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 environment.

    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 of mutation.

    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hairspray and food colours, were discovered to be mutagenic and were withdrawn from the market.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.

Illustration after Griffiths et al. ©1996; All text material ©2014 by Steven M. Carr