The Ames test is a biological assay to assess the mutagenic potential of chemical compounds. A positive test indicates that the chemical might act as a carcinogen (although a number of false-positives and false-negatives are known). As cancer is often linked to DNA damage, the test also serves as a quick assay to estimate the carcinogenic potential of a compound since it is difficult to ascertain whether standard carcinogen assays on rodents were successful. The procedure is described in a series of papers from the early 1970s by Bruce Ames and his group at the University of California, Berkeley.
The test uses several strains of the bacterium Salmonella typhimurium that carry mutations in genes involved in histidine synthesis i.e. it is a auxotrophic mutant, so that they require histidine for growth. The variable being tested is the mutagen's ability to cause a reversion to growth on a histidine-free medium. The tester strains are specially constructed to have both frameshift and point mutations in the genes required to synthesize histidine, which allows for the detection of mutagens acting via different mechanisms. Some compounds are quite specific, causing reversions in just one or two strains. The tester strains also carry mutations in the genes responsible for lipopolysaccharide synthesis, making the cell wall of the bacteria more permeable, and in the excision repair system to make the test more sensitive. Rat liver extract is optionally added to simulate the effect of metabolism, as some compounds, like benzo[a]pyrene, are not mutagenic themselves but their metabolic products are.
The bacteria are spread on an agar plate with a small amount of histidine. This small amount of histidine in the growth medium allows the bacteria to grow for an initial time and have the opportunity to mutate. When the histidine is depleted only bacteria that have mutated to gain the ability to produce its own histidine will survive. The plate is incubated for 48 hours. The mutagenicity of a substance is proportional to the number of colonies observed.
As Salmonella is a prokaryote, it is not a perfect model for humans. An adapted in vitro model has been made for eukaryotic cells, for example yeast structure. The original test also doesn't count for metabolites that are formed by in the hepatic system. Modified tests can include liver S9 fraction to help recreate the system and observe whether the parent molecule's metabolites formed in the hepatic system are positive. Further tests would be needed to determine the specific metabolite that causes a positive Ames to further any drugs development.
Drugs that contain the nitrate moiety sometimes come back positive for Ames when they are indeed safe. Nitroglycerin is an example that gives a positive Ames yet is still used in treatment today. The conditions of the Ames test are dosed at very high concentrations and with nitrate compounds that can potentially generate nitric oxide (NO), an important signal molecule, will give a false positive. Long toxicology and outcome studies are needed with such compounds to disprove a positive Ames test.
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