. . . "Similarly, a ??-galactosidase detection system can be used with, for example, the chromogenic substrate o-nitrophenyl-??-D-galactopyranoside (ONPG) to yield a soluble product detectable at 410 nm, or a urease detection system can be used with, for example, a substrate such as urea-bromocresol purple (Sigma Immunochemicals, St. Louis, Mo.)." . .