Most of the drinking water in this country is now tested using this invention, known as "defined substrate technology" or DST, which operates by feeding the bacteria a special compound which is a combined food and indicator. As the organisms metabolize the food part of the molecule, they "spit out" the indicator, which then changes color or becomes fluorescent. Since only the bacteria of interest can metabolize the food portion of the molecule, the test is not affected by even large numbers of other bacteria because the nutrients in the medium are carefully defined to allow the growth of the target organism but not other organisms.