The great majority of carcinogenic substances, that is, substances that cause cancer in humans and animals, are also mutagenic in bacteria. This fact has been used to develop an initial screening procedure for carcinogens; instead of the expensive and time-consuming process of exposing laboratory animals (not to mention the moral issues involved), a substance can be tested on bacteria to see if it induces mutations.
The Ames Test assesses the ability of a substance to cause reverse mutations in aux-otrophic strains of Salmonella that have lost the ability to synthesise the amino acid his-tidine (his-). Rates of back mutation (assessed by the ability to grow in a histidine-free medium) are compared in the presence and absence of the test substance (Figure 11.23). A reversion to his+ at a rate higher than that of the control indicates a mutagen. Many substances are procarcinogens, only becoming mutagenic/carcinogenic after metabolic conversion by mammals; in order to test for these, an extract of rat liver is added to the experimental system as a source of the necessary enzymes.
his strain of Salmonella +
Plate onto minimal medium (no histidine)
Some spontaneous His+ colonies
Significantly greater number of colonies than control suggests a mutagen
Figure 11.23 The Ames test. The test assesses the ability of a mutagen to bring about reverse mutations in a strain of S. typhimurium auxotrophic for histidine. Since it is possible that a small number of revertants may occur due to spontaneous mutation, results are compared with a control plate with no added mutagen
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