AquaVial^TM EX uses a system of dyes designed to change color in the presence of β–glucuronidase (for E coli detection) or β–Galactosidase (for Total Coliform detection). The sample is incubated in a selective nutrient media and for 5 to 9 hours (depending on the sensitivity level desired) at 35-40 C (95-105 F), or 18-24 hour at room temperature. After the incubation, a contrast agent is added that kills the live bacteria and reacts with metabolites produced during the incubation. This reaction results in a change of color of the solution from yellow to purple or indigo. The intensity of the color produced is directly proportional to the number of bacteria found in the sample.