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Dehydrated Culture Media

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ENDO AGAR BASE

Code: CM0479

A modified medium requiring the addition of basic fuchsin to form Endo Agar.

Typical Formula*

gm/litre

Peptone

10.0

Lactose

10.0

Di-potassium phosphate

3.5

Sodium sulphite

2.5

Agar

10.0

pH 7.5 ± 0.2 @ 25°C

 
* Adjusted as required to meet performance standards

Directions
Suspend 36g in 1 litre of distilled water. Add 6ml (or as directed by the supplier) of a 10% w/v solution of basic fuchsin BR0050 (50:50 ethanol/distilled water). Bring to the boil to dissolve completely. Sterilise by autoclaving at 121°C for 15 minutes. Mix well before pouring. Plates should be stored in the dark to preserve their pale pink colour.

BASIC FUCHSIN IS A POTENTIAL CARCINOGEN AND CARE SHOULD BE TAKEN TO AVOID INHALATION OF THE POWDERED DYE AND CONTAMINATION OF THE SKIN.

Description
Endo Agar is a long established medium which was originally devised for the isolation of the typhoid bacillus. More reliable media for this purpose have since been evolved, and the medium is now used for the differentiation of lactose fermenting and non-lactose fermenting intestinal organisms, particularly during confirmation of the presumptive test for coliforms. Production of both acid and aldehyde by lactose fermenting organisms, such as Escherichia coli, gives rise to the characteristic red coloration of the colony and the surrounding medium.

Technique
For the confirmation of presumptive tests with liquid media, subculture tubes showing gas, or acid and gas formation, onto an Endo Agar plate. Incubate for 24 hours at 35°C.

Lactose fermenting coliforms (e.g. Escherichia coli) give rise to deep red colonies which colour the surrounding medium and possess a golden metallic sheen.

Non-lactose fermenters form colourless translucent colonies, against the pink to colourless medium.

Storage conditions and Shelf life
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
Store the prepared plates at 2-8°C away from light.

Appearance
Dehydrated medium: Straw coloured, free-flowing powder
Prepared medium: Straw coloured gel

Quality control

Positive controls:

Expected results

Escherichia coli ATCC® 25922 *Good growth; red colonies, red medium, golden sheen

Enterobacter aerogenes ATCC® 13048 *

Good growth; pink / red coloured colonies no sheen

Negative control:

 
Staphylococcus aureus ATCC® 25923 *Inhibited
* This organism is available as a Culti-Loop®

Precautions
Weigh out the basic fuchsin (BR0050) in a fume cupboard and avoid inhalation of the powder or contamination of the skin.
Keep the prepared medium away from light to avoid photo-oxidation.
Endo Agar is quoted by the American Public Health Association as a `Standard Methods’ medium for use in water1 and dairy products2. Windle Taylor3 recommended the medium for the isolation and differentiation of coli-aerogenes bacteria from water.

References

1. American Public Health Association (1998) Standard Methods for the Examination of Water and Wastewater. 20th Edn. APHA Inc. Washington DC.
2. American Public Health Association (1978) Standard Methods for the Examination of Dairy Products. 14th Edn. APHA Inc. Washington DC.
3. Windle Taylor E. (1958) `The Examination of Waters and Water Supplies’ 7th ed., Churchill Ltd., London, pp. 417. 440-441, 780-781.

 
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