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The bacterial reverse mutation test, commonly known as the Ames assay, uses amino acid-requiring strains of Salmonella typhimurium (S. typhimurium) and Escherichia coli (E. coli) to detect point mutations, which involve substitution, addition, or deletion of 1 or a few DNA base pairs. The principle of this bacterial reverse-mutation test is that it detects chemicals that induce mutations which revert mutations present in the tester strains and restore the functional capability of the bacteria to synthesize an essential amino acid. The revertant bacteria are detected by their ability to grow in the absence of the amino acid required by the parent tester strain.

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