Bacteria are little geniuses at identifying molecules. They're also prolific, cheap, and easy to manipulate, which makes them ideal workers. Using genetically modified, nonharmful strains of E. coli, chemist Sylvia Daunert of the University of Kentucky has designed a prototype biosensor system capable of detecting a variety of drinking-water toxins, including arsenic, anthrax, lead and PCBs. The bacteria are housed in the tip of a fiber-optic cable, which dangles in the drinking-water supply. When they detect a toxin, the bacteria glow; the light they produce is carried along the fiber to a monitoring station, where its intensity is measured to determine the precise concentration of toxic molecules (sensitive down to parts-per-billion scale).