(for β-galactosidase in bacteria) the organism is grown in a buffered peptone medium containing D-nitrophenyl-β-D-galactopyranoside (ONPG): production of β-galactosidase is indicated by the appearance of a yellow color. Used to differentiate Salmonella (positive) from Arizona (negative), and Neisseria lactamicus (positive) from N. meningitidis (negative).
