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Application(s): Industrial, Food, Dairy
Organism(s): Enterobacteriaceae / Coliforms, E.coli

Modified Lauryl Sulphate Tryptose Broth with MUG & Tryptophan (ISO) is used for the presumptive enumeration of Escherichia coli from milk and milk products using the Most Probable Number (MPN) technique according to ISO 11866-1:2005.

The original Lauryl Tryptose Broth described by Mallmann and Darby (1941) has been modified to incorporate 4-Methylumbelliferyl-β-D-glucuronide (MUG) and tryptophan allowing presumptive E. coli to be enumerated from presumptive coliforms. The addition of MUG to the medium allows the positive discrimination of E. coli strains. As the majority of E. coli produce the β-glucuronidase enzyme, they are able to hydrolyse MUG, releasing a fluorogenic compound. Tryptophan acts as a substrate for the indole test. Tubes which fluoresce under UV are confirmed for E. coli by a positive indole reaction when indole (Kovac´s) reagent is added to the tube. Tubes showing gas formation are identified as being positive for presumptive coliforms.

Phosphate buffers and sodium chloride improve gas production by slow lactose fermenting organisms whilst sodium lauryl sulphate acts as a selective agent for the inhibition of non-coliform organisms.

Modified Lauryl Sulphate Tryptose Broth with MUG & Tryptophan (ISO) is described in ISO 11866-1:2005 and is used in the HPA National Standard Method D5 for Enumeration of coliforms and presumptive Escherichia coli by the Most Probable Number (MPN) technique.