKGROUND

This test is among one of the biochemical test of IMViC tests which is used to differentiate to the gram-negative bacilli in the family Enterobacteriaceae.

Principle: The medium citrate agar is used to test an ability of an organism to use citrate as a source of carbon. Bacteria growing on this medium will produce an enzyme citrate permease, that will convert citrate to pyruvate. Then pyruvate enter the metabolic cycle and generates energy. When the organism metabolize citrate, the ammonium salts present are broken down to ammonia that in turn enhances the alkanity. This in converts the colour of the bromothymol indicator present in the medium from green to blue.

So, the basic objective of this test is to test for the utilization of citrate as sole source of carbon.

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REQUIREMENTS

Sample:         Bacterial sample

Media:          Simmon citrate agar

              Water

Apparatus:   Test tube

      Inoculation tube

      Incubator

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PROCEDURE

Prepare Simmons citrate agar medium and autoclave it. Then after it becomes little cool, prepare the slant. One for control and another for inoculating organism. Then in the test tube for inoculation take s small amount of organism in the inoculating loop and streak it in the slant agar. Then incubate it at 37^oC for about 24 hours.¹ Then after incubation observe for the colour observation. If the colour changes to blue colour then it is citrate positive, if no colour change then it is citrate negative.²

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CONCLUSION

Citrate utilization test is used for identification of gram negative pathogens of the Enterobacteriaceae and other environmental organisms.

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REFERENCES

1. McFadden JF. Biochemical Tests for Identification of Medical Bacteria, 2nd ed. Williams and Wilkins, Baltimore.1980.
   
2. Bachoon, Dave S, Wendy A. Dustman. Microbiology Laboratory Manual. Ed. Michael Stranz. Mason, OH: Cengage Learning. Exercise 15, “Normal Flora of the Intestinal Tract” Print. 2008.