Mutations that show themselves as phenotypic changes are easily observed in microbial populations. The rapid growth, rate and short generation time of microbes makes possible the identification of the few cells that have experienced a change in their DNA as a result of exposure to a mutagenic agent.
Image Source: elements.geoscienceworld.org
By exposing a population of microbes to a test chemical and counting the number of mutants that appear after culturing, it is possible to determine whether the chemical is a mutagen and at what concentration permanent changes are produced in the DNA.
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Because many mutagenic agents are also carcinogenic (cancer causing) to the eukaryotic cells of humans and animals, the carcinogenicity and mutagenicity of a chemical can be tested in much the same way. Such a carcinogenicity test has been developed by Bruce Ames and is known as the Ames test.
The evidence that many cancers are caused by exposure to man-made and natural chemicals in our environment is mounting. Between 60 and 90 percent of cancer cases are believed to be environmentally induced by carcinogenic agents such as radiation and chemicals.
Cosmetics, cleaning agents, food additive, industrial materials, adhesives, and many other synthetic materials all contain new chemicals that are constantly being introduced into the environment at levels never before experienced.
Each year approximately 70,000 different chemicals become commercially available and ultimately may be ingested, inhaled, or absorbed into the body. The Ames test is a short-term test (2 days) of the potential carcinogenicity of many of these chemical compounds.
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Ames and his coworkers have produced a mutation in Salmonella that prevents its growth under normal conditions. However, if another mutation is caused by a test chemical, the microbe will be able to grow no agar plates. If the chemical does not cause this mutation, there will be no growth.
The test is run by exposing the bacteria to a number of dilutions of the suspect chemical, the microbes are mixed with a liver extract to simulate animal conditions, and the mixture is plated out on agar plates.
The colonies growing on the plate are counted and compared to a control of a known carcinogen. A positive Ames test (appearance of colonies) indicates that the chemical can cause mutations and may be carcinogenic, however, a negative test may not mean that the chemical is safe to use because some carcinogenic agents do not cause mutations, and no carcinogenic test is perfect.
Bacterial and human DNA is not the same and may not be affected by a chemical in the same way. To substantiate the result of the Ames test, long-term animal tests, which may take a year or longer, should be run on the suspect chemical? A number of chemical companies, including Dow and Dupont, have adopted the Ames test as part of their testing procedures. They have taken the position that new chemicals should be “guilty” of being carcinogens until proven “innocent”.
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Tests on chemicals already in use are also being run; however, there is a resistance to ban them from use without running both the short-term Ames test and long-term animal test. For example, Tris, the flame retardant used in children’s pajamas and other clothing, was identified by Ames and his associates in 1975 as a carcinogenic chemical that could be absorbed into the body.
However, until animal tests confirmed the short-term Ames test, the chemical continued to be used. These animal tests required almost two years of research and evaluation.
The same experience has also occurred with the chemicals used in hair dyes. Pretesting new chemicals before they are mass produced and marketed may significantly reduce the incidence of cancer.