ONPG

ONPG (Ortho-nitrophenyl beta-D-galactopyranoside) is a synthetic colourless compound (galactoside) structurally
similar to lactose (1). beta-galactosidase cleaves ONPG to galactose and o-nitrophenyl, a yellow compound. The ONPG
test is especially useful in the rapid identification of cryptic lactose fermenters (late fermenters). Since members of
family Enterobacteriaceae are routinely grouped according to their lactose fermenting ability the ONPG test is
significant here. 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. The first
enzyme permease, facilitates the entry of lactose molecules into the bacterial cell while the second enzyme, betagalactosidase,
hydrolyzes the lactose to yield glucose and galactose. True non-lactose fermenters lack both enzymes;
however, some organisms lack permease but possess betagalactosidase. These organisms are late lactose fermenters.

Testing for ONPG.