Endo LES Agar

Description

The moderate selectivity is due to the formation of a fuchsine-sulfite compound. This compound reacts with the acetaldehyde formed in the lactose fermentation and frees the fuchsin dye that colours the bacterial colony. The strains that produce large amounts of the metabolite, like E. coli, can crystallize the fuchsin on the colony, giving rise to the characteristic green metallic sheen.

Colonies of coliforms, which ferment lactose, are pink to pale red, with or without green metallic sheen: marked reddening of the medium may occur. Colonies of other enteric bacilli, including Salmonella, and of non-lactose-fermenters are about the same colour as the medium, being almost colourless to faint pink.

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.

Technique

Collect, dilute and prepare samples and volumes to be filtered as required according to specifications, directives, official standard regulations and/or expected results.

Filter the sample through a 0.45 µm pore membrane and apply it onto the surface of the agar.

Incubate the plates aerobically at 36 ± 1 °C for 18-24 h.

(Incubation times greater than those mentioned above or different incubation temperatures may be required depending on the sample, on the specifications, etc.)

After incubation, enumerate all the colonies developed on the membrane.

Calculate total microbial count per mL of sample by multiplying the average number of colonies per membrane by the inverse dilution factor. Report results as colony forming units (CFU) per mL along with incubation time and temperature.

Confirmation of E. coli (dark red colonies with a characteristic metallic sheen) is required.

Physical/chemical control

Microbiological control

Membrane filtration / practical range 100 ± 20 CFU; min. 50 CFU (productivity) / 10⁴-10⁶ CFU for selectivity.

Aerobiosis. Incubation at 37 ± 1 °C, reading after 24 ± 3 h.

Sterility control

Incubation 48 hours at 30-35 °C and 48 hours at 20-25 °C: NO GROWTH.

Check at 7 days after incubation in same conditions.

• APHA/AWWA/WEF (1985) Standard Methods for the examination of water and wastewater. 15th ed. APHA Washington. DC. USA.
• APHA (1967) Standard methods for the examination of dairy products. 12th ed. APHA Washington. DC. USA.
• ATLAS, R.M. (1995) Handbook of Microbiological Media for the Examination of Food. CRC Press. Boca Raton. Fla. USA.
• ATLAS, R.M. & R.C. PARKS (1993) Handbook of Microbiological Media. CRC Press. London.
• ENDO, S. (1904) Über ein verfahren zum Nachweis von typhusbazillen. Zbl. Bakt. Hyg. Abt. I Orig. 35:109.
• ISO 11133:2014/Amd 1:2018. Microbiology of food, animal feed and water. Preparation, production, storage and performance testing of culture media.
• WINDLE TAYLOR, E. (1958) The examination of water and water supplies. 7th ed. Churchill Ltd. London.