Enteric Fermentation Base

Intended Use

Enteric Fermentation Base is used with added carbohydrate and indicator for differentiating microorganisms based on fermentation reactions.

Summary and Explanation

The fermentative properties of bacteria are valuable criteria in their identification.^1-4 A basal medium for determining the fermentation reactions of microorganisms must be capable of supporting growth of test organisms and be free from fermentable carbohydrates. Enteric Fermentation Base is prepared according to the formula described by Edwards and Ewing.⁵

Principles of the Procedure

Beef extract and peptone provide the carbon and nitrogen sources required for good growth of a wide variety of organisms. Sodium chloride maintains the osmotic balance of the medium. The microorganisms tested are differentiated by their ability to ferment a particular carbohydrate that has been added to the Enteric Fermentation Base. The fermentation and resultant acid production are indicated by a change in color of the pH indicator (Andrade’s indicator) which is also added to the Enteric Fermentation Base.

User Quality Control

Identity Specifications
Enteric Fermentation Base
Dehydrated Appearance: Light tan, free-flowing, homogeneous.
Solution:                       1.8% solution, soluble in purified water. Solution is light amber,
                                   clear.
Prepared Medium (plain)
+ Andrade’s Indicator:    Light pinkish-amber, clear.
Reaction of 1.8%
Solution at 25°C:           pH 7.2 ± 0.1
Cultural Response
Enteric Fermentation Base
Prepare the medium per label directions, without and with 1% dextrose. Inoculate with fresh cultures and incubate at 35 ± 2°C for 18-24 hours. Acid production is indicated by a
change in color from light amber to dark pink or red. Check for gas production in at least 3% of the volume of the fermentation vial.

Formula

Enteric Fermentation Base
Approximate Formula* Per Liter
Beef Extract.................................................................. 3.0 g
Peptone..................................................................... 10.0 g
Sodium Chloride........................................................... 5.0 g
*Adjusted and/or supplemented as required to meet performance criteria.

Directions for Preparation from Dehydrated Product

1. Suspend 18 g of the powder in 1 L of purified water. Mix thoroughly.
2. Add 10 mL of Andrade’s indicator.
3. Heat with frequent agitation and boil for 1 minute to completely dissolve the powder.
4. Autoclave at 121°C for 15 minutes.
5. Cool to 45-50°C in a water bath.
6. Aseptically add 0.5% or 1% of sterile carbohydrate (see table).

 7. Dispense 9 mL amounts into test tubes containing inverted vials (Durham tubes).

8. Test samples of the finished product for performance using stable, typical control cultures.

Procedure

For a complete discussion on identification of Enterobacteriaceae, refer to the appropriate procedures outlined in the references.^1-4,6

Expected Results

A positive result for gas includes production in at least 3% of the volume of the fermentation tube. A positive reaction for acid is a change in color from light amber to dark pink or red.

Limitations of the Procedure

Negative tubes remain colorless and should be observed regularly for a total of 30 days.

*Store at 2-8° C.

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