The Ames test's principle is to determine whether a substance is mutagenic by testing its capacity to revert mutations present in the tester mutant bacteria and restore its ability to synthesize an essential amino acid required for growth. For example, tester mutant bacteria his- S. typhimurium carries a mutation that makes them incapable of producing the amino acid histidine, and without it, they cannot grow. If a substance were mutagenic, exposure to the tester mutant bacteria would cause a reverse mutation to his+ S. typhimurium. The bacteria would regain its ability to synthesize histidine such that it can grow on a histidine-free medium.