This is a Provisional monograph.

MacConkey broth (MacConkey, 1905) was originally used for the detection of coliforms and Escherichia coli in water and this was replaced by the defined substrate medium (Gray, 1964) modified by the PHLS in 1969. Samples are inoculated into the medium as a multiple tube system and all broths are incubated at 37°C for 24 h. Tubes or bottles showing acid production are sub-cultured to MacConkey agar and lactose-fermenting isolates are confirmed by acid production or possession of β-galactosidase and lack of cytochrome oxidase at 37°C for coliforms and lactose fermentation or possession of β-galactosidase and indole production at 44°C for thermotolerant coliforms and Escherichia coli, respectively. The medium may be made up as double or single strength.