To determine if an agent is mutagenic, researchers use testing methods
that can monitor whether or not an agent increases the rate of
mutation. Many different kinds of tests have been used to evaluate
mutagenicity. One common method is the Ames test, which was
developed by Bruce Ames. This test uses strains of a bacterium,
Salmonella typhimurium, that cannot synthesize the amino acid
histidine. These strains contain a point mutation within a gene that
encodes an enzyme required for histidine biosynthesis. The mutation
renders the enzyme inactive. Therefore, the bacteria cannot
grow on petri plates unless histidine has been added to the growth
medium. However, a second mutation—a rev ersion—may occur
that restores the ability to synthesize histidine. In other words,
a second mutation can cause a reversion back to the wild-type
condition. The Ames test monitors the rate at which this second
mutation occurs, thereby indicating whether an agent increases the
mutation rate above the spontaneous rate.