The Ames test is used to detect mutagenic substances. It utilises genetically engineered strains of Salmonella that are sensitive to mutations caused by a wide range of chemicals. Specifically, these strains have mutations in their histidine operons, which prevent them from synthesising histidine, an amino acid essential for bacterial growth and replication.

When a population is grown on an agar plate with a trace amount of histidine, most of the bacteria die. However, some use the trace amount to reproduce and spontaneously revert to the histidine-independent wild type, allowing them to synthesise histidine on their own. Each generation produced in this way exhibits a natural spontaneous mutation frequency that can be quantified.

The mutations in the histidine operon, which make the bacteria dependent on an external supply of histidine, act as hotspots for mutagens to act upon. By comparing the number of revertant colonies in the presence of a mutagen (after 48 hours at 37°C) to the number of revertant colonies on a control plate, we can determine the mutagenic potential of different substances.

However, bacteria lack some of the mammalian metabolic reactions, such as oxidation and reduction. For this reason, we may need to add rat liver products to mimic these reactions and assess the mutagenic behaviour of substances formed as intermediate products during these processes.