ONPG Discs are used for the rapid detection of b-galactosidase activity in microorganisms, specially to identify late lactose fermenters quickly. ONPG is a synthetic colourless compound (galactoside) structurally similar to lactose. Since members of family Enterobacteriaceae are routinely grouped according to their lactose fermenting ability the ONPG test is significant. ONPG discs are sterile filter paper discs impregnated with ONPG. ONPG is similar in structure to lactose. The presence of two enzymes is required to demonstrate lactose fermentation in a conventional test. Citrobacter freundii ATCC 8090 Positive reaction, yellow colour; Proteus vulgaris ATCC 13315 Negative reaction: no colour change