Reversion mutations in the Ames Test
    The dish on the left was plated with about 109  his- bacteria. In this control experiment, in the absence of the amino acid Histidine, such bacteria will not grow, except for a small number (<102) of white colonies derived from single bacteria that have undergone spontaneous reversion mutations to his+. In the experiment on the right, the filter-paper disc in the middle of the dish contains a chemical being tested for mutagenic properties. As it diffuses outward, the chemical first kills all the bacteria (clear area around the disc), but at lower concentration induces reversion mutations in many 103s of revertant ( his+) colonies. Note that as the concentration of the chemical drops off towards the outer walls of the plate, the frequency of revertant colonies falls to about the same as in the control.

    The Ames Test measures mutagenicity as the difference between the induced and spontaneous rates of reversion mutation.


All text material ©2024 by Steven M. Carr