EC Medium

Intended Use

EC Medium is a culture medium for the detection of coliform bacteria at 35°C and of Escherichia coli at an elevated temperature (44.5 or 45.5°C).

Summary and Explanation

EC Medium was devised by Hajna and Perry¹ and is used for the examination of water, milk, shellfish and other material for evidence of fecal pollution. Tennant et al. reported on the use of this medium for the estimation of E. coli densities in seawater and shellfish.² Fishbein and Surkiewicz used the EC confirmation test for recovery of E. coli from frozen foods and nut meats and reported that the test worked optimally when conducted at 45.5°C with incubation being limited to 24 hours.³
EC Medium is recommended for use in the fecal coliform Most Probable Number (MPN) procedure for the examination of water, wastewater and foods.^4,5 The procedure employing EC Medium provides information regarding the source of the coliform group (fecal or nonfecal) when used as a confirmatory test.⁶ It should not be used for the direct isolation of coliforms since prior enrichment in a presumptive medium for optimal recovery of fecal coliforms is required.

Principles of the Procedure

EC Medium contains peptone as a source of nutrients. Lactose provides fermentable carbohydrate for the growth of coliforms. Bile salts are inhibitory for gram-positive bacteria, particularly bacilli and fecal streptococci. The medium has a strong potassium phosphate buffering system to control the pH in the presence of considerable fermentative action. Sodium chloride maintains the osmotic balance of the medium.

User Quality Control

Identity Specifications
EC Medium
Dehydrated Appearance: Light beige, freeflowing,homogeneous.
Solution:                       3.7% solution, soluble in purified water upon warming. Solution
                                   is light amber, clear.
Prepared Appearance:    Light amber, clear.
Reaction of 3.7%
Solution at 25°C:           pH 6.9 ± 0.2
Cultural Response
EC Medium
Prepare the medium per label directions. Inoculate and incubate tubes with fermentation vials at 44.5 ± 0.2°C for 24 ± 2 hours.

Formula

EC Medium
Approximate Formula* Per Liter
Tryptose..................................................................... 20.0 g
Lactose........................................................................ 5.0 g
Bile Salts No. 3.............................................................. 1.5 g
Dipotassium Phosphate.................................................. 4.0 g
Monopotassium Phosphate............................................. 1.5 g
Sodium Chloride............................................................ 5.0 g
*Adjusted and/or supplemented as required to meet performance criteria.

Directions for Preparation from Dehydrated Product

1. Dissolve 37 g of the powder in 1 L of purified water. Mix thoroughly.
2. Warm slightly to completely dissolve the powder.
3. Dispense into tubes containing inverted fermentation vials.
4. Autoclave at 121°C for 15 minutes.
5. Test samples of the finished product for performance using stable, typical control cultures.

Procedure

Refer to the various compendia for the specific procedures employing EC Medium.^4-8

Expected Results

Refer to the compendia for the results expected when using this medium for the detection of coliforms and E. coli.^4-8

Limitations of the Procedure

False-negative reactions in recovering coliforms from water supplies can occur due to low pH, refrigeration and use of bactericidal or bacteriostatic agents.⁹

*Store at 2-8° C.

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