The o-Nitrophenyl-β-D-Galactopyranoside (ONPG) test is a biochemical test used for the detection of the enzyme β-galactosidase in bacteria. It is mainly applied for differentiating late lactose fermenting organisms from non-lactose fermenters. It is based on the ability of the organism to hydrolyse a synthetic lactose analogue when the required enzyme is present. The test is especially useful when organisms fail to ferment lactose rapidly on routine culture media.
Lactose fermentation normally requires two enzymes. One enzyme is β-galactoside permease which helps in the transport of lactose into the bacterial cell and the other enzyme…
