This monograph has been assessed by members of the IUMS–ICFMH Working Party on Culture Media and given ‘Approved’ status.

This medium, designed for the enumeration of bacteria of the coli–aerogenes group, is derived from MacConkey's original formula (1905). It relies on the use of the selective inhibitory components crystal violet and bile salts and the indicator system lactose and neutral red. Thus the growth of many unwanted organisms is suppressed, while tentative identification of sought bacteria can be made. Organisms which rapidly attack lactose produce purple colonies surrounded by purple haloes. Non-fermenters or late lactose fermenters produce pale colonies with greenish zones. Some other related Gram-negative bacteria may grow, but may be suppressed by the overlay procedure. Selectivity can also be increased by incubation under anaerobic conditions and at elevated temperatures, i.e. >42°C (Mossel and Vega, 1973).