The Ames test is a biological assay used in genetics, generally genetic toxicology , to test for mutagenic properties of a chemical compound. A compound is said to be mutagenic if it causes a change in the DNA (deoxyriboneucleic acid) of a living cell or organism. The test is named after its inventor, Bruce Ames.
This assay is carried out using strains of bacteria, generally Escherichia coli or Salmonella that already have a single mutation, for example, a strain that cannot produce histidine, an amino acid that is essential for the bacterium to grow if not provided externally with essential nutrients. Cultures of the bacteria are grown in an agar containing dish so that a "lawn" of bacteria is present. The experimental cultures are exposed to the agent to be tested while the positive control cultures are exposed to a known mutagen to confirm that there has been no contamination of the strain. Strains of bacteria are available which have been genetically modified such that only a certain type of mutation (i.e. a base pair mutation or a frameshift mutation) will cause the strand to revert to a normal state, not requiring nutrients to grow. If the mutation screened for has in fact occurred dense spots in the colonies will form. A certain number of spots may form due to random mutation not caused by the agent; therefore, data analysis using control dishes is necessary. Occasionally a tested agent will be toxic enough to simply kill the bacterial culture in which case a "thin lawn" is observed.
In some Ames assays an S-9 mix is added. The S-9 mix contains liver enzymes, generally from a rat. These enzymes can metabolize the agent being tested in order to predict the mutagenic properties within a living system, specifically the source of the S-9 enzymes.
Studies have shown that the majority of substances shown to be mutagenic by an Ames assay will be carcinogenic in humans.