The Ames test for carcinogens, based on mutagenicity. The Ames test uses a strain of Salmonella typhimurium lacking an enzyme needed to synthesize histidine; the bacterium is plated on a histidine-free medium. (a) In the absence of the chemical to be tested, a few cells spontaneously develop a reversion mutation and form colonies. (b) The chemical to be tested is soaked into a filter-paper disk placed in the center of the plate, and the chemical diffuses to create a concentration gradient in the plate. (c) Identical nutrient plates are inoculated with an equal number of cells, but have progressively lower concentrations of the putative mutagen on the filter-paper disk (from left to right). At lower concentrations, the clear zone around the disk is smaller because fewer cells are killed by the chemical.