Endo Agar

Intended Use

Endo Agar is a differential and slightly selective culture medium for the detection of coliform and other enteric microorganisms.

Summary and Explanation

The majority of the enteric plating media developed in the early years of the 20th century utilized either mixtures of bile salts or individual salts as selective agents to achieve inhibition of gram-positive species. In 1904, Endo reported the development of a culture medium for the differentiation of lactose fermenters from the nonfermenters in which no bile salts were used.1 Inhibition of gram-positive microorganisms was achieved by the sodium sulfite and basic fuchsin contained in the formulation. Endo’s Fuchsin Sulphite Infusion Agar was the
original name for this medium,2 which is known today as Endo Agar. It was developed initially in order to facilitate the isolation and identification of the typhoid bacillus.
The original formula has been modified extensively since its introduction. The meat infusions have been replaced by a peptic digest of animal tissue. The dye composition and
concentration also have been adjusted.
Over the years, Endo Agar has been an important medium in the microbiological examination of potable water and wastewater, dairy products and foods; however, the current
compendia of standard methods for the examination of these materials recommend alternative media formulations.3-5

Principles of the Procedure

The selectivity of Endo Agar is due to the sodium sulfite/basic fuchsin combination, which results in the suppression of gram-positive microorganisms. It is classified as only slightly
selective since other media contain more potent inhibitors of the gram-positive microorganisms. Coliforms ferment the lactose, produce pink to rose-red colonies and similar coloration
of the medium. The colonies of organisms that do not ferment lactose are colorless to faint against the pink background of the medium.

Formula

Endo Agar
Approximate Formula* Per Liter
Dipotassium Phosphate................................................. 3.5 g
Peptic Digest of Animal Tissue...................................... 10.0 g
Agar.......................................................................... 15.0 g
Lactose...................................................................... 10.0 g
Sodium Sulfite.............................................................. 2.5 g
Basic Fuchsin................................................................ 0.5 g
*Adjusted and/or supplemented as required to meet performance criteria.

User Quality Control

Identity Specifications
Endo Agar
Dehydrated Appearance: Fine, homogeneous powder that may contain a
                                   large amount of minute to small dark particles.
Solution:                       4.15% solution, soluble in purified water upon
                                   boiling. Solution is light to medium, pink rose to
                                   tan rose trace orange, moderately hazy to hazy.
                                   May contain a moderate amount of small dark
                                  red particles and a large amount of minute dark
                                  red particles.
Prepared Appearance:   Light to medium, pink rose to tan rose trace
                                  orange, moderately hazy to hazy. May contain
                                  a moderate amount of small dark red particles
                                  and a large amount of minute dark red particles.
Reaction of 4.15%
Solution at 25°C:          pH 7.5 ± 0.2
Cultural Response
Endo Agar
Prepare the medium per label directions. Inoculate and incubate at 35 ± 2°C for 48 hours.

Directions for Preparation from Dehydrated Product

1. Suspend 41.5 g of the powder in 1 L of purified water. Mix thoroughly.
2. Heat with frequent agitation and boil for 1 minute to completely dissolve the powder.
3. Autoclave at 121°C for 15 minutes.
4. Cool to 45-50°C. Resuspend precipitate by gentle mixing before use. Endo Agar should be prepared as needed.
5. Test samples of the finished product for performance using stable, typical control cultures.

Procedure

Use standard procedures to obtain isolated colonies from specimens. A nonselective medium should also be streaked to increase the chance of recovery when the population of gramnegative organisms is low and to provide an indication of other organisms present in the specimen. Incubate plates, protected from light, at 35 ± 2°C for 18-24 hours. If negative after 24 hours, reincubate an additional 24 hours.

Expected Results

Typical colonial morphology on Endo Agar is as follows:
Escherichia coli..................Pink to rose-red, green metallic sheen
Enterobacter/Klebsiella.......Large, mucoid, pink
Proteus............................Colorless to pale pink
Salmonella.......................Colorless to pale pink
Shigella............................Colorless to pale pink
Pseudomonas...................Irregular, colorless
Gram-positive bacteria.....No growth to slight growth

*Store at 2-8°C.

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