The Ames Test for
mammalian environmental mutagenicity
The Ames Test
combines a bacterial revertant
mutation assay with a simulation of mammalian metabolism to
produce a highly sensitive test for mutagenic chemicals in the
A rat liver homogenate is prepared to produce a
metabolically active extract (S9).
[Above] The extract is combined with a strain of his- Salmonella
bacteria: in the absence of histidine,
the bacteria are unable to grow on minimal medium (control result). [Below] The homogenate and bacterial
strain are combined with a suspected mutagenic substance (X).
The induction of revertant colonies indicates that some his- bacteria have mutated
(reverted) to his+ ,
and therefore that substance X is a mutagen.
Different bacterial strains are sensitive to different types
Initial experiments used the reversion assay
without a liver homogenate. However, mutagenicity unlike
toxicity is not the result of ingestion of a suspect
substance, but rather its accumulation and that of its breakdown
products in the body. Use of a liver homogenate simulates the
metabolic breakdown of the suspected mutagen in a mammalian
system, and more accurately predicts mutagenicity of substances
ingested by humans. For example, sodium
nitrate (NaNO3), which occurs naturally in
smoked meats such as bacon, hot dogs, ham, etc., is not itself
mutagenic. However, when acted upon by HCl in the stomach, it is converted to nitrous acid (HNO2), which has been demonstrated to be a
powerful mutagen by the Ames Test.
Bruce Ames (1928 - ) and his
undergraduate students tested large numbers of commercial products
in student labs at UC Berkeley when the test was first introduced
in the 1970s. Many common items, such as hairspray and food colours, were discovered to be mutagenic and were
withdrawn from the market. Ames also established that many
mutagenic compounds are also carcinogenic, an early
indication that changes in DNA sequence led to cancer.
Griffiths et al. ©1996; All text material ©2014 by Steven M. Carr