The fluctuation assay uses the same concept as the Ames plate incorporation assay but is in liquid form. Bacterial strains are exposed to test chemical in media containing trace amounts of histidine, biotin, and tryptophan. After exposure, media containing a pH indicator is employed and the resultant mixture pipetted into multi-well plates. If a DNA mutation occurred during exposure, the color of the media changes due to acidification by microbial growth. Wells that contain a colony, or appear yellow, are considered revertants.