ONPG Disks are used to detect the presence of β-galactosidase, an enzyme found in lactose-fermenting organisms. Lactose utilization depends upon two enzymes: β-galactoside permease, which catalyzes transport of lactose into the cell, and β-galactosidase, which breaks down lactose into galactose and glucose. β-Galactosidase is not lactose specific and can act on simple galactosides including the ONPG (o-nitrophenyl-β-D-galactopyranose) substrate. ONPG hydrolysis results in the release of galactose, and the yellow chromogenic compound, o-nitrophenol. The test substrate, ONPG, does not depend on an induced or constitutive permease enzyme to enter the cell, therefore reactions are rapid and occur within a 24-hour period even for late lactose fermenters.
Sterile filter paper discs (50 pcs, diameter 6 mm) impregnated with o-nitrophenyl-β-D-galacto