Koser Citrate Medium

Intended Use

Koser Citrate Medium is used for differentiating Escherichia coli from Enterobacter aerogenes based on citrate utilization.

Summary and Explanation

In 1923, the work of Koser demonstrated that coli-aerogenes bacteria could be differentiated by their use of certain salts of organic acids.¹ Koser found that the sodium salt of citric acid (sodium citrate) is used as a source of carbon by E. aerogenes and not by E. coli. Biochemical identification schemes for identifying E. coli frequently include Koser citrate.
E. coli is an important member of the coliform group of bacteria. The coliforms are described as aerobic and facultatively anaerobic gram-negative non-sporeforming bacilli that ferment lactose and form acid and gas at 35°C within 48 hours. Procedures to detect, enumerate and presumptively identify coliforms are used in testing foods and dairy products.^2-5 Presumptive identification is confirmed by performing biochemical tests that specifically identify E. coli.

Principles of the Procedure

Koser Citrate Medium is prepared with chemically pure salts and tested to determine that no sources of carbon (other than sodium citrate) or nitrogen (other than ammonium salts) are present. Bacteria that are able to use citrate as their carbon source will grow in the medium and cause turbidity.

User Quality Control

Identity Specifications
Koser Citrate Medium
Dehydrated Appearance: White, free-flowing, homogeneous.
Solution:                        0.57% solution, soluble in purified water. Solution
                                     is colorless, clear.
Prepared Appearance:    Colorless, clear.
Reaction of 0.57%
Solution at 25°C:            pH 6.7 ± 0.2
Cultural Response
Koser Citrate Medium
Prepare the medium per label directions. Inoculate and incubate at 35 ± 2°C for 18-24 hours.

Formula

Koser Citrate Medium
Approximate Formula* Per Liter
Sodium Ammonium Phosphate.................................... 1.5 g
Monopotassium Phosphate.......................................... 1.0 g
Magnesium Sulfate..................................................... 0.2 g
Sodium Citrate.......................................................... 3.0 g
*Adjusted and/or supplemented as required to meet performance criteria.

Directions for Preparation from Dehydrated Product

1. Dissolve 5.7 g of the powder in 1 L of purified water.
2. Autoclave at 121°C for 15 minutes.
3. Test samples of the finished product for performance using stable, typical control cultures.

Procedure

1. Transfer growth from a single colony or a loopful of liquid suspension and inoculate the broth medium.
2. Incubate at 35 ± 2°C for 18-24 hours.

Limitations of the Procedure

Positive:...................Turbidity
Negative:.................Clear, no turbidity

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