Lauryl Sulfate Chromogenic Broth

The combination of chromogenic compounds within Lauryl Sulfate Broth provides a double indicator system. This medium contains a phosphate buffer to ensure the high growth of the total number of coliforms. Lauryl sulfate inhibits Gram-positive bacteria. Coliforms and E. coli contain beta-galactosidase which cleaves the chromogenic substrate. The enzyme which cleaves MUG is highly specific to E. coli, making the simultaneous detection of total coliforms and E. coli possible. IPTG stimulates the synthesis and increases the activity of beta-galactosidase.

The color change from amber to blue-greenish due to the reaction of the chromogenic substrate indicates the presence of coliforms. Blue fluorescence under UV light allows the rapid detection of E. coli due to the MUG.

Tryptophan promotes the indole reaction after adding Kovac's Reagent (LC-5205). This reagent detects the microorganisms capable of cleaving the tryptophan. When E. coli is present in the medium, indole is liberated and reacts with 4-dimethylaminobenzaldehyde to form a dark red dye.

Typical formula in g/L, adjusted and/or supplemented as required to meet performance criteria.

Preparation

Suspend 17 grams of the medium in one liter of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Dispense into appropriate containers and sterilize in the autoclave at 121 °C for 15 minutes.

Instructions for use

Inoculate and incubate at 35 ± 2 °C for 18-24 hours. Check the tubes under UV light (366 nm). Light blue fluorescence indicates the presence of E. coli.

• MANAFI, M., KNEIFEL, F., a. BASCON, S.: Fluorogenic and chromogenic substrates used in bacterial diagnosis. Microbiol. Rev. 55; 335-348 (1991).
• OSSMER, R.: Simultaneous Detection of Total Coliforms and E. coli-Fluorocult LMX-Broth. - 15th International Symposium/FOOD MICRO 1993. The International Committee on Food Microbiology and Hygiene, Bingen/Rhine (1993).