Aldol® 515 beta-D-galactopyranoside is a chromo- and fluorigenicindicator for beta-galactosidase activity. The colorless product iscleaved by lactose-utilizing bacteria in liquid media or on agar platesand yields an intense red color. In addition, red fluorescence signalsare generated if a fluorescence enhancer or a suitable matrix ispresent. Aldol® 518 beta-D-galactopyranoside can also be used asdetecting agent for beta-galactosidase activity in reporter gene assaysor for identification of positive transformants on agar plates whenusing the α-complementation approach.Color and fluorescence formation is due to on an intramolecular aldolcondensation after enzymatic cleavage. The reaction proceeds equallywell in the presence and absence of oxygen, i.e., in aerobic andanaerobic cultures. Well detectable color/fluorescence of bacterialcolonies or liquid cultures is obtained within 16-48 h. In the presenceof purified beta-galactosidase, color and fluorescence develops withinminutes to few hours.