What is indicated by a positive ONPG test result?

A positive ONPG (Ortho-Nitrophenyl-β-D-Galactopyranoside) test is indicated by the production of a yellow color. This test is used to detect the presence of the enzyme β-galactosidase, which can hydrolyze ONPG into galactose and ortho-nitrophenol. The ortho-nitrophenol released during this reaction is yellow in color, which is why the presence of a yellow color after the test is what signifies a positive result.

In the context of microbiology, a positive ONPG test is particularly important for differentiating members of the Enterobacteriaceae family. For instance, it helps distinguish lactose fermenters, like Escherichia coli, from non-lactose fermenters, such as Salmonella and Shigella, which do not produce the enzyme and thus do not yield a color change. The presence of a yellow color confirms that the organism being tested is capable of utilizing lactose and producing the enzyme β-galactosidase.