Cat. no. R135 Koser Citrate Broth, 13x100mm Tube, 4ml 20 tubes/box


Hardy Diagnostics' Koser Citrate Broth is recommended for the differentiation of Escherichia coli from Enterobacter aerogenes based on citrate utilization. This broth is used primarily for coliform determinations from water and wastewater samples.


The coliform group of bacteria are the subject of continued interest because they are so widely recognized as sanitary indicators of water quality, food spoilage, food-production plants and restaurants. Some coliforms occur naturally and in great abundance in the soil, on plants, and in aquatic environments. Species in the genera Enterobacter are largely accepted as nonfecal in origin, whereas species of Escherichia are commonly recognized as indicators of fecal contamination. Consequently, procedures for detecting, enumerating and presumptively identifying between these types of coliforms are widely used in the food, water and dairy industries. (1-4,9)

In the early 1920s, Koser reported finding a significant characteristic in differentiating strains from these two genera; he found that sodium citrate can be used as a sole source of carbon by Enterobacter aerogenes , but not by Escherichia coli . (5,7,8) In the absence of alternate carbon and nitrogen sources, E. coli fails to grow. (10) Koser later reported the correlation of citrate utilization along with other differential characteristics as pertaining to the organism's natural habitat; a trait not readily acquired or lost. (6)

Koser developed citrate medium to distinguish between Escherichia coli and Enterobacter aerogenes . Hardy Diagnostics' Koser Citrate Broth is based on this formulation and contains sodium citrate as a single source of carbon; ammonium phosphate salts act as a source of nitrogen; magnesium sulfate is an inorganic co-factor that facilitates metabolic reactions, and monopotassium phosphate is a buffering agent. Bacteria able to use citrate as a sole carbon source will grow in the medium and cause turbidity. The formulation is listed in Official Methods of Analysis by the AOAC and Standard Methods for the Examination of Water and Wastewater by the APHA for the identification of microorganisms found in water. (1,2)


Ingredients per liter of deionized water:*

Sodium Citrate 3.0gm
Sodium Ammonium Phosphate 1.5gm
Monopotassium Phosphate 1.0gm
Magnesium Sulfate 0.2gm

Final pH 6.7 +/- 0.2 at 25ºC.

* Adjusted and/or supplemented as required to meet performance criteria.


Storage: Upon receipt store at 2-30ºC. away from direct light. Media should not be used if there are any signs of deterioration, discoloration, contamination, or if the expiration date has passed. Product is light and temperature sensitive; protect from light, excessive heat, moisture, and freezing.



1. Obtain a loopful of liquid suspension or growth of a single colony and inoculate the broth medium.

2. Incubate tubes with loose caps at 35 +/- 2ºC. for 18-24 hours.


Compare results to an uninoculated tube. Bacteria able to utilize citrate as their carbon source will grow in the medium and produce a cloudiness or turbidity. Bacteria unable to utilize citrate will not grow and the broth should remain clear.


Use of a heavy inoculum upon initial culture could be misinterpreted as a positive test for growth. It is recommended that only a single loopful of liquid suspension or colony be used to inoculate the broth.

The presence of contaminating chemicals, however small, introduced with the inoculum may result in the erroneous growth of E. coli or other negative microorganisms. Extreme care is needed not to include any carryover of various media contaminants, including agar. (10)


Standard microbiological supplies and equipment such as loops, swabs, applicator sticks, other culture media, incinerators, and incubators, etc., as well as serological and biochemical reagents, are not provided.


Test Organisms Inoculation Method* Incubation Results
Time Temperature Atmosphere
Enterobacter aerogenes
ATCC ® 13048
A 18-24hr 35°C Aerobic Growth
Escherichia coli
ATCC ® 25922
A 18-24hr 35°C Aerobic Partial to complete inhibition

Check for signs of contamination and deterioration. Users of commercially prepared media may be required to perform quality control testing with at least one known organism to demonstrate growth or a positive reaction; and at least one organism to demonstrate inhibition or a negative reaction (where applicable). Refer to the following keywords, in the Hardy Diagnostics software program HUGO™, for more information on QC: "Introduction to QC", "QC of Finished Product", and "The CLSI (NCCLS) Standard and Recommendations for User QC of Media". Also see listed references for more information. (1-4,9)

Physical Appearance

Koser Citrate Broth should appear clear, and colorless.


1. American Public Health Association. Standard Methods for the Examination of Water and Wastewater, APHA, Washington, D.C.

2. Association of Official Analytical Chemists. Official Methods of Analysis , AOAC, Washington, D.C.

3. American Public Health Association. Standard Methods for the Examination of Dairy Products, APHA, Washington, D.C.

4. APHA Technical Committee on Microbiological Methods for Foods. Compendium of Methods for the Microbiological Examination of Foods, APHA, Washington, D.C.

5. Koser, S.A. 1923. Utilization of the Salts of Organic Acids by the Colon-Aerogenes Group. J. Bacteriol. ; 8:493-520.

6. Koser, S.A. 1924. Correlation of Citrate Utilization by Members of the Colon-Aerogenes Group with Other Differential Characteristics and with Habitat. J. Bacteriol. ; 9:59-77.

7. Koser, S.A. 1924. Differential Tests for Colon-Aerogenes Group in Relation to Sanitary Quality of Water. J. Infectious Diseases. ; 35:14-22.

8. Koser, S.A. 1924. A New Differential Test for Members of the Colon Group of Bacteria. J. Am. Water Works Assoc. ; 12:200-205.

9. U.S. Food and Drug Administration. Bacteriological Analytical Manual. AOAC, Arlington, VA. www.fda.gov/Food/ScienceResearch/LaboratoryMethods/BacteriologicalAnalyticalManualBAM/default.htm .

10. Vaughn, R.H., J.T. Osborne, G.T. Wedding, J. Tabachnick, C.G. Beisel and T. Braxton. 1950. The Utilization of Citrate by Escherichia coli . J. Bacteriol. ; 60(2):119-127

ATCC is a registered trademark of the American Type Culture Collection.