ONPG test

(for β-galactosidase in bacteria) a pure culture of a bacterial isolate is grown on a lactose-rich medium, then transferred to a substrate containing the test substance o-nitrophenyl-β-D-galactopyranoside (ONPG). ONPG does not require permease to enter the cell, so the test is independent of the presence of permease in the bacteria being tested, and is useful for identification of late lactose fermenters. If the bacterial strain possesses β-galactosidase, the ONPG will be split to form o-nitrophenol, yielding a yellow color. The test is used in diagnostic microbiology, particularly in identification of Enterobacteriaceae, such as in distinguishing certain Citrobacter species and Salmonella enterica subsp. arizonae from other species of Salmonella.