Endo Agar Or Endo's Medium Agar ; Definition, Composition, Microbiology, Preparation, Uses

Endo Agar, also known as Endo's Medium, is a selective and differential medium used to isolate and identify Gram-negative bacteria, particularly coliforms, based on their ability to ferment lactose.

Composition
A typical Endo Agar medium includes:

Peptone: 10.0 g/L

Lactose: 10.0 g/L

Dipotassium Phosphate: 3.5 g/L

Sodium Sulfite: 2.5 g/L

Basic Fuchsin: 0.5 g/L

Agar: 15.0 g/L

Final pH: 7.5 ± 0.2 at 25°C

Principle
Endo Agar differentiates Gram-negative bacteria based on lactose fermentation. The medium contains peptone, which provides essential nutrients for bacterial growth. Sodium sulfite and basic fuchsin make the medium selective by inhibiting Gram-positive organisms. Lactose-fermenting bacteria produce aldehydes that react with sodium sulfite and basic fuchsin, resulting in red colonies with a metallic sheen. Non-lactose-fermenting bacteria form clear, colorless colonies.

Procedure
Preparation: Suspend 41.5 grams of the medium in 1 liter of distilled water. Heat to boiling to dissolve completely. Sterilize by autoclaving at 121°C for 15 minutes. Mix well before pouring into sterile Petri plates.

Inoculation: Streak the test organism onto the surface of the agar.

Incubation: Incubate the plates at 35-37°C for 18-24 hours.

Observation: Examine the plates for colony morphology and color changes.

Results
Lactose Fermenters: Red colonies with a metallic sheen (e.g., Escherichia coli).

Non-Lactose Fermenters: Clear, colorless colonies (e.g., Salmonella species).

Uses
Endo Agar is used for:

Isolation and identification of coliforms from clinical and non-clinical samples.

Differentiation of lactose-fermenting and non-lactose-fermenting Gram-negative bacteria.

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