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 is seen
to induce reversion
mutations many 103s
of revertant
( his+)
colonies. Note that concentration of the chemical drops off
towards the outer edge 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.