Description : O.N.P.G Medium is used to distinguish between delayed lactose fermenters by microorganisms and true lactose nonfermentation.Two enymes are required for lactose fermentation to take place An induced intracellular enzyme,galactosidase, which hydrolyses Lactose and A permease which regulates the uptake of Lactose into the cells. Lactose nonfermenters possess neither enzyme. Delayed lactose fermenters are deficient in permease. ONPG broth turns yellow in the presence of beta galactosidase