What is the purpose of the ONPG test?

The ONPG test is specifically designed to detect slow lactose fermenters. This test utilizes o-nitrophenyl-β-D-galactopyranoside as a substrate, which is cleaved by the enzyme β-galactosidase, releasing o-nitrophenol and resulting in a visible yellow color in the medium. Organisms that can metabolize lactose, even if they do so slowly or only in small quantities, will produce β-galactosidase, leading to a positive ONPG test.

This is particularly relevant for distinguishing between lactose fermenters and non-fermenters in bacterial identification. It caters specifically to those bacteria that may not show rapid fermentation with conventional lactose media but still possess the ability to utilize lactose over time. Thus, the ONPG test is crucial for identifying organisms that fall into this category, aiding in the accurate classification and identification of different bacterial species in a laboratory setting.