A device for differentially enumerating colonies of coliform and Escherichia coli microorganisms is provided. The device comprises a water-impermeable first sheet; a water-impermeable second sheet attached to the first sheet; a dry, rehydratable culture medium comprising a lactose-fermentation indicator system, a β-D-glucuronidase indicator system that comprises a plurality of compounds that enhance β-glucuronidase enzyme activity in E. coli, a redox indicator system and a first cold-water soluble gelling agent adhered to the first sheet, the culture medium disposed in a microbial growth zone; and a second cold-water-soluble gelling agent adhered to the second sheet. The microbial growth zone is disposed between the first sheet and the second sheet. Methods of using the device are also provided.