What enzyme does the ONPG test use to allow organisms to be classified as lactose fermenters?

### β-galactosidase

## Step1: Understanding the ONPG Test### The ONPG test is a biochemical test used to determine if an organism can ferment lactose. It involves the use of ONPG (ortho-Nitrophenyl-β-galactoside), which is a substrate for the enzyme β-galactosidase.## Step2: Role of β-galactosidase### β-galactosidase is the enzyme that breaks down lactose into glucose and galactose. If an organism produces β-galactosidase, it will hydrolyze ONPG, resulting in a color change due to the release of ortho-nitrophenol, indicating lactose fermentation.