What is the Ames test?

The Ames test is a bacterial assay used to determine the mutagenicity of a given chemical compound (i.e., its ability to cause mutations in the DNA). This assay uses mutant strains of bacteria, his- S. typhimurium or trp- E. coli, that are incapable of producing the corresponding amino acid necessary for its growth. When these mutant bacteria are cultured on media lacking that specific amino acid, they cannot grow, and no colonies are produced. This method tests the capability of the chemical in question to reverse this mutation in the mutant bacteria. If the chemical is mutagenic, the mutant bacteria will regain their ability to synthesize the corresponding amino acid and grow. For example, his- S. typhimurium would reverse mutate to his+ S. typhimurium and regain its ability to synthesize histidine, such that it can grow on a histidine-free medium. The formation of colonies is indicative of a positive Ames test and confirmation that the chemical is mutagenic.