Reversion mutations in his- Salmonella bacteria
    The dish on the left was plated with ~109  his- Salmonella bacteria, which cannot grow in the absence of the amino acid histidine. In this control experiment, the small number (~102) of white colonies are derived from single bacteria that have undergone spontaneous reversion mutations to his+. The reversion test is thus extremely sensitive, because it can detect mutation rates as small as 1/109-2 = 10-7/ cell.

    In the experiment on the right, the disc at the center of the dish contains a mutagenic chemical. As it diffuses outward, the chemical at high concentration is toxic and at first kills all the bacteria (clear circle around the disc), but at lower concentrations gives rise to induced reversion mutations, seen as many 103s of revertant ( his+) colonies. As the concentration continues to decrease towards the outer edge of the plate, the frequency of revertant colonies falls to about the same as in the control experiment at left. In cell culture, it is therefore possible to measure the precise degree of mutagenicity at a range of concentrations.

    The Ames Test uses the bacterial reversion assay to
measure mutagenicity as the difference between the induced and spontaneous rates of reversion mutation at various concentrations of the mutagenic substance.

All text material 2014 by Steven M. Carr