A complex bacterial growth medium containing two pH-sensitive dyes, eosin and methylene blue, and a sugar such as lactose. Lac+ cells preferentially metabolize lactose oxidatively, resulting in excretion of hydrogen ions; under low pH, the dyes become deep purple. Lac− cells that use amino acids as a source of energy release NH3, thus raising pH; this decolorizes the dyes and produces a white colony. These color differences occur for all metabolizable sugars and also for many other carbon sources.
Subjects: Genetics and Genomics.