o-Nitrophenyl-b-D-Galactopyranoside (ONPG) Test Overview In order for bacteria to ferment lactose, two enzymes, permease and beta-galactosidase, must be present. Lactose is transported into the bacterial cell wall by permease and subsequently hydrolyzed by beta-galactosidase into glucose and galactose. bacteria are able to digest glucose and galactose. Some organisms, however, appear to be late or non-fermenters … Read more