Endo LES Agar Base

Like Endo Agar, it uses fuchsin to differentiate between lactose-fermenting and lactose non-fermenting bacteria. Acetaldehyde production by lactose-fermenting organisms such as Escherichia coli produces characteristic red colonies and a red surrounding area, marked by its reaction with sodium sulfite in the presence of fuchsin. Lactose non-fermenters form colorless, transparent colonies.

Casein and meat peptones, and tryptose provide nitrogen, vitamins, minerals and amino acids essential for growth. Yeast extract is a source of vitamins, particularly of the B-group. Lactose is the fermentable carbohydrate providing carbon and energy. Potassium phosphates act as a buffer system. Sodium deoxycholate inhibits growth of Gram-positive bacteria. Sodium lauryl sulfate partially inhibits organisms other than coliforms. Sodium chloride supplies essential electrolytes for transport and osmotic balance. Bacteriological agar is the solidifying agent.

Note: Basic fuchsin is a potential carcinogen and precautions should be taken to avoid inhalation of the dye powder as well as contact with skin.

Preparation

Suspend 50.25 grams of the medium in one liter of distilled water. Add 8 mL of an alcoholic solution at 10% (w/v) of basic fuchsin in 95% ethanol. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Sterilize in autoclave at 121 °C for 15 minutes. Cool to 50 °C, mix well and dispense into plates.

Instructions for use

Use the membrane filter technique to inoculate filters and pre-incubate on pads saturated with Lauryl Sulfate Broth (LC-1310) at 35 ± 2 °C for 1.5-2.5 hours. Transfer filters to plates of Endo LES Agar Base and incubate at 35 ± 2 °C for 18-24 hours.

Rapid lactose fermenters produce red colonies with a metallic sheen. Slow lactose fermenters produce red colonies. Lactose non-fermenters produce colorless colonies.

Caution: on exposure to oxygen, the plated medium gradually becomes red due to the oxidation of sulfite and can thus no longer be used. The oxidized culture medium (very intense red) should not be used because it decreases productivity of the culture medium.

• APHA (1980) Standard Methods for the Examination of Water and Wastewater. 15th Ed. Washington, D.C.