AquaVial^TM EX uses a system of dyes designed to change color in the presence of Beta-Glucoronidaze, an enzyme produced by the large majority of E. coli strains. The sample is incubated in a selective nutrient media and for 5 to 9 hours (depending on the sensitivity level desired). 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 indigo. The intensity of the color produced is directly proportional to the number of bacteria found in the sample.