Description
ONPG Test is a rapid biochemical test for detecting bacterial beta-galactosidase activity, particularly during differentiation of Gram-negative enteric bacteria.
Beta-galactosidase hydrolyses colourless ONPG to o-nitrophenol, producing a yellow reaction. This allows late lactose fermenters to be distinguished from true non-fermenters without relying on lactose permease.
A pure 18–24-hour culture is suspended in the test tube and incubated at 35 ± 2°C. Results are examined after four hours and up to 24 hours; yellow indicates a positive reaction.
