Toxicology. A test to determine if a substance is a potential mutagen or carcinogen. The Ames test is performed on a auxotrophic strain of bacteria (Salmonella typhimurium). The auxotrophic bacteria lost the ability to grow on medium without the addition of the amino acid histidin. After exposure to a mutagen some of the bacteria (revertants) can grow and form colonies on the histidin-free medium due to a new mutation.