Measurement of active constitutive beta-D-glucosidase (esculinase) in the presence of sodium desoxycholate.

The hydrolysis of esculin in the presence of bile has been utilized for many years for the identification of bacteria. It is especially useful in differentiating species of the genus Streptococcus. The procedure is a two-step one. First, the bacterium must grow in a particular concentration of bile, and second, it must hydrolyze esculin. The hydrolysis of esculin has traditionally been determined by the brown-black color that results when one of the hydrolysate products, esculetin, reacts with iron in the medium. The procedure requires incubation for 24 h or more. A method was developed based on the measurement of constitutive beta-glucosidase (esculinase) with the repression of this enzyme by bile equivalent (sodium desoxycholate) that required only 30 min. p-Nitrophenyl-beta-D-glucopyranoside was the esculinase substrate, and sodium desoxycholate was substituted for bile salts. After inoculation, a yellow color was equivalent to the brown-black seen in the 40% bile-esculin reaction. The reagent was dispensed in test tubes and was stable for 6 months. The 30-min procedure correlated well with the conventional 24-h bile-esculin agar tube. Streptococcus pneumoniae could also be identified because of the rapid lysis it exhibited in the substrate solution.