Abstract: A test medium and method for detecting, quantifying, identifying and differentiating up to four (4) separate biological materials in a test sample. A test medium is disclosed which allows quantifying and differentiating under ambient light aggregates of biological entities producing specific enzymes, which might include general coliforms, E. coli, Aeromonas, and Salmonella in a single test medium. A new class of nonchromogenic substrates is disclosed which produce a substantially black, non-diffusible precipitate. This precipitate is not subject to interference from other chromogenic substrates present in the test medium. In one embodiment, the substrates are selected such that E. coli colonies present in the test medium show as substantially black, general coliforms colonies show in the test medium as a blue-violet color, Aeromonas colonies present in the test medium show as a generally red-pink color, and Salmonella colonies show as a generally teal-green color.