Phenol red (phenol-sulfon-phthalein) with a pH range of 6.8 to 8.4, is an excellent indicator.
Bacterial metabolic activity generates acidic products which cause a reactions from red to
yellow. In the case of mineral base, ammonium sulfate is the sole source of nitrogen, and the
various types of sugars added later on serve as a source of carbon in this medium. A positive
reaction is indicated by a precipitate after several days of incubation at room temperature.