EC Broth with MUG (also known as Modified EC Broth with 4-Methylumbelliferyl-β-D-Glucuronide or mEC + MUG Broth) is a selective enrichment liquid medium designed for the presumptive detection, isolation, and enumeration of Escherichia coli, particularly β-glucuronidase-positive strains, in water, food, dairy, and environmental samples.

Developed as an enhancement to standard EC Broth (Hajna and Perry, 1951), this formulation incorporates MUG as a fluorogenic substrate that, when hydrolysed by the β-glucuronidase enzyme of most E. coli strains, produces bright blue fluorescence under long-wave UV light (366 nm), enabling rapid presumptive screening without the need for immediate subculture. The base formulation contains bile salts for Gram-positive inhibition and lactose for gas production in Durham tubes.

A buffered phosphate system stabilises pH and supports recovery of stressed cells, including organisms recovered from chlorinated water or low-nutrient environments. Incubation at elevated temperature (44.5 ± 0.2 °C for 24 ± 2 hours) enhances selectivity for thermotolerant E. coli and fecal coliforms. EC Broth with MUG is widely used in Most Probable Number (MPN) methods and aligns with APHA, EPA Method 1604, ISO 16649-2, and FDA-BAM approaches for E. coli detection.

EC Broth with MUG selectively enriches thermotolerant coliforms through bile salt inhibition of competing Gram-positive flora. Lactose fermentation results in acid and gas production, which may be detected using Durham tubes. MUG serves as a fluorogenic substrate for β-glucuronidase, an enzyme expressed by most E. coli strains. Hydrolysis of MUG releases a fluorescent compound that is detectable under long-wave UV light, allowing rapid presumptive differentiation of E. coli-positive cultures.